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Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)

This article was archived around: 1 Nov 2008 10:00:10 GMT

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[Last modified July 3, 2004 by scs.] This article is Copyright 1990-2004 by Steve Summit. Content from the book _C Programming FAQs: Frequently Asked Questions_ is made available here by permission of the author and the publisher as a service to the community. It is intended to complement the use of the published text and is protected by international copyright laws. The on-line content may be accessed freely for personal use but may not be republished without permission. Certain topics come up again and again on this newsgroup. They are good questions, and the answers may not be immediately obvious, but each time they recur, much net bandwidth and reader time is wasted on repetitive responses, and on tedious corrections to any incorrect answers which may unfortunately be posted. This article, which is posted monthly, attempts to answer these common questions definitively and succinctly, so that net discussion can move on to more constructive topics without continual regression to first principles. No mere newsgroup article can substitute for thoughtful perusal of a full-length tutorial or language reference manual. Anyone interested enough in C to be following this newsgroup should also be interested enough to read and study one or more such manuals, preferably several times. Some C books and compiler manuals are unfortunately inadequate; a few even perpetuate some of the myths which this article attempts to refute. Several noteworthy books on C are listed in this article's bibliography; see also questions 18.9 and 18.10. Many of the questions and answers are cross-referenced to these books, for further study by the interested and dedicated reader. If you have a question about C which is not answered in this article, you might first try to answer it by checking a few of the referenced books, or one of the expanded versions mentioned below, before posing your question to the net at large. There are many people on the net who are happy to answer questions, but the volume of repetitive answers posted to one question, as well as the growing number of questions as the net attracts more readers, can become oppressive. If you have questions or comments prompted by this article, please reply by mail rather than following up -- this article is meant to decrease net traffic, not increase it. Besides listing frequently-asked questions, this article also summarizes frequently-posted answers. Even if you know all the answers, it's worth skimming through this list once in a while, so that when you see one of its questions unwittingly posted, you won't have to waste time answering. (However, this is a large and heavy document, so don't assume that everyone on the net has managed to read all of it in detail, and please don't roll it up and thwack people over the head with it just because they missed their answer in it.) This article was last modified on July 3, 2004, and its travels may have taken it far from its original home on Usenet. It may, however, be out-of-date, particularly if you are looking at a printed copy or one retrieved from a tertiary archive site or CD-ROM. You should be able to obtain the most up-to-date copy at http://www.eskimo.com/~scs/C-faq/top.html or http://www.faqs.org/faqs/ , or via ftp from ftp://rtfm.mit.edu/. (See also question 20.40.) Since this list is modified from time to time, its question numbers may not match those in older or newer copies which are in circulation, so be careful when referring to FAQ list entries by number alone. (Also, this article was produced for free redistribution. You should not need to pay anyone for a copy of it.) Several other versions of this document are available. Posted along with it are an abridged version and (when there are changes) a list of differences with respect to the previous version. A hypertext version is available on the web at the aforementioned URL. For those who might prefer a bound, hardcopy version, a book-length version has been published by Addison-Wesley (ISBN 0-201-84519-9). The hypertext and book versions include additional questions and more detailed answers, so you might want to check one of them if you still have questions after reading this posted list. This article can always be improved. Your input is welcome. Send your comments to scs@eskimo.com . The questions answered here are divided into several categories: 1. Declarations and Initializations 2. Structures, Unions, and Enumerations 3. Expressions 4. Pointers 5. Null Pointers 6. Arrays and Pointers 7. Memory Allocation 8. Characters and Strings 9. Boolean Expressions and Variables 10. C Preprocessor 11. ANSI/ISO Standard C 12. Stdio 13. Library Functions 14. Floating Point 15. Variable-Length Argument Lists 16. Strange Problems 17. Style 18. Tools and Resources 19. System Dependencies 20. Miscellaneous Bibliography Acknowledgements (The question numbers within each section are not always continuous, because they are aligned with the aforementioned book-length version, which contains even more questions.) Herewith, some frequently-asked questions and their answers: Section 1. Declarations and Initializations 1.1: How should I decide which integer type to use? A: If you might need large values (above 32,767 or below -32,767), use long. Otherwise, if space is very important (i.e. if there are large arrays or many structures), use short. Otherwise, use int. If well-defined overflow characteristics are important and negative values are not, or if you want to steer clear of sign- extension problems when manipulating bits or bytes, use one of the corresponding unsigned types. (Beware when mixing signed and unsigned values in expressions, though.) Although character types (especially unsigned char) can be used as "tiny" integers, doing so is sometimes more trouble than it's worth, due to unpredictable sign extension and increased code size. (Using unsigned char can help; see question 12.1 for a related problem.) A similar space/time tradeoff applies when deciding between float and double. None of the above rules apply if pointers to the variable must have a particular type. If for some reason you need to declare something with an *exact* size (usually the only good reason for doing so is when attempting to conform to some externally-imposed storage layout, but see question 20.5), be sure to encapsulate the choice behind an appropriate typedef, such as those in C99's <inttypes.h>. If you need to manipulate huge values, larger than the guaranteed range of C's built-in types, see question 18.15d. References: K&R1 Sec. 2.2 p. 34; K&R2 Sec. 2.2 p. 36, Sec. A4.2 pp. 195-6, Sec. B11 p. 257; ISO Sec. 5.2.4.2.1, Sec. 6.1.2.5; H&S Secs. 5.1,5.2 pp. 110-114. 1.4: What should the 64-bit type be on a machine that can support it? A: The new C99 Standard specifies type long long as effectively being at least 64 bits, and this type has been implemented by a number of compilers for some time. (Others have implemented extensions such as __longlong.) On the other hand, it's also appropriate to implement type short int as 16, int as 32, and long int as 64 bits, and some compilers do. See also question 18.15d. References: C9X Sec. 5.2.4.2.1, Sec. 6.1.2.5. 1.7: What's the best way to declare and define global variables and functions? A: First, though there can be many "declarations" (and in many translation units) of a single global variable or function, there must be exactly one "definition", where the definition is the declaration that actually allocates space, and provides an initialization value, if any. The best arrangement is to place each definition in some relevant .c file, with an external declaration in a header (".h") file, which is included wherever the declaration is needed. The .c file containing the definition should also #include the same header file, so the compiler can check that the definition matches the declarations. This rule promotes a high degree of portability: it is consistent with the requirements of the ANSI C Standard, and is also consistent with most pre-ANSI compilers and linkers. (Unix compilers and linkers typically use a "common model" which allows multiple definitions, as long as at most one is initialized; this behavior is mentioned as a "common extension" by the ANSI Standard, no pun intended.) It is possible to use preprocessor tricks to arrange that a line like DEFINE(int, i); need only be entered once in one header file, and turned into a definition or a declaration depending on the setting of some macro, but it's not clear if this is worth the trouble. It's especially important to put global declarations in header files if you want the compiler to catch inconsistent declarations for you. In particular, never place a prototype for an external function in a .c file: it wouldn't generally be checked for consistency with the definition, and an incompatible prototype is worse than useless. See also questions 10.6 and 18.8. References: K&R1 Sec. 4.5 pp. 76-7; K&R2 Sec. 4.4 pp. 80-1; ISO Sec. 6.1.2.2, Sec. 6.7, Sec. 6.7.2, Sec. G.5.11; Rationale Sec. 3.1.2.2; H&S Sec. 4.8 pp. 101-104, Sec. 9.2.3 p. 267; CT&P Sec. 4.2 pp. 54-56. 1.11: What does extern mean in a function declaration? A: It can be used as a stylistic hint to indicate that the function's definition is probably in another source file, but there is no formal difference between extern int f(); and int f(); References: ISO Sec. 6.1.2.2, Sec. 6.5.1; Rationale Sec. 3.1.2.2; H&S Secs. 4.3,4.3.1 pp. 75-6. 1.12: What's the auto keyword good for? A: Nothing; it's archaic. See also question 20.37. References: K&R1 Sec. A8.1 p. 193; ISO Sec. 6.1.2.4, Sec. 6.5.1; H&S Sec. 4.3 p. 75, Sec. 4.3.1 p. 76. 1.14: I can't seem to define a linked list successfully. I tried typedef struct { char *item; NODEPTR next; } *NODEPTR; but the compiler gave me error messages. Can't a structure in C contain a pointer to itself? A: Structures in C can certainly contain pointers to themselves; the discussion and example in section 6.5 of K&R make this clear. The problem with the NODEPTR example is that the typedef has not yet been defined at the point where the "next" field is declared. To fix this code, first give the structure a tag (e.g. "struct node"). Then, declare the "next" field as a simple "struct node *", or disentangle the typedef declaration from the structure definition, or both. One corrected version would be struct node { char *item; struct node *next; }; typedef struct node *NODEPTR; and there are at least three other equivalently correct ways of arranging it. A similar problem, with a similar solution, can arise when attempting to declare a pair of typedef'ed mutually referential structures. See also question 2.1. References: K&R1 Sec. 6.5 p. 101; K&R2 Sec. 6.5 p. 139; ISO Sec. 6.5.2, Sec. 6.5.2.3; H&S Sec. 5.6.1 pp. 132-3. 1.21: How do I construct and understand declarations of complicated types such as "array of N pointers to functions returning pointers to functions returning pointers to char"? A: There are at least three ways of answering this question: 1. char *(*(*a[N])())(); 2. Build the declaration up incrementally, using typedefs: typedef char *pc; /* pointer to char */ typedef pc fpc(); /* function returning pointer to char */ typedef fpc *pfpc; /* pointer to above */ typedef pfpc fpfpc(); /* function returning... */ typedef fpfpc *pfpfpc; /* pointer to... */ pfpfpc a[N]; /* array of... */ 3. Use the cdecl program, which turns English into C and vice versa: cdecl> declare a as array of pointer to function returning pointer to function returning pointer to char char *(*(*a[])())() cdecl can also explain complicated declarations, help with casts, and indicate which set of parentheses the parameters go in (for complicated function definitions, like the one above). See question 18.1. A good book on C should explain how to read these complicated declarations "inside out" to understand them ("declaration mimics use"). The pointer-to-function declarations in the examples above have not included parameter type information. When the parameters have complicated types, declarations can *really* get messy. (Modern versions of cdecl can help here, too.) References: K&R2 Sec. 5.12 p. 122; ISO Sec. 6.5ff (esp. Sec. 6.5.4); H&S Sec. 4.5 pp. 85-92, Sec. 5.10.1 pp. 149-50. 1.25: My compiler is complaining about an invalid redeclaration of a function, but I only define it once and call it once. A: Functions which are called without a declaration in scope, perhaps because the first call precedes the function's definition, are assumed to be declared as returning int (and without any argument type information), leading to discrepancies if the function is later declared or defined otherwise. All functions should be (and non-int functions must be) declared before they are called. Another possible source of this problem is that the function has the same name as another one declared in some header file. See also questions 11.3 and 15.1. References: K&R1 Sec. 4.2 p. 70; K&R2 Sec. 4.2 p. 72; ISO Sec. 6.3.2.2; H&S Sec. 4.7 p. 101. 1.25b: What's the right declaration for main()? Is void main() correct? A: See questions 11.12a through 11.15. (But no, it's not correct.) 1.30: What am I allowed to assume about the initial values of variables and arrays which are not explicitly initialized? If global variables start out as "zero", is that good enough for null pointers and floating-point zeroes? A: Uninitialized variables with "static" duration (that is, those declared outside of functions, and those declared with the storage class static), are guaranteed to start out as zero, just as if the programmer had typed "= 0". Therefore, such variables are implicitly initialized to the null pointer (of the correct type; see also section 5) if they are pointers, and to 0.0 if they are floating-point. Variables with "automatic" duration (i.e. local variables without the static storage class) start out containing garbage, unless they are explicitly initialized. (Nothing useful can be predicted about the garbage.) These rules do apply to arrays and structures (termed "aggregates"); arrays and structures are considered "variables" as far as initialization is concerned. Dynamically-allocated memory obtained with malloc() and realloc() is likely to contain garbage, and must be initialized by the calling program, as appropriate. Memory obtained with calloc() is all-bits-0, but this is not necessarily useful for pointer or floating-point values (see question 7.31, and section 5). References: K&R1 Sec. 4.9 pp. 82-4; K&R2 Sec. 4.9 pp. 85-86; ISO Sec. 6.5.7, Sec. 7.10.3.1, Sec. 7.10.5.3; H&S Sec. 4.2.8 pp. 72-3, Sec. 4.6 pp. 92-3, Sec. 4.6.2 pp. 94-5, Sec. 4.6.3 p. 96, Sec. 16.1 p. 386. 1.31: This code, straight out of a book, isn't compiling: int f() { char a[] = "Hello, world!"; } A: Perhaps you have an old, pre-ANSI compiler, which doesn't allow initialization of "automatic aggregates" (i.e. non-static local arrays, structures, or unions). See also question 11.29. 1.31b: What's wrong with this initialization? char *p = malloc(10); My compiler is complaining about an "invalid initializer", or something. A: Is the declaration of a static or non-local variable? Function calls are allowed in initializers only for automatic variables (that is, for local, non-static variables). 1.32: What is the difference between these initializations? char a[] = "string literal"; char *p = "string literal"; My program crashes if I try to assign a new value to p[i]. A: A string literal can be used in two slightly different ways. As an array initializer (as in the declaration of char a[] in the question), it specifies the initial values of the characters in that array. Anywhere else, it turns into an unnamed, static array of characters, which may be stored in read-only memory, and which therefore cannot necessarily be modified. In an expression context, the array is converted at once to a pointer, as usual (see section 6), so the second declaration initializes p to point to the unnamed array's first element. (For compiling old code, some compilers have a switch controlling whether string literals are writable or not.) See also questions 1.31, 6.1, 6.2, 6.8, and 11.8b. References: K&R2 Sec. 5.5 p. 104; ISO Sec. 6.1.4, Sec. 6.5.7; Rationale Sec. 3.1.4; H&S Sec. 2.7.4 pp. 31-2. 1.34: I finally figured out the syntax for declaring pointers to functions, but now how do I initialize one? A: Use something like extern int func(); int (*fp)() = func; When the name of a function appears in an expression, it "decays" into a pointer (that is, it has its address implicitly taken), much as an array name does. A prior, explicit declaration for the function (perhaps in a header file) is normally needed. The implicit external function declaration that can occur when a function is called does not help when a function name's only use is for its value. See also questions 1.25 and 4.12. Section 2. Structures, Unions, and Enumerations 2.1: What's the difference between these two declarations? struct x1 { ... }; typedef struct { ... } x2; A: The first form declares a "structure tag"; the second declares a "typedef". The main difference is that you subsequently refer to the first type as "struct x1" and the second simply as "x2". That is, the second declaration is of a slightly more abstract type -- its users don't necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it. 2.2: Why doesn't struct x { ... }; x thestruct; work? A: C is not C++. Typedef names are not automatically generated for structure tags. See also questions 1.14 and 2.1. 2.3: Can a structure contain a pointer to itself? A: Most certainly. See also question 1.14. 2.4: How can I implement opaque (abstract) data types in C? A: One good way is for clients to use structure pointers (perhaps additionally hidden behind typedefs) which point to structure types which are not publicly defined. It's legal to declare and use "anonymous" structure pointers (that is, pointers to structures of incomplete type), as long as no attempt is made to access the members -- which of course is exactly the point of an opaque type. 2.4b: Is there a good way of simulating OOP-style inheritance, or other OOP features, in C? A: It's straightforward to implement simple "methods" by placing function pointers in structures. You can make various clumsy, brute-force attempts at inheritance using the preprocessor or by having structures contain "base types" as initial subsets, but it won't be perfect. There's obviously no operator overloading, and overriding (i.e. of "methods" in "derived classes") would have to be done by hand. Obviously, if you need "real" OOP, you'll want to use a language that supports it, such as C++. 2.6: I came across some code that declared a structure like this: struct name { int namelen; char namestr[1]; }; and then did some tricky allocation to make the namestr array act like it had several elements. Is this legal or portable? A: This technique is popular, although Dennis Ritchie has called it "unwarranted chumminess with the C implementation." An official interpretation has deemed that it is not strictly conforming with the C Standard, although it does seem to work under all known implementations. (Compilers which check array bounds carefully might issue warnings.) Another possibility is to declare the variable-size element very large, rather than very small; in the case of the above example: ... char namestr[MAXSIZE]; where MAXSIZE is larger than any name which will be stored. However, it looks like this technique is disallowed by a strict interpretation of the Standard as well. Furthermore, either of these "chummy" structures must be used with care, since the programmer knows more about their size than the compiler does. C99 introduces the concept of a "flexible array member", which allows the size of an array to be omitted if it is the last member in a structure, thus providing a well-defined solution. References: Rationale Sec. 3.5.4.2; C9X Sec. 6.5.2.1. 2.8: Is there a way to compare structures automatically? A: No. There is not a good way for a compiler to implement structure comparison (i.e. to support the == operator for structures) which is consistent with C's low-level flavor. A simple byte-by-byte comparison could founder on random bits present in unused "holes" in the structure (see question 2.12). A field-by-field comparison might require unacceptable amounts of repetitive code for large structures. If you need to compare two structures, you'll have to write your own function to do so, field by field. References: K&R2 Sec. 6.2 p. 129; Rationale Sec. 3.3.9; H&S Sec. 5.6.2 p. 133. 2.10: How can I pass constant values to functions which accept structure arguments? A: Traditional C had no way of generating anonymous structure values; you had to use a temporary structure variable or a little structure-building function. C99 introduces "compound literals", one form of which provides for structure constants. For example, to pass a constant coordinate pair to a hypothetical plotpoint() function which expects a struct point, you can call plotpoint((struct point){1, 2}); Combined with "designated initializers" (another C99 feature), it is also possible to specify member values by name: plotpoint((struct point){.x=1, .y=2}); See also question 4.10. References: C9X Sec. 6.3.2.5, Sec. 6.5.8. 2.11: How can I read/write structures from/to data files? A: It is relatively straightforward to write a structure out using fwrite(): fwrite(&somestruct, sizeof somestruct, 1, fp); and a corresponding fread invocation can read it back in. However, data files so written will *not* be portable (see questions 2.12 and 20.5). Also, if the structure contains any pointers, only the pointer values will be written, and they are most unlikely to be valid when read back in. Finally, note that for widespread portability you must use the "b" flag when opening the files; see question 12.38. A more portable solution, though it's a bit more work initially, is to write a pair of functions for writing and reading a structure, field-by-field, in a portable (perhaps even human- readable) way. References: H&S Sec. 15.13 p. 381. 2.12: My compiler is leaving holes in structures, which is wasting space and preventing "binary" I/O to external data files. Why? Can I turn this off, or otherwise control the alignment of structure fields? A: Those "holes" provide "padding", which may be needed in order to preserve the "alignment" of later fields of the structure. For efficient access, most processors prefer (or require) that multibyte objects (e.g. structure members of any type larger than char) not sit at arbitrary memory addresses, but rather at addresses which are multiples of 2 or 4 or the object size. Your compiler may provide an extension to give you explicit control over struct alignment (perhaps involving a #pragma; see question 11.20), but there is no standard method. See also question 20.5. References: K&R2 Sec. 6.4 p. 138; H&S Sec. 5.6.4 p. 135. 2.13: Why does sizeof report a larger size than I expect for a structure type, as if there were padding at the end? A: Padding at the end of a structure may be necessary to preserve alignment when an array of contiguous structures is allocated. Even when the structure is not part of an array, the padding remains, so that sizeof can always return a consistent size. See also question 2.12 above. References: H&S Sec. 5.6.7 pp. 139-40. 2.14: How can I determine the byte offset of a field within a structure? A: ANSI C defines the offsetof() macro in <stddef.h>, which lets you compute the offset of field f in struct s as offsetof(struct s, f). If for some reason you have to code this sort of thing yourself, one possibility is #define offsetof(type, f) ((size_t) \ ((char *)&((type *)0)->f - (char *)(type *)0)) This implementation is not 100% portable; some compilers may legitimately refuse to accept it. References: ISO Sec. 7.1.6; Rationale Sec. 3.5.4.2; H&S Sec. 11.1 pp. 292-3. 2.15: How can I access structure fields by name at run time? A: Keep track of the field offsets as computed using the offsetof() macro (see question 2.14). If structp is a pointer to an instance of the structure, and field f is an int having offset offsetf, f's value can be set indirectly with *(int *)((char *)structp + offsetf) = value; 2.18: This program works correctly, but it dumps core after it finishes. Why? struct list { char *item; struct list *next; } /* Here is the main program. */ main(argc, argv) { ... } A: A missing semicolon causes main() to be declared as returning a structure. (The connection is hard to see because of the intervening comment.) Since structure-valued functions are usually implemented by adding a hidden return pointer, the generated code for main() tries to accept three arguments, although only two are passed (in this case, by the C start-up code). See also questions 10.9 and 16.4. References: CT&P Sec. 2.3 pp. 21-2. 2.20: Can I initialize unions? A: In the original ANSI C, an initializer was allowed only for the first-named member of a union. C99 introduces "designated initializers" which can be used to initialize any member. References: K&R2 Sec. 6.8 pp. 148-9; ISO Sec. 6.5.7; C9X Sec. 6.5.8; H&S Sec. 4.6.7 p. 100. 2.22: What's the difference between an enumeration and a set of preprocessor #defines? A: There is little difference. The C Standard says that enumerations may be freely intermixed with other integral types, without errors. (If, on the other hand, such intermixing were disallowed without explicit casts, judicious use of enumerations could catch certain programming errors.) Some advantages of enumerations are that the numeric values are automatically assigned, that a debugger may be able to display the symbolic values when enumeration variables are examined, and that they obey block scope. (A compiler may also generate nonfatal warnings when enumerations are indiscriminately mixed, since doing so can still be considered bad style.) A disadvantage is that the programmer has little control over those nonfatal warnings; some programmers also resent not having control over the sizes of enumeration variables. References: K&R2 Sec. 2.3 p. 39, Sec. A4.2 p. 196; ISO Sec. 6.1.2.5, Sec. 6.5.2, Sec. 6.5.2.2, Annex F; H&S Sec. 5.5 pp. 127-9, Sec. 5.11.2 p. 153. 2.24: Is there an easy way to print enumeration values symbolically? A: No. You can write a little function to map an enumeration constant to a string. (For debugging purposes, a good debugger should automatically print enumeration constants symbolically.) Section 3. Expressions 3.1: Why doesn't this code: a[i] = i++; work? A: The subexpression i++ causes a side effect -- it modifies i's value -- which leads to undefined behavior since i is also referenced elsewhere in the same expression, and there's no way to determine whether the reference (in a[i] on the left-hand side) should be to the old or the new value. (Note that although the language in K&R suggests that the behavior of this expression is unspecified, the C Standard makes the stronger statement that it is undefined -- see question 11.33.) References: K&R1 Sec. 2.12; K&R2 Sec. 2.12; ISO Sec. 6.3; H&S Sec. 7.12 pp. 227-9. 3.2: Under my compiler, the code int i = 7; printf("%d\n", i++ * i++); prints 49. Regardless of the order of evaluation, shouldn't it print 56? A: Although the postincrement and postdecrement operators ++ and -- perform their operations after yielding the former value, the implication of "after" is often misunderstood. It is *not* guaranteed that an increment or decrement is performed immediately after giving up the previous value and before any other part of the expression is evaluated. It is merely guaranteed that the update will be performed sometime before the expression is considered "finished" (before the next "sequence point," in ANSI C's terminology; see question 3.8). In the example, the compiler chose to multiply the previous value by itself and to perform both increments later. The behavior of code which contains multiple, ambiguous side effects has always been undefined. (Loosely speaking, by "multiple, ambiguous side effects" we mean any combination of increment, decrement, and assignment operators in a single expression which causes the same object either to be modified twice or modified and then inspected. This is a rough definition; see question 3.8 for a precise one, and question 11.33 for the meaning of "undefined.") Don't even try to find out how your compiler implements such things (contrary to the ill-advised exercises in many C textbooks); as K&R wisely point out, "if you don't know *how* they are done on various machines, that innocence may help to protect you." References: K&R1 Sec. 2.12 p. 50; K&R2 Sec. 2.12 p. 54; ISO Sec. 6.3; H&S Sec. 7.12 pp. 227-9; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp. 120-1. 3.3: I've experimented with the code int i = 3; i = i++; on several compilers. Some gave i the value 3, and some gave 4. Which compiler is correct? A: There is no correct answer; the expression is undefined. See questions 3.1, 3.8, 3.9, and 11.33. (Also, note that neither i++ nor ++i is the same as i+1. If you want to increment i, use i=i+1, i+=1, i++, or ++i, not some combination. See also question 3.12b.) 3.3b: Here's a slick expression: a ^= b ^= a ^= b It swaps a and b without using a temporary. A: Not portably, it doesn't. It attempts to modify the variable a twice between sequence points, so its behavior is undefined. For example, it has been reported that when given the code int a = 123, b = 7654; a ^= b ^= a ^= b; the SCO Optimizing C compiler (icc) sets b to 123 and a to 0. See also questions 3.1, 3.8, 10.3, and 20.15c. 3.4: Can I use explicit parentheses to force the order of evaluation I want? Even if I don't, doesn't precedence dictate it? A: Not in general. Operator precedence and explicit parentheses impose only a partial ordering on the evaluation of an expression. In the expression f() + g() * h() although we know that the multiplication will happen before the addition, there is no telling which of the three functions will be called first. When you need to ensure the order of subexpression evaluation, you may need to use explicit temporary variables and separate statements. References: K&R1 Sec. 2.12 p. 49, Sec. A.7 p. 185; K&R2 Sec. 2.12 pp. 52-3, Sec. A.7 p. 200. 3.5: But what about the && and || operators? I see code like "while((c = getchar()) != EOF && c != '\n')" ... A: There is a special "short-circuiting" exception for these operators: the right-hand side is not evaluated if the left-hand side determines the outcome (i.e. is true for || or false for &&). Therefore, left-to-right evaluation is guaranteed, as it also is for the comma operator. Furthermore, all of these operators (along with ?:) introduce an extra internal sequence point (see question 3.8). References: K&R1 Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1; K&R2 Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8; ISO Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15; H&S Sec. 7.7 pp. 217-8, Sec. 7.8 pp. 218-20, Sec. 7.12.1 p. 229; CT&P Sec. 3.7 pp. 46-7. 3.8: How can I understand these complex expressions? What's a "sequence point"? A: A sequence point is a point in time (at the end of the evaluation of a full expression, or at the ||, &&, ?:, or comma operators, or just before a function call) at which the dust has settled and all side effects are guaranteed to be complete. The ANSI/ISO C Standard states that Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored. The second sentence can be difficult to understand. It says that if an object is written to within a full expression, any and all accesses to it within the same expression must be directly involved in the computation of the value to be written. This rule effectively constrains legal expressions to those in which the accesses demonstrably precede the modification. For example, i = i + 1 is legal, but not a[i] = i++ (see question 3.1). See also question 3.9 below. References: ISO Sec. 5.1.2.3, Sec. 6.3, Sec. 6.6, Annex C; Rationale Sec. 2.1.2.3; H&S Sec. 7.12.1 pp. 228-9. 3.9: So given a[i] = i++; we don't know which cell of a[] gets written to, but i does get incremented by one, right? A: Not necessarily! Once an expression or program becomes undefined, *all* aspects of it become undefined. See questions 3.2, 3.3, 11.33, and 11.35. 3.12a: What's the difference between ++i and i++? A: If your C book doesn't explain, get a better one. Briefly: ++i adds one to the stored value of i and "returns" the new, incremented value to the surrounding expression; i++ adds one to i but returns the prior, unincremented value. 3.12b: If I'm not using the value of the expression, should I use ++i or i++ to increment a variable? A: Since the two forms differ only in the value yielded, they are entirely equivalent when only their side effect is needed. (However, the prefix form is preferred in C++.) See also question 3.3. References: K&R1 Sec. 2.8 p. 43; K&R2 Sec. 2.8 p. 47; ISO Sec. 6.3.2.4, Sec. 6.3.3.1; H&S Sec. 7.4.4 pp. 192-3, Sec. 7.5.8 pp. 199-200. 3.14: Why doesn't the code int a = 1000, b = 1000; long int c = a * b; work? A: Under C's integral promotion rules, the multiplication is carried out using int arithmetic, and the result may overflow or be truncated before being promoted and assigned to the long int left-hand side. Use an explicit cast to force long arithmetic: long int c = (long int)a * b; Notice that (long int)(a * b) would *not* have the desired effect. A similar problem can arise when two integers are divided, with the result assigned to a floating-point variable; the solution is similar, too. References: K&R1 Sec. 2.7 p. 41; K&R2 Sec. 2.7 p. 44; ISO Sec. 6.2.1.5; H&S Sec. 6.3.4 p. 176; CT&P Sec. 3.9 pp. 49-50. 3.16: I have a complicated expression which I have to assign to one of two variables, depending on a condition. Can I use code like this? ((condition) ? a : b) = complicated_expression; A: No. The ?: operator, like most operators, yields a value, and you can't assign to a value. (In other words, ?: does not yield an "lvalue".) If you really want to, you can try something like *((condition) ? &a : &b) = complicated_expression; although this is admittedly not as pretty. References: ISO Sec. 6.3.15; H&S Sec. 7.1 pp. 179-180. Section 4. Pointers 4.2: I'm trying to declare a pointer and allocate some space for it, but it's not working. What's wrong with this code? char *p; *p = malloc(10); A: The pointer you declared is p, not *p. When you're manipulating the pointer itself (for example when you're setting it to make it point somewhere), you just use the name of the pointer: p = malloc(10); It's when you're manipulating the pointed-to memory that you use * as an indirection operator: *p = 'H'; See also questions 1.21, 7.1, 7.3c, and 8.3. References: CT&P Sec. 3.1 p. 28. 4.3: Does *p++ increment p, or what it points to? A: The postfix ++ and -- operators essentially have higher precedence than the prefix unary operators. Therefore, *p++ is equivalent to *(p++); it increments p, and returns the value which p pointed to before p was incremented. To increment the value pointed to by p, use (*p)++ (or perhaps ++*p, if the order of the side effect doesn't matter). References: K&R1 Sec. 5.1 p. 91; K&R2 Sec. 5.1 p. 95; ISO Sec. 6.3.2, Sec. 6.3.3; H&S Sec. 7.4.4 pp. 192-3, Sec. 7.5 p. 193, Secs. 7.5.7,7.5.8 pp. 199-200. 4.5: I have a char * pointer that happens to point to some ints, and I want to step it over them. Why doesn't ((int *)p)++; work? A: In C, a cast operator does not mean "pretend these bits have a different type, and treat them accordingly"; it is a conversion operator, and by definition it yields an rvalue, which cannot be assigned to, or incremented with ++. (It is either an accident or a deliberate but nonstandard extension if a particular compiler accepts expressions such as the above.) Say what you mean: use p = (char *)((int *)p + 1); or (since p is a char *) simply p += sizeof(int); When possible, however, you should choose appropriate pointer types in the first place, rather than trying to treat one type as another. References: K&R2 Sec. A7.5 p. 205; ISO Sec. 6.3.4; Rationale Sec. 3.3.2.4; H&S Sec. 7.1 pp. 179-80. 4.8: I have a function which accepts, and is supposed to initialize, a pointer: void f(int *ip) { static int dummy = 5; ip = &dummy; } But when I call it like this: int *ip; f(ip); the pointer in the caller remains unchanged. A: Are you sure the function initialized what you thought it did? Remember that arguments in C are passed by value. The called function altered only the passed copy of the pointer. You'll either want to pass the address of the pointer (the function will end up accepting a pointer-to-a-pointer), or have the function return the pointer. See also questions 4.9 and 4.11. 4.9: Can I use a void ** pointer as a parameter so that a function can accept a generic pointer by reference? A: Not portably. There is no generic pointer-to-pointer type in C. void * acts as a generic pointer only because conversions (if necessary) are applied automatically when other pointer types are assigned to and from void *'s; these conversions cannot be performed (the correct underlying pointer type is not known) if an attempt is made to indirect upon a void ** value which points at a pointer type other than void *. 4.10: I have a function extern int f(int *); which accepts a pointer to an int. How can I pass a constant by reference? A call like f(&5); doesn't seem to work. A: In C99, you can use a "compound literal": f((int[]){5}); Prior to C99, you couldn't do this directly; you had to declare a temporary variable, and then pass its address to the function: int five = 5; f(&five); See also questions 2.10, 4.8, and 20.1. 4.11: Does C even have "pass by reference"? A: Not really. Strictly speaking, C always uses pass by value. You can simulate pass by reference yourself, by defining functions which accept pointers and then using the & operator when calling, and the compiler will essentially simulate it for you when you pass an array to a function (by passing a pointer instead, see question 6.4 et al.). However, C has nothing truly equivalent to formal pass by reference or C++ reference parameters. (On the other hand, function-like preprocessor macros can provide a form of "pass by name".) See also questions 4.8 and 20.1. References: K&R1 Sec. 1.8 pp. 24-5, Sec. 5.2 pp. 91-3; K&R2 Sec. 1.8 pp. 27-8, Sec. 5.2 pp. 95-7; ISO Sec. 6.3.2.2; H&S Sec. 9.5 pp. 273-4. 4.12: I've seen different syntax used for calling functions via pointers. What's the story? A: Originally, a pointer to a function had to be "turned into" a "real" function, with the * operator (and an extra pair of parentheses, to keep the precedence straight), before calling: int r, func(), (*fp)() = func; r = (*fp)(); It can also be argued that functions are always called via pointers, and that "real" function names always decay implicitly into pointers (in expressions, as they do in initializations; see question 1.34). This reasoning means that r = fp(); is legal and works correctly, whether fp is the name of a function or a pointer to one. (The usage has always been unambiguous; there is nothing you ever could have done with a function pointer followed by an argument list except call the function pointed to.) The ANSI C Standard essentially adopts the latter interpretation, meaning that the explicit * is not required, though it is still allowed. See also question 1.34. References: K&R1 Sec. 5.12 p. 116; K&R2 Sec. 5.11 p. 120; ISO Sec. 6.3.2.2; Rationale Sec. 3.3.2.2; H&S Sec. 5.8 p. 147, Sec. 7.4.3 p. 190. 4.15: How do I convert an int to a char *? I tried a cast, but it's not working. A: It depends on what you're trying to do. If you tried a cast but it's not working, you're probably trying to convert an integer to a string, in which case see question 13.1. If you're trying to convert an integer to a character, see question 8.6. If you're trying to set a pointer to point to a particular memory address, see question 19.25. Section 5. Null Pointers 5.1: What is this infamous null pointer, anyway? A: The language definition states that for each pointer type, there is a special value -- the "null pointer" -- which is distinguishable from all other pointer values and which is "guaranteed to compare unequal to a pointer to any object or function." That is, the address-of operator & will never yield a null pointer, nor will a successful call to malloc(). (malloc() does return a null pointer when it fails, and this is a typical use of null pointers: as a "special" pointer value with some other meaning, usually "not allocated" or "not pointing anywhere yet.") A null pointer is conceptually different from an uninitialized pointer. A null pointer is known not to point to any object or function; an uninitialized pointer might point anywhere. See also questions 1.30, 7.1, and 7.31. As mentioned above, there is a null pointer for each pointer type, and the internal values of null pointers for different types may be different. Although programmers need not know the internal values, the compiler must always be informed which type of null pointer is required, so that it can make the distinction if necessary (see questions 5.2, 5.5, and 5.6 below). References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ISO Sec. 6.2.2.3; Rationale Sec. 3.2.2.3; H&S Sec. 5.3.2 pp. 121-3. 5.2: How do I get a null pointer in my programs? A: According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following fragments are perfectly legal: char *p = 0; if(p != 0) (See also question 5.3.) However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer. To generate a null pointer in a function call context, an explicit cast may be required, to force the 0 to be recognized as a pointer. For example, the Unix system call execl takes a variable-length, null-pointer-terminated list of character pointer arguments, and is correctly called like this: execl("/bin/sh", "sh", "-c", "date", (char *)0); If the (char *) cast on the last argument were omitted, the compiler would not know to pass a null pointer, and would pass an integer 0 instead. (Note that many Unix manuals get this example wrong.) When function prototypes are in scope, argument passing becomes an "assignment context," and most casts may safely be omitted, since the prototype tells the compiler that a pointer is required, and of which type, enabling it to correctly convert an unadorned 0. Function prototypes cannot provide the types for variable arguments in variable-length argument lists however, so explicit casts are still required for those arguments. (See also question 15.3.) It is probably safest to properly cast all null pointer constants in function calls, to guard against varargs functions or those without prototypes. Summary: Unadorned 0 okay: Explicit cast required: initialization function call, no prototype in scope assignment variable argument in comparison varargs function call function call, prototype in scope, fixed argument References: K&R1 Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R2 Sec. A7.10 p. 207, Sec. A7.17 p. 209; ISO Sec. 6.2.2.3; H&S Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171. 5.3: Is the abbreviated pointer comparison "if(p)" to test for non- null pointers valid? What if the internal representation for null pointers is nonzero? A: When C requires the Boolean value of an expression, a false value is inferred when the expression compares equal to zero, and a true value otherwise. That is, whenever one writes if(expr) where "expr" is any expression at all, the compiler essentially acts as if it had been written as if((expr) != 0) Substituting the trivial pointer expression "p" for "expr", we have if(p) is equivalent to if(p != 0) and this is a comparison context, so the compiler can tell that the (implicit) 0 is actually a null pointer constant, and use the correct null pointer value. There is no trickery involved here; compilers do work this way, and generate identical code for both constructs. The internal representation of a null pointer does *not* matter. The boolean negation operator, !, can be described as follows: !expr is essentially equivalent to (expr)?0:1 or to ((expr) == 0) which leads to the conclusion that if(!p) is equivalent to if(p == 0) "Abbreviations" such as if(p), though perfectly legal, are considered by some to be bad style (and by others to be good style; see question 17.10). See also question 9.2. References: K&R2 Sec. A7.4.7 p. 204; ISO Sec. 6.3.3.3, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5; H&S Sec. 5.3.2 p. 122. 5.4: What is NULL and how is it defined? A: As a matter of style, many programmers prefer not to have unadorned 0's scattered through their programs. Therefore, the preprocessor macro NULL is defined (by <stdio.h> and several other headers) as a null pointer constant, typically 0 or ((void *)0) (see also question 5.6). A programmer who wishes to make explicit the distinction between 0 the integer and 0 the null pointer constant can then use NULL whenever a null pointer is required. Using NULL is a stylistic convention only; the preprocessor turns NULL back into 0 which is then recognized by the compiler, in pointer contexts, as before. In particular, a cast may still be necessary before NULL (as before 0) in a function call argument. The table under question 5.2 above applies for NULL as well as 0 (an unadorned NULL is equivalent to an unadorned 0). NULL should be used *only* as a pointer constant; see question 5.9. References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102; ISO Sec. 7.1.6, Sec. 6.2.2.3; Rationale Sec. 4.1.5; H&S Sec. 5.3.2 p. 122, Sec. 11.1 p. 292. 5.5: How should NULL be defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer? A: The same as on any other machine: as 0 (or some version of 0; see question 5.4). Whenever a programmer requests a null pointer, either by writing "0" or "NULL", it is the compiler's responsibility to generate whatever bit pattern the machine uses for that null pointer. Therefore, #defining NULL as 0 on a machine for which internal null pointers are nonzero is as valid as on any other: the compiler must always be able to generate the machine's correct null pointers in response to unadorned 0's seen in pointer contexts. See also questions 5.2, 5.10, and 5.17. References: ISO Sec. 7.1.6; Rationale Sec. 4.1.5. 5.6: If NULL were defined as follows: #define NULL ((char *)0) wouldn't that make function calls which pass an uncast NULL work? A: Not in the most general case. The complication is that there are machines which use different internal representations for pointers to different types of data. The suggested definition would make uncast NULL arguments to functions expecting pointers to characters work correctly, but pointer arguments of other types could still (in the absence of prototypes) be problematical, and legal constructions such as FILE *fp = NULL; could fail. Nevertheless, ANSI C allows the alternate definition #define NULL ((void *)0) for NULL. Besides potentially helping incorrect programs to work (but only on machines with homogeneous pointers, thus questionably valid assistance), this definition may catch programs which use NULL incorrectly (e.g. when the ASCII NUL character was really intended; see question 5.9). At any rate, ANSI function prototypes ensure that most (though not quite all; see question 5.2) pointer arguments are converted correctly when passed as function arguments, so the question is largely moot. References: Rationale Sec. 4.1.5. 5.9: If NULL and 0 are equivalent as null pointer constants, which should I use? A: Many programmers believe that NULL should be used in all pointer contexts, as a reminder that the value is to be thought of as a pointer. Others feel that the confusion surrounding NULL and 0 is only compounded by hiding 0 behind a macro, and prefer to use unadorned 0 instead. There is no one right answer. (See also questions 9.2 and 17.10.) C programmers must understand that NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable. Any usage of NULL (as opposed to 0) should be considered a gentle reminder that a pointer is involved; programmers should not depend on it (either for their own understanding or the compiler's) for distinguishing pointer 0's from integer 0's. NULL should *not* be used when another kind of 0 is required, even though it might work, because doing so sends the wrong stylistic message. (Furthermore, ANSI allows the definition of NULL to be ((void *)0), which will not work at all in non- pointer contexts.) In particular, do not use NULL when the ASCII null character (NUL) is desired. Provide your own definition #define NUL '\0' if you must. References: K&R1 Sec. 5.4 pp. 97-8; K&R2 Sec. 5.4 p. 102. 5.10: But wouldn't it be better to use NULL (rather than 0), in case the value of NULL changes, perhaps on a machine with nonzero internal null pointers? A: No. (Using NULL may be preferable, but not for this reason.) Although symbolic constants are often used in place of numbers because the numbers might change, this is *not* the reason that NULL is used in place of 0. Once again, the language guarantees that source-code 0's (in pointer contexts) generate null pointers. NULL is used only as a stylistic convention. See questions 5.5 and 9.2. 5.12: I use the preprocessor macro #define Nullptr(type) (type *)0 to help me build null pointers of the correct type. A: This trick, though popular and superficially attractive, does not buy much. It is not needed in assignments or comparisons; see question 5.2. (It does not even save keystrokes.) See also questions 9.1 and 10.2. 5.13: This is strange. NULL is guaranteed to be 0, but the null pointer is not? A: When the term "null" or "NULL" is casually used, one of several things may be meant: 1. The conceptual null pointer, the abstract language concept defined in question 5.1. It is implemented with... 2. The internal (or run-time) representation of a null pointer, which may or may not be all-bits-0 and which may be different for different pointer types. The actual values should be of concern only to compiler writers. Authors of C programs never see them, since they use... 3. The null pointer constant, which is a constant integer 0 (see question 5.2). It is often hidden behind... 4. The NULL macro, which is #defined to be 0 (see question 5.4). Finally, as red herrings, we have... 5. The ASCII null character (NUL), which does have all bits zero, but has no necessary relation to the null pointer except in name; and... 6. The "null string," which is another name for the empty string (""). Using the term "null string" can be confusing in C, because an empty string involves a null ('\0') character, but *not* a null pointer, which brings us full circle... This article uses the phrase "null pointer" (in lower case) for sense 1, the token "0" or the phrase "null pointer constant" for sense 3, and the capitalized word "NULL" for sense 4. 5.14: Why is there so much confusion surrounding null pointers? Why do these questions come up so often? A: C programmers traditionally like to know a lot (perhaps more than they need to) about the underlying machine implementation. The fact that null pointers are represented both in source code, and internally to most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL) may seem to suggest that the value could change some day, or on some weird machine. The construct "if(p == 0)" is easily misread as calling for conversion of p to an integral type, rather than 0 to a pointer type, before the comparison. Finally, the distinction between the several uses of the term "null" (listed in question 5.13 above) is often overlooked. One good way to wade out of the confusion is to imagine that C used a keyword (perhaps "nil", like Pascal) as a null pointer constant. The compiler could either turn "nil" into the appropriate type of null pointer when it could unambiguously determine that type from the source code, or complain when it could not. Now in fact, in C the keyword for a null pointer constant is not "nil" but "0", which works almost as well, except that an uncast "0" in a non-pointer context generates an integer zero instead of an error message, and if that uncast 0 was supposed to be a null pointer constant, the resulting program may not work. 5.15: I'm confused. I just can't understand all this null pointer stuff. A: Here are two simple rules you can follow: 1. When you want a null pointer constant in source code, use "0" or "NULL". 2. If the usage of "0" or "NULL" is an argument in a function call, cast it to the pointer type expected by the function being called. The rest of the discussion has to do with other people's misunderstandings, with the internal representation of null pointers (which you shouldn't need to know), and with the complexities of function prototypes. (Taking those complexities into account, we find that rule 2 is conservative, of course; but it doesn't hurt.) Understand questions 5.1, 5.2, and 5.4, and consider 5.3, 5.9, 5.13, and 5.14, and you'll do fine. 5.16: Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to be represented internally by zeroes? A: If for no other reason, doing so would be ill-advised because it would unnecessarily constrain implementations which would otherwise naturally represent null pointers by special, nonzero bit patterns, particularly when those values would trigger automatic hardware traps for invalid accesses. Besides, what would such a requirement really accomplish? Proper understanding of null pointers does not require knowledge of the internal representation, whether zero or nonzero. Assuming that null pointers are internally zero does not make any code easier to write (except for a certain ill-advised usage of calloc(); see question 7.31). Known-zero internal pointers would not obviate casts in function calls, because the *size* of the pointer might still be different from that of an int. (If "nil" were used to request null pointers, as mentioned in question 5.14 above, the urge to assume an internal zero representation would not even arise.) 5.17: Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types? A: The Prime 50 series used segment 07777, offset 0 for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to all the extant poorly-written C code which made incorrect assumptions. Older, word-addressed Prime machines were also notorious for requiring larger byte pointers (char *'s) than word pointers (int *'s). The Eclipse MV series from Data General has three architecturally supported pointer formats (word, byte, and bit pointers), two of which are used by C compilers: byte pointers for char * and void *, and word pointers for everything else. Some Honeywell-Bull mainframes use the bit pattern 06000 for (internal) null pointers. The CDC Cyber 180 Series has 48-bit pointers consisting of a ring, segment, and offset. Most users (in ring 11) have null pointers of 0xB00000000000. It was common on old CDC ones- complement machines to use an all-one-bits word as a special flag for all kinds of data, including invalid addresses. The old HP 3000 series uses a different addressing scheme for byte addresses than for word addresses; like several of the machines above it therefore uses different representations for char * and void * pointers than for other pointers. The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair <NIL, 0> (basically a nonexistent <object, offset> handle) as a C null pointer. Depending on the "memory model" in use, 8086-family processors (PC compatibles) may use 16-bit data pointers and 32-bit function pointers, or vice versa. Some 64-bit Cray machines represent int * in the lower 48 bits of a word; char * additionally uses some of the upper 16 bits to indicate a byte address within a word. References: K&R1 Sec. A14.4 p. 211. 5.20: What does a run-time "null pointer assignment" error mean? A: This message, which typically occurs with MS-DOS compilers, means that you've written, via a null pointer, to an invalid location -- probably offset 0 in the default data segment. See also question 16.8. Section 6. Arrays and Pointers 6.1: I had the definition char a[6] in one source file, and in another I declared extern char *a. Why didn't it work? A: In one source file you defined an array of characters and in the other you declared a pointer to characters. The declaration extern char *a simply does not match the actual definition. The type pointer-to-type-T is not the same as array-of-type-T. Use extern char a[]. References: ISO Sec. 6.5.4.2; CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5. 6.2: But I heard that char a[] was identical to char *a. A: Not at all. (What you heard has to do with formal parameters to functions; see question 6.4.) Arrays are not pointers. The array declaration char a[6] requests that space for six characters be set aside, to be known by the name "a". That is, there is a location named "a" at which six characters can sit. The pointer declaration char *p, on the other hand, requests a place which holds a pointer, to be known by the name "p". This pointer can point almost anywhere: to any char, or to any contiguous array of chars, or nowhere (see also questions 5.1 and 1.30). As usual, a picture is worth a thousand words. The declarations char a[] = "hello"; char *p = "world"; would initialize data structures which could be represented like this: +---+---+---+---+---+---+ a: | h | e | l | l | o |\0 | +---+---+---+---+---+---+ +-----+ +---+---+---+---+---+---+ p: | *======> | w | o | r | l | d |\0 | +-----+ +---+---+---+---+---+---+ It is useful to realize that a reference like x[3] generates different code depending on whether x is an array or a pointer. Given the declarations above, when the compiler sees the expression a[3], it emits code to start at the location "a", move three past it, and fetch the character there. When it sees the expression p[3], it emits code to start at the location "p", fetch the pointer value there, add three to the pointer, and finally fetch the character pointed to. In other words, a[3] is three places past (the start of) the object *named* a, while p[3] is three places past the object *pointed to* by p. In the example above, both a[3] and p[3] happen to be the character 'l', but the compiler gets there differently. (The essential difference is that the values of an array like a and a pointer like p are computed differently *whenever* they appear in expressions, whether or not they are being subscripted, as explained further in the next question.) See also question 1.32. References: K&R2 Sec. 5.5 p. 104; CT&P Sec. 4.5 pp. 64-5. 6.3: So what is meant by the "equivalence of pointers and arrays" in C? A: Much of the confusion surrounding arrays and pointers in C can be traced to a misunderstanding of this statement. Saying that arrays and pointers are "equivalent" means neither that they are identical nor even interchangeable. What it means is that array and pointer arithmetic is defined such that a pointer can be conveniently used to access an array or to simulate an array. Specifically, the cornerstone of the equivalence is this key definition: An lvalue of type array-of-T which appears in an expression decays (with three exceptions) into a pointer to its first element; the type of the resultant pointer is pointer-to-T. That is, whenever an array appears in an expression, the compiler implicitly generates a pointer to the array's first element, just as if the programmer had written &a[0]. (The exceptions are when the array is the operand of a sizeof or & operator, or is a string literal initializer for a character array.) As a consequence of this definition, the compiler doesn't apply the array subscripting operator [] that differently to arrays and pointers, after all. In an expression of the form a[i], the array decays into a pointer, following the rule above, and is then subscripted just as would be a pointer variable in the expression p[i] (although the eventual memory accesses will be different, as explained in question 6.2). If you were to assign the array's address to the pointer: p = a; then p[3] and a[3] would access the same element. See also questions 6.8 and 6.14. References: K&R1 Sec. 5.3 pp. 93-6; K&R2 Sec. 5.3 p. 99; ISO Sec. 6.2.2.1, Sec. 6.3.2.1, Sec. 6.3.6; H&S Sec. 5.4.1 p. 124. 6.4: Then why are array and pointer declarations interchangeable as function formal parameters? A: It's supposed to be a convenience. Since arrays decay immediately into pointers, an array is never actually passed to a function. Allowing pointer parameters to be declared as arrays is a simply a way of making it look as though an array was being passed, perhaps because the parameter will be used within the function as if it were an array. Specifically, any parameter declarations which "look like" arrays, e.g. void f(char a[]) { ... } are treated by the compiler as if they were pointers, since that is what the function will receive if an array is passed: void f(char *a) { ... } This conversion holds only within function formal parameter declarations, nowhere else. If the conversion bothers you, avoid it; many programmers have concluded that the confusion it causes outweighs the small advantage of having the declaration "look like" the call or the uses within the function. See also question 6.21. References: K&R1 Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R2 Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; ISO Sec. 6.5.4.3, Sec. 6.7.1, Sec. 6.9.6; H&S Sec. 9.3 p. 271; CT&P Sec. 3.3 pp. 33-4. 6.7: How can an array be an lvalue, if you can't assign to it? A: The ANSI C Standard defines a "modifiable lvalue," which an array is not. References: ISO Sec. 6.2.2.1; Rationale Sec. 3.2.2.1; H&S Sec. 7.1 p. 179. 6.8: Practically speaking, what is the difference between arrays and pointers? A: Arrays automatically allocate space, but can't be relocated or resized. Pointers must be explicitly assigned to point to allocated space (perhaps using malloc), but can be reassigned (i.e. pointed at different objects) at will, and have many other uses besides serving as the base of blocks of memory. Due to the so-called equivalence of arrays and pointers (see question 6.3), arrays and pointers often seem interchangeable, and in particular a pointer to a block of memory assigned by malloc is frequently treated (and can be referenced using []) exactly as if it were a true array. See questions 6.14 and 6.16. (Be careful with sizeof, though.) See also questions 1.32 and 20.14. 6.9: Someone explained to me that arrays were really just constant pointers. A: This is a bit of an oversimplification. An array name is "constant" in that it cannot be assigned to, but an array is *not* a pointer, as the discussion and pictures in question 6.2 should make clear. See also questions 6.3 and 6.8. 6.11: I came across some "joke" code containing the "expression" 5["abcdef"] . How can this be legal C? A: Yes, Virginia, array subscripting is commutative in C. This curious fact follows from the pointer definition of array subscripting, namely that a[e] is identical to *((a)+(e)), for *any* two expressions a and e, as long as one of them is a pointer expression and one is integral. This unsuspected commutativity is often mentioned in C texts as if it were something to be proud of, but it finds no useful application outside of the Obfuscated C Contest (see question 20.36). References: Rationale Sec. 3.3.2.1; H&S Sec. 5.4.1 p. 124, Sec. 7.4.1 pp. 186-7. 6.12: Since array references decay into pointers, if arr is an array, what's the difference between arr and &arr? A: The type. In Standard C, &arr yields a pointer, of type pointer-to-array- of-T, to the entire array. (In pre-ANSI C, the & in &arr generally elicited a warning, and was generally ignored.) Under all C compilers, a simple reference (without an explicit &) to an array yields a pointer, of type pointer-to-T, to the array's first element. (See also questions 6.3, 6.13, and 6.18.) References: ISO Sec. 6.2.2.1, Sec. 6.3.3.2; Rationale Sec. 3.3.3.2; H&S Sec. 7.5.6 p. 198. 6.13: How do I declare a pointer to an array? A: Usually, you don't want to. When people speak casually of a pointer to an array, they usually mean a pointer to its first element. Instead of a pointer to an array, consider using a pointer to one of the array's elements. Arrays of type T decay into pointers to type T (see question 6.3), which is convenient; subscripting or incrementing the resultant pointer will access the individual members of the array. True pointers to arrays, when subscripted or incremented, step over entire arrays, and are generally useful only when operating on arrays of arrays, if at all. (See question 6.18.) If you really need to declare a pointer to an entire array, use something like "int (*ap)[N];" where N is the size of the array. (See also question 1.21.) If the size of the array is unknown, N can in principle be omitted, but the resulting type, "pointer to array of unknown size," is useless. See also question 6.12 above. References: ISO Sec. 6.2.2.1. 6.14: How can I set an array's size at run time? How can I avoid fixed-sized arrays? A: The equivalence between arrays and pointers (see question 6.3) allows a pointer to malloc'ed memory to simulate an array quite effectively. After executing #include <stdlib.h> int *dynarray; dynarray = malloc(10 * sizeof(int)); (and if the call to malloc succeeds), you can reference dynarray[i] (for i from 0 to 9) almost as if dynarray were a conventional, statically-allocated array (int a[10]). The only difference is that sizeof will not give the size of the "array". See also questions 1.31b, 6.16, and 7.7. 6.15: How can I declare local arrays of a size matching a passed-in array? A: Until recently, you couldn't; array dimensions in C traditionally had to be compile-time constants. However, C99 introduces variable-length arrays (VLA's) which solve this problem; local arrays may have sizes set by variables or other expressions, perhaps involving function parameters. (gcc has provided parameterized arrays as an extension for some time.) If you can't use C99 or gcc, you'll have to use malloc(), and remember to call free() before the function returns. See also questions 6.14, 6.16, 6.19, 7.22, and maybe 7.32. References: ISO Sec. 6.4, Sec. 6.5.4.2; C9X Sec. 6.5.5.2. 6.16: How can I dynamically allocate a multidimensional array? A: The traditional solution is to allocate an array of pointers, and then initialize each pointer to a dynamically-allocated "row." Here is a two-dimensional example: #include <stdlib.h> int **array1 = malloc(nrows * sizeof(int *)); for(i = 0; i < nrows; i++) array1[i] = malloc(ncolumns * sizeof(int)); (In real code, of course, all of malloc's return values would be checked. You can also use sizeof(*array1) and sizeof(**array1) instead of sizeof(int *) and sizeof(int).) You can keep the array's contents contiguous, at the cost of making later reallocation of individual rows more difficult, with a bit of explicit pointer arithmetic: int **array2 = malloc(nrows * sizeof(int *)); array2[0] = malloc(nrows * ncolumns * sizeof(int)); for(i = 1; i < nrows; i++) array2[i] = array2[0] + i * ncolumns; In either case, the elements of the dynamic array can be accessed with normal-looking array subscripts: arrayx[i][j] (for 0 <= i < nrows and 0 <= j < ncolumns). If the double indirection implied by the above schemes is for some reason unacceptable, you can simulate a two-dimensional array with a single, dynamically-allocated one-dimensional array: int *array3 = malloc(nrows * ncolumns * sizeof(int)); However, you must now perform subscript calculations manually, accessing the i,jth element with array3[i * ncolumns + j]. (A macro could hide the explicit calculation, but invoking it would require parentheses and commas which wouldn't look exactly like multidimensional array syntax, and the macro would need access to at least one of the dimensions, as well. See also question 6.19.) Yet another option is to use pointers to arrays: int (*array4)[NCOLUMNS] = malloc(nrows * sizeof(*array4)); but the syntax starts getting horrific and at most one dimension may be specified at run time. With all of these techniques, you may of course need to remember to free the arrays (which may take several steps; see question 7.23) when they are no longer needed, and you cannot necessarily intermix dynamically-allocated arrays with conventional, statically-allocated ones (see question 6.20, and also question 6.18). Finally, in C99 you can use a variable-length array. All of these techniques can also be extended to three or more dimensions. References: C9X Sec. 6.5.5.2. 6.17: Here's a neat trick: if I write int realarray[10]; int *array = &realarray[-1]; I can treat "array" as if it were a 1-based array. A: Although this technique is attractive (and was used in old editions of the book _Numerical Recipes in C_), it is not strictly conforming to the C Standard. Pointer arithmetic is defined only as long as the pointer points within the same allocated block of memory, or to the imaginary "terminating" element one past it; otherwise, the behavior is undefined, *even if the pointer is not dereferenced*. The code above could fail if, while subtracting the offset, an illegal address were generated (perhaps because the address tried to "wrap around" past the beginning of some memory segment). References: K&R2 Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7 pp. 205-6; ISO Sec. 6.3.6; Rationale Sec. 3.2.2.3. 6.18: My compiler complained when I passed a two-dimensional array to a function expecting a pointer to a pointer. A: The rule (see question 6.3) by which arrays decay into pointers is *not* applied recursively. An array of arrays (i.e. a two- dimensional array in C) decays into a pointer to an array, not a pointer to a pointer. Pointers to arrays can be confusing, and must be treated carefully; see also question 6.13. If you are passing a two-dimensional array to a function: int array[NROWS][NCOLUMNS]; f(array); the function's declaration must match: void f(int a[][NCOLUMNS]) { ... } or void f(int (*ap)[NCOLUMNS]) /* ap is a pointer to an array */ { ... } In the first declaration, the compiler performs the usual implicit parameter rewriting of "array of array" to "pointer to array" (see questions 6.3 and 6.4); in the second form the pointer declaration is explicit. Since the called function does not allocate space for the array, it does not need to know the overall size, so the number of rows, NROWS, can be omitted. The width of the array is still important, so the column dimension NCOLUMNS (and, for three- or more dimensional arrays, the intervening ones) must be retained. If a function is already declared as accepting a pointer to a pointer, it is almost certainly meaningless to pass a two- dimensional array directly to it. See also questions 6.12 and 6.15. References: K&R1 Sec. 5.10 p. 110; K&R2 Sec. 5.9 p. 113; H&S Sec. 5.4.3 p. 126. 6.19: How do I write functions which accept two-dimensional arrays when the width is not known at compile time? A: It's not always easy. One way is to pass in a pointer to the [0][0] element, along with the two dimensions, and simulate array subscripting "by hand": void f2(int *aryp, int nrows, int ncolumns) { ... array[i][j] is accessed as aryp[i * ncolumns + j] ... } This function could be called with the array from question 6.18 as f2(&array[0][0], NROWS, NCOLUMNS); It must be noted, however, that a program which performs multidimensional array subscripting "by hand" in this way is not in strict conformance with the ANSI C Standard; according to an official interpretation, the behavior of accessing (&array[0][0])[x] is not defined for x >= NCOLUMNS. C99 allows variable-length arrays, and once compilers which accept C99's extensions become widespread, VLA's will probably become the preferred solution. (gcc has supported variable- sized arrays for some time.) When you want to be able to use a function on multidimensional arrays of various sizes, one solution is to simulate all the arrays dynamically, as in question 6.16. See also questions 6.18, 6.20, and 6.15. References: ISO Sec. 6.3.6; C9X Sec. 6.5.5.2. 6.20: How can I use statically- and dynamically-allocated multidimensional arrays interchangeably when passing them to functions? A: There is no single perfect method. Given the declarations int array[NROWS][NCOLUMNS]; int **array1; /* ragged */ int **array2; /* contiguous */ int *array3; /* "flattened" */ int (*array4)[NCOLUMNS]; with the pointers initialized as in the code fragments in question 6.16, and functions declared as void f1a(int a[][NCOLUMNS], int nrows, int ncolumns); void f1b(int (*a)[NCOLUMNS], int nrows, int ncolumns); void f2(int *aryp, int nrows, int ncolumns); void f3(int **pp, int nrows, int ncolumns); where f1a() and f1b() accept conventional two-dimensional arrays, f2() accepts a "flattened" two-dimensional array, and f3() accepts a pointer-to-pointer, simulated array (see also questions 6.18 and 6.19), the following calls should work as expected: f1a(array, NROWS, NCOLUMNS); f1b(array, NROWS, NCOLUMNS); f1a(array4, nrows, NCOLUMNS); f1b(array4, nrows, NCOLUMNS); f2(&array[0][0], NROWS, NCOLUMNS); f2(*array, NROWS, NCOLUMNS); f2(*array2, nrows, ncolumns); f2(array3, nrows, ncolumns); f2(*array4, nrows, NCOLUMNS); f3(array1, nrows, ncolumns); f3(array2, nrows, ncolumns); The following calls would probably work on most systems, but involve questionable casts, and work only if the dynamic ncolumns matches the static NCOLUMNS: f1a((int (*)[NCOLUMNS])(*array2), nrows, ncolumns); f1a((int (*)[NCOLUMNS])(*array2), nrows, ncolumns); f1b((int (*)[NCOLUMNS])array3, nrows, ncolumns); f1b((int (*)[NCOLUMNS])array3, nrows, ncolumns); It must again be noted that passing &array[0][0] (or, equivalently, *array) to f2() is not strictly conforming; see question 6.19. If you can understand why all of the above calls work and are written as they are, and if you understand why the combinations that are not listed would not work, then you have a *very* good understanding of arrays and pointers in C. Rather than worrying about all of this, one approach to using multidimensional arrays of various sizes is to make them *all* dynamic, as in question 6.16. If there are no static multidimensional arrays -- if all arrays are allocated like array1 or array2 in question 6.16 -- then all functions can be written like f3(). 6.21: Why doesn't sizeof properly report the size of an array when the array is a parameter to a function? A: The compiler pretends that the array parameter was declared as a pointer (see question 6.4), and sizeof reports the size of the pointer. References: H&S Sec. 7.5.2 p. 195. Section 7. Memory Allocation 7.1: Why doesn't this fragment work? char *answer; printf("Type something:\n"); gets(answer); printf("You typed \"%s\"\n", answer); A: The pointer variable answer, which is handed to gets() as the location into which the response should be stored, has not been set to point to any valid storage. That is, we cannot say where the pointer answer points. (Since local variables are not initialized, and typically contain garbage, it is not even guaranteed that answer starts out as a null pointer. See questions 1.30 and 5.1.) The simplest way to correct the question-asking program is to use a local array, instead of a pointer, and let the compiler worry about allocation: #include <stdio.h> #include <string.h> char answer[100], *p; printf("Type something:\n"); fgets(answer, sizeof answer, stdin); if((p = strchr(answer, '\n')) != NULL) *p = '\0'; printf("You typed \"%s\"\n", answer); This example also uses fgets() instead of gets(), so that the end of the array cannot be overwritten. (See question 12.23. Unfortunately for this example, fgets() does not automatically delete the trailing \n, as gets() would.) It would also be possible to use malloc() to allocate the answer buffer. 7.2: I can't get strcat() to work. I tried char *s1 = "Hello, "; char *s2 = "world!"; char *s3 = strcat(s1, s2); but I got strange results. A: As in question 7.1 above, the main problem here is that space for the concatenated result is not properly allocated. C does not provide an automatically-managed string type. C compilers allocate memory only for objects explicitly mentioned in the source code (in the case of strings, this includes character arrays and string literals). The programmer must arrange for sufficient space for the results of run-time operations such as string concatenation, typically by declaring arrays, or by calling malloc(). strcat() performs no allocation; the second string is appended to the first one, in place. Therefore, one fix would be to declare the first string as an array: char s1[20] = "Hello, "; Since strcat() returns the value of its first argument (s1, in this case), the variable s3 is superfluous; after the call to strcat(), s1 contains the result. The original call to strcat() in the question actually has two problems: the string literal pointed to by s1, besides not being big enough for any concatenated text, is not necessarily writable at all. See question 1.32. References: CT&P Sec. 3.2 p. 32. 7.3: But the man page for strcat() says that it takes two char *'s as arguments. How am I supposed to know to allocate things? A: In general, when using pointers you *always* have to consider memory allocation, if only to make sure that the compiler is doing it for you. If a library function's documentation does not explicitly mention allocation, it is usually the caller's problem. The Synopsis section at the top of a Unix-style man page or in the ANSI C standard can be misleading. The code fragments presented there are closer to the function definitions used by an implementor than the invocations used by the caller. In particular, many functions which accept pointers (e.g. to structures or strings) are usually called with a pointer to some object (a structure, or an array -- see questions 6.3 and 6.4) which the caller has allocated. Other common examples are time() (see question 13.12) and stat(). 7.3b: I just tried the code char *p; strcpy(p, "abc"); and it worked. How? Why didn't it crash? A: You got lucky, I guess. The memory randomly pointed to by the uninitialized pointer p happened to be writable by you, and apparently was not already in use for anything vital. See also question 11.35. 7.3c: How much memory does a pointer variable allocate? A: That's a pretty misleading question. When you declare a pointer variable, as in char *p; you (or, more properly, the compiler) have allocated only enough memory to hold the pointer itself; that is, in this case you have allocated sizeof(char *) bytes of memory. But you have not yet allocated *any* memory for the pointer to point to. See also questions 7.1 and 7.2. 7.5a: I have a function that is supposed to return a string, but when it returns to its caller, the returned string is garbage. A: Make sure that the pointed-to memory is properly allocated. For example, make sure you have *not* done something like char *itoa(int n) { char retbuf[20]; /* WRONG */ sprintf(retbuf, "%d", n); return retbuf; /* WRONG */ } One fix (which is imperfect, especially if the function in question is called recursively, or if several of its return values are needed simultaneously) would be to declare the return buffer as static char retbuf[20]; See also questions 7.5b, 12.21, and 20.1. References: ISO Sec. 6.1.2.4. 7.5b: So what's the right way to return a string or other aggregate? A: The returned pointer should be to a statically-allocated buffer (as in the answer to question 7.5a), or to a buffer passed in by the caller, or to memory obtained with malloc(), but *not* to a local (automatic) array. See also question 20.1. 7.6: Why am I getting "warning: assignment of pointer from integer lacks a cast" for calls to malloc()? A: Have you #included <stdlib.h>, or otherwise arranged for malloc() to be declared properly? See also question 1.25. References: H&S Sec. 4.7 p. 101. 7.7: Why does some code carefully cast the values returned by malloc to the pointer type being allocated? A: Before ANSI/ISO Standard C introduced the void * generic pointer type, these casts were typically required to silence warnings (and perhaps induce conversions) when assigning between incompatible pointer types. Under ANSI/ISO Standard C, these casts are no longer necessary, and in fact modern practice discourages them, since they can camouflage important warnings which would otherwise be generated if malloc() happened not to be declared correctly; see question 7.6 above. (However, the casts are typically seen in C code which for one reason or another is intended to be compatible with C++, where explicit casts from void * are required.) References: H&S Sec. 16.1 pp. 386-7. 7.7c: In a call to malloc(), what does an error like "Cannot convert `void *' to `int *'" mean? A: It means you're using a C++ compiler instead of a C compiler. See question 7.7. 7.8: I see code like char *p = malloc(strlen(s) + 1); strcpy(p, s); Shouldn't that be malloc((strlen(s) + 1) * sizeof(char))? A: It's never necessary to multiply by sizeof(char), since sizeof(char) is, by definition, exactly 1. (On the other hand, multiplying by sizeof(char) doesn't hurt, and in some circumstances may help by introducing a size_t into the expression.) See also question 8.9. References: ISO Sec. 6.3.3.4; H&S Sec. 7.5.2 p. 195. 7.11: How can I dynamically allocate arrays? A: See questions 6.14 and 6.16. 7.14: I've heard that some operating systems don't actually allocate malloc'ed memory until the program tries to use it. Is this legal? A: It's hard to say. The Standard doesn't say that systems can act this way, but it doesn't explicitly say that they can't, either. References: ISO Sec. 7.10.3. 7.16: I'm allocating a large array for some numeric work, using the line double *array = malloc(300 * 300 * sizeof(double)); malloc() isn't returning null, but the program is acting strangely, as if it's overwriting memory, or malloc() isn't allocating as much as I asked for, or something. A: Notice that 300 x 300 is 90,000, which will not fit in a 16-bit int, even before you multiply it by sizeof(double). If you need to allocate this much memory, you'll have to be careful. If size_t (the type accepted by malloc()) is a 32-bit type on your machine, but int is 16 bits, you might be able to get away with writing 300 * (300 * sizeof(double)) (see question 3.14). Otherwise, you'll have to break your data structure up into smaller chunks, or use a 32-bit machine or compiler, or use some nonstandard memory allocation functions. See also question 19.23. 7.17: I've got 8 meg of memory in my PC. Why can I only seem to malloc 640K or so? A: Under the segmented architecture of PC compatibles, it can be difficult to use more than 640K with any degree of transparency, especially under MS-DOS. See also question 19.23. 7.19: My program is crashing, apparently somewhere down inside malloc, but I can't see anything wrong with it. Is there a bug in malloc()? A: It is unfortunately very easy to corrupt malloc's internal data structures, and the resulting problems can be stubborn. The most common source of problems is writing more to a malloc'ed region than it was allocated to hold; a particularly common bug is to malloc(strlen(s)) instead of strlen(s) + 1. Other problems may involve using pointers to memory that has been freed, freeing pointers twice, freeing pointers not obtained from malloc, or trying to realloc a null pointer (see question 7.30). See also questions 7.26, 16.8, and 18.2. 7.20: You can't use dynamically-allocated memory after you free it, can you? A: No. Some early documentation for malloc() stated that the contents of freed memory were "left undisturbed," but this ill- advised guarantee was never universal and is not required by the C Standard. Few programmers would use the contents of freed memory deliberately, but it is easy to do so accidentally. Consider the following (correct) code for freeing a singly-linked list: struct list *listp, *nextp; for(listp = base; listp != NULL; listp = nextp) { nextp = listp->next; free(listp); } and notice what would happen if the more-obvious loop iteration expression listp = listp->next were used, without the temporary nextp pointer. References: K&R2 Sec. 7.8.5 p. 167; ISO Sec. 7.10.3; Rationale Sec. 4.10.3.2; H&S Sec. 16.2 p. 387; CT&P Sec. 7.10 p. 95. 7.21: Why isn't a pointer null after calling free()? How unsafe is it to use (assign, compare) a pointer value after it's been freed? A: When you call free(), the memory pointed to by the passed pointer is freed, but the value of the pointer in the caller probably remains unchanged, because C's pass-by-value semantics mean that called functions never permanently change the values of their arguments. (See also question 4.8.) A pointer value which has been freed is, strictly speaking, invalid, and *any* use of it, even if it is not dereferenced, can theoretically lead to trouble, though as a quality of implementation issue, most implementations will probably not go out of their way to generate exceptions for innocuous uses of invalid pointers. References: ISO Sec. 7.10.3; Rationale Sec. 3.2.2.3. 7.22: When I call malloc() to allocate memory for a pointer which is local to a function, do I have to explicitly free() it? A: Yes. Remember that a pointer is different from what it points to. Local variables are deallocated when the function returns, but in the case of a pointer variable, this means that the pointer is deallocated, *not* what it points to. Memory allocated with malloc() always persists until you explicitly free it. In general, for every call to malloc(), there should be a corresponding call to free(). 7.23: I'm allocating structures which contain pointers to other dynamically-allocated objects. When I free a structure, do I also have to free each subsidiary pointer? A: Yes. In general, you must arrange that each pointer returned from malloc() be individually passed to free(), exactly once (if it is freed at all). A good rule of thumb is that for each call to malloc() in a program, you should be able to point at the call to free() which frees the memory allocated by that malloc() call. See also question 7.24. 7.24: Must I free allocated memory before the program exits? A: You shouldn't have to. A real operating system definitively reclaims all memory and other resources when a program exits. Nevertheless, some personal computers are said not to reliably recover memory, and all that can be inferred from the ANSI/ISO C Standard is that this is a "quality of implementation issue." References: ISO Sec. 7.10.3.2. 7.25: I have a program which mallocs and later frees a lot of memory, but I can see from the operating system that memory usage doesn't actually go back down. A: Most implementations of malloc/free do not return freed memory to the operating system, but merely make it available for future malloc() calls within the same program. 7.26: How does free() know how many bytes to free? A: The malloc/free implementation remembers the size of each block as it is allocated, so it is not necessary to remind it of the size when freeing. 7.27: So can I query the malloc package to find out how big an allocated block is? A: Unfortunately, there is no standard or portable way. (Some compilers provide nonstandard extensions.) 7.30: Is it legal to pass a null pointer as the first argument to realloc()? Why would you want to? A: ANSI C sanctions this usage (and the related realloc(..., 0), which frees), although several earlier implementations do not support it, so it may not be fully portable. Passing an initially-null pointer to realloc() can make it easier to write a self-starting incremental allocation algorithm. References: ISO Sec. 7.10.3.4; H&S Sec. 16.3 p. 388. 7.31: What's the difference between calloc() and malloc()? Is it safe to take advantage of calloc's zero-filling? Does free() work on memory allocated with calloc(), or do you need a cfree()? A: calloc(m, n) is essentially equivalent to p = malloc(m * n); memset(p, 0, m * n); The zero fill is all-bits-zero, and does *not* therefore guarantee useful null pointer values (see section 5 of this list) or floating-point zero values. free() is properly used to free the memory allocated by calloc(). References: ISO Sec. 7.10.3 to 7.10.3.2; H&S Sec. 16.1 p. 386, Sec. 16.2 p. 386; PCS Sec. 11 pp. 141,142. 7.32: What is alloca() and why is its use discouraged? A: alloca() allocates memory which is automatically freed when the function which called alloca() returns. That is, memory allocated with alloca is local to a particular function's "stack frame" or context. alloca() cannot be written portably, and is difficult to implement on machines without a conventional stack. Its use is problematical (and the obvious implementation on a stack-based machine fails) when its return value is passed directly to another function, as in fgets(alloca(100), 100, stdin). For these reasons, alloca() is not Standard and cannot be used in programs which must be widely portable, no matter how useful it might be. Now that C99 supports variable-length arrays (VLA's), they can be used to more cleanly accomplish most of the tasks which alloca() used to be put to. See also question 7.22. References: Rationale Sec. 4.10.3. Section 8. Characters and Strings 8.1: Why doesn't strcat(string, '!'); work? A: There is a very real difference between characters and strings, and strcat() concatenates *strings*. Characters in C are represented by small integers corresponding to their character set values (see also question 8.6 below). Strings are represented by arrays of characters; you usually manipulate a pointer to the first character of the array. It is never correct to use one when the other is expected. To append a ! to a string, use strcat(string, "!"); See also questions 1.32, 7.2, and 16.6. References: CT&P Sec. 1.5 pp. 9-10. 8.2: I'm checking a string to see if it matches a particular value. Why isn't this code working? char *string; ... if(string == "value") { /* string matches "value" */ ... } A: Strings in C are represented as arrays of characters, and C never manipulates (assigns, compares, etc.) arrays as a whole. The == operator in the code fragment above compares two pointers -- the value of the pointer variable string and a pointer to the string literal "value" -- to see if they are equal, that is, if they point to the same place. They probably don't, so the comparison never succeeds. To compare two strings, you generally use the library function strcmp(): if(strcmp(string, "value") == 0) { /* string matches "value" */ ... } 8.3: If I can say char a[] = "Hello, world!"; why can't I say char a[14]; a = "Hello, world!"; A: Strings are arrays, and you can't assign arrays directly. Use strcpy() instead: strcpy(a, "Hello, world!"); See also questions 1.32, 4.2, and 7.2. 8.6: How can I get the numeric (character set) value corresponding to a character, or vice versa? A: In C, characters are represented by small integers corresponding to their values in the machine's character set. Therefore, you don't need a conversion function: if you have the character, you have its value. To convert back and forth between the digit characters and the corresponding integers in the range 0-9, add or subtract the constant '0' (that is, the character value '0'). See also questions 13.1 and 20.10. 8.9: I think something's wrong with my compiler: I just noticed that sizeof('a') is 2, not 1 (i.e. not sizeof(char)). A: Perhaps surprisingly, character constants in C are of type int, so sizeof('a') is sizeof(int) (though this is another area where C++ differs). See also question 7.8. References: ISO Sec. 6.1.3.4; H&S Sec. 2.7.3 p. 29. Section 9. Boolean Expressions and Variables 9.1: What is the right type to use for Boolean values in C? Why isn't it a standard type? Should I use #defines or enums for the true and false values? A: C does not provide a standard Boolean type, in part because picking one involves a space/time tradeoff which can best be decided by the programmer. (Using an int may be faster, while using char may save data space. Smaller types may make the generated code bigger or slower, though, if they require lots of conversions to and from int.) The choice between #defines and enumeration constants for the true/false values is arbitrary and not terribly interesting (see also questions 2.22 and 17.10). Use any of #define TRUE 1 #define YES 1 #define FALSE 0 #define NO 0 enum bool {false, true}; enum bool {no, yes}; or use raw 1 and 0, as long as you are consistent within one program or project. (An enumeration may be preferable if your debugger shows the names of enumeration constants when examining variables.) Some people prefer variants like #define TRUE (1==1) #define FALSE (!TRUE) or define "helper" macros such as #define Istrue(e) ((e) != 0) These don't buy anything (see question 9.2 below; see also questions 5.12 and 10.2). 9.2: Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered "true" in C? What if a built-in logical or relational operator "returns" something other than 1? A: It is true (sic) that any nonzero value is considered true in C, but this applies only "on input", i.e. where a Boolean value is expected. When a Boolean value is generated by a built-in operator, it is guaranteed to be 1 or 0. Therefore, the test if((a == b) == TRUE) would work as expected (as long as TRUE is 1), but it is obviously silly. In fact, explicit tests against TRUE and FALSE are generally inappropriate, because some library functions (notably isupper(), isalpha(), etc.) return, on success, a nonzero value which is not necessarily 1. (Besides, if you believe that "if((a == b) == TRUE)" is an improvement over "if(a == b)", why stop there? Why not use "if(((a == b) == TRUE) == TRUE)"?) A good rule of thumb is to use TRUE and FALSE (or the like) only for assignment to a Boolean variable or function parameter, or as the return value from a Boolean function, but never in a comparison. The preprocessor macros TRUE and FALSE (and, of course, NULL) are used for code readability, not because the underlying values might ever change. (See also questions 5.3 and 5.10.) Although the use of macros like TRUE and FALSE (or YES and NO) seems clearer, Boolean values and definitions can be sufficiently confusing in C that some programmers feel that TRUE and FALSE macros only compound the confusion, and prefer to use raw 1 and 0 instead. (See also question 5.9.) References: K&R1 Sec. 2.6 p. 39, Sec. 2.7 p. 41; K&R2 Sec. 2.6 p. 42, Sec. 2.7 p. 44, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206; ISO Sec. 6.3.3.3, Sec. 6.3.8, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5; H&S Sec. 7.5.4 pp. 196-7, Sec. 7.6.4 pp. 207-8, Sec. 7.6.5 pp. 208-9, Sec. 7.7 pp. 217-8, Sec. 7.8 pp. 218-9, Sec. 8.5 pp. 238-9, Sec. 8.6 pp. 241-4; "What the Tortoise Said to Achilles". 9.3: Is if(p), where p is a pointer, a valid conditional? A: Yes. See question 5.3. Section 10. C Preprocessor 10.2: Here are some cute preprocessor macros: #define begin { #define end } What do y'all think? A: Bleah. See also section 17. 10.3: How can I write a generic macro to swap two values? A: There is no good answer to this question. If the values are integers, a well-known trick using exclusive-OR could perhaps be used, but it will not work for floating-point values or pointers, or if the two values are the same variable. (See questions 3.3b and 20.15c.) If the macro is intended to be used on values of arbitrary type (the usual goal), it cannot use a temporary, since it does not know what type of temporary it needs (and would have a hard time picking a name for it if it did), and standard C does not provide a typeof operator. The best all-around solution is probably to forget about using a macro, unless you're willing to pass in the type as a third argument. 10.4: What's the best way to write a multi-statement macro? A: The usual goal is to write a macro that can be invoked as if it were a statement consisting of a single function call. This means that the "caller" will be supplying the final semicolon, so the macro body should not. The macro body cannot therefore be a simple brace-enclosed compound statement, because syntax errors would result if it were invoked (apparently as a single statement, but with a resultant extra semicolon) as the if branch of an if/else statement with an explicit else clause. The traditional solution, therefore, is to use #define MACRO(arg1, arg2) do { \ /* declarations */ \ stmt1; \ stmt2; \ /* ... */ \ } while(0) /* (no trailing ; ) */ When the caller appends a semicolon, this expansion becomes a single statement regardless of context. (An optimizing compiler will remove any "dead" tests or branches on the constant condition 0, although lint may complain.) If all of the statements in the intended macro are simple expressions, with no declarations or loops, another technique is to write a single, parenthesized expression using one or more comma operators. (For an example, see the first DEBUG() macro in question 10.26.) This technique also allows a value to be "returned." References: H&S Sec. 3.3.2 p. 45; CT&P Sec. 6.3 pp. 82-3. 10.6: I'm splitting up a program into multiple source files for the first time, and I'm wondering what to put in .c files and what to put in .h files. (What does ".h" mean, anyway?) A: As a general rule, you should put these things in header (.h) files: macro definitions (preprocessor #defines) structure, union, and enumeration declarations typedef declarations external function declarations (see also question 1.11) global variable declarations It's especially important to put a declaration or definition in a header file when it will be shared between several other files. (In particular, never put external function prototypes in .c files. See also question 1.7.) On the other hand, when a definition or declaration should remain private to one .c file, it's fine to leave it there. See also questions 1.7 and 10.7. References: K&R2 Sec. 4.5 pp. 81-2; H&S Sec. 9.2.3 p. 267; CT&P Sec. 4.6 pp. 66-7. 10.7: Is it acceptable for one header file to #include another? A: It's a question of style, and thus receives considerable debate. Many people believe that "nested #include files" are to be avoided: the prestigious Indian Hill Style Guide (see question 17.9) disparages them; they can make it harder to find relevant definitions; they can lead to multiple-definition errors if a file is #included twice; they can lead to increased compilation time; and they make manual Makefile maintenance very difficult. On the other hand, they make it possible to use header files in a modular way (a header file can #include what it needs itself, rather than requiring each #includer to do so); a tool like grep (or a tags file) makes it easy to find definitions no matter where they are; a popular trick along the lines of: #ifndef HFILENAME_USED #define HFILENAME_USED ...header file contents... #endif (where a different bracketing macro name is used for each header file) makes a header file "idempotent" so that it can safely be #included multiple times; and automated Makefile maintenance tools (which are a virtual necessity in large projects anyway; see question 18.1) handle dependency generation in the face of nested #include files easily. See also question 17.10. References: Rationale Sec. 4.1.2. 10.8a: What's the difference between #include <> and #include "" ? A: The <> syntax is typically used with Standard or system-supplied headers, while "" is typically used for a program's own header files. 10.8b: What are the complete rules for header file searching? A: The exact behavior is implementation-defined (which means that it is supposed to be documented; see question 11.33). Typically, headers named with <> syntax are searched for in one or more standard places. Header files named with "" syntax are first searched for in the "current directory," then (if not found) in the same standard places. Traditionally (especially under Unix compilers), the current directory is taken to be the directory containing the file containing the #include directive. Under other compilers, however, the current directory (if any) is the directory in which the compiler was initially invoked. Check your compiler documentation. References: K&R2 Sec. A12.4 p. 231; ISO Sec. 6.8.2; H&S Sec. 3.4 p. 55. 10.9: I'm getting strange syntax errors on the very first declaration in a file, but it looks fine. A: Perhaps there's a missing semicolon at the end of the last declaration in the last header file you're #including. See also questions 2.18, 11.29, and 16.1b. 10.10b: I'm #including the right header file for the library function I'm using, but the linker keeps saying it's undefined. A: See question 13.25. 10.11: I'm compiling a program, and I seem to be missing one of the header files it requires. Can someone send me a copy? A: There are several situations, depending on what sort of header file it is that's "missing". If the missing header file is a standard one, there's a problem with your compiler. You'll need to contact your vendor, or someone knowledgeable about your particular compiler, for help. The situation is more complicated in the case of nonstandard headers. Some are completely system- or compiler-specific. Some are completely unnecessary, and should be replaced by their Standard equivalents. (For example, instead of <malloc.h>, use <stdlib.h>.) Other headers, such as those associated with popular add-on libraries, may be reasonably portable. Standard headers exist in part so that definitions appropriate to your compiler, operating system, and processor can be supplied. You cannot just pick up a copy of someone else's header file and expect it to work, unless that person is using exactly the same environment. You may actually have a portability problem (see section 19), or a compiler problem. Otherwise, see question 18.16. 10.12: How can I construct preprocessor #if expressions which compare strings? A: You can't do it directly; preprocessor #if arithmetic uses only integers. An alternative is to #define several macros with symbolic names and distinct integer values, and implement conditionals on those. See also question 20.17. References: K&R2 Sec. 4.11.3 p. 91; ISO Sec. 6.8.1; H&S Sec. 7.11.1 p. 225. 10.13: Does the sizeof operator work in preprocessor #if directives? A: No. Preprocessing happens during an earlier phase of compilation, before type names have been parsed. Instead of sizeof, consider using the predefined constants in ANSI's <limits.h>, if applicable, or perhaps a "configure" script. (Better yet, try to write code which is inherently insensitive to type sizes; see also question 1.1.) References: ISO Sec. 5.1.1.2, Sec. 6.8.1; H&S Sec. 7.11.1 p. 225. 10.14: Can I use an #ifdef in a #define line, to define something two different ways? A: No. You can't "run the preprocessor on itself," so to speak. What you can do is use one of two completely separate #define lines, depending on the #ifdef setting. References: ISO Sec. 6.8.3, Sec. 6.8.3.4; H&S Sec. 3.2 pp. 40-1. 10.15: Is there anything like an #ifdef for typedefs? A: Unfortunately, no. You may have to keep sets of preprocessor macros (e.g. MY_TYPE_DEFINED) recording whether certain typedefs have been declared. (See also question 10.13.) References: ISO Sec. 5.1.1.2, Sec. 6.8.1; H&S Sec. 7.11.1 p. 225. 10.16: How can I use a preprocessor #if expression to tell if a machine is big-endian or little-endian? A: You probably can't. (Preprocessor arithmetic uses only long integers, and there is no concept of addressing.) Are you sure you need to know the machine's endianness explicitly? Usually it's better to write code which doesn't care. See also question 20.9. References: ISO Sec. 6.8.1; H&S Sec. 7.11.1 p. 225. 10.18: I inherited some code which contains far too many #ifdef's for my taste. How can I preprocess the code to leave only one conditional compilation set, without running it through the preprocessor and expanding all of the #include's and #define's as well? A: There are programs floating around called unifdef, rmifdef, and scpp ("selective C preprocessor") which do exactly this. See question 18.16. 10.19: How can I list all of the predefined identifiers? A: There's no standard way, although it is a common need. gcc provides a -dM option which works with -E, and other compilers may provide something similar. If the compiler documentation is unhelpful, the most expedient way is probably to extract printable strings from the compiler or preprocessor executable with something like the Unix strings utility. Beware that many traditional system-specific predefined identifiers (e.g. "unix") are non-Standard (because they clash with the user's namespace) and are being removed or renamed. 10.20: I have some old code that tries to construct identifiers with a macro like #define Paste(a, b) a/**/b but it doesn't work any more. A: It was an undocumented feature of some early preprocessor implementations (notably Reiser's) that comments disappeared entirely and could therefore be used for token pasting. ANSI affirms (as did K&R1) that comments are replaced with white space. However, since the need for pasting tokens was demonstrated and real, ANSI introduced a well-defined token- pasting operator, ##, which can be used like this: #define Paste(a, b) a##b See also question 11.17. References: ISO Sec. 6.8.3.3; Rationale Sec. 3.8.3.3; H&S Sec. 3.3.9 p. 52. 10.22: Why is the macro #define TRACE(n) printf("TRACE: %d\n", n) giving me the warning "macro replacement within a string literal"? It seems to be expanding TRACE(count); as printf("TRACE: %d\count", count); A: See question 11.18. 10.23-4: I'm having trouble using macro arguments inside string literals, using the `#' operator. A: See questions 11.17 and 11.18. 10.25: I've got this tricky preprocessing I want to do and I can't figure out a way to do it. A: C's preprocessor is not intended as a general-purpose tool. (Note also that it is not guaranteed to be available as a separate program.) Rather than forcing it to do something inappropriate, consider writing your own little special-purpose preprocessing tool, instead. You can easily get a utility like make(1) to run it for you automatically. If you are trying to preprocess something other than C, consider using a general-purpose preprocessor. (One older one available on most Unix systems is m4.) 10.26: How can I write a macro which takes a variable number of arguments? A: One popular trick is to define and invoke the macro with a single, parenthesized "argument" which in the macro expansion becomes the entire argument list, parentheses and all, for a function such as printf(): #define DEBUG(args) (printf("DEBUG: "), printf args) if(n != 0) DEBUG(("n is %d\n", n)); The obvious disadvantage is that the caller must always remember to use the extra parentheses. gcc has an extension which allows a function-like macro to accept a variable number of arguments, but it's not standard. Other possible solutions are to use different macros (DEBUG1, DEBUG2, etc.) depending on the number of arguments, or to play tricky games with commas: #define DEBUG(args) (printf("DEBUG: "), printf(args)) #define _ , DEBUG("i = %d" _ i); C99 introduces formal support for function-like macros with variable-length argument lists. The notation ... can appear at the end of the macro "prototype" (just as it does for varargs functions), and the pseudomacro __VA_ARGS__ in the macro definition is replaced by the variable arguments during invocation. Finally, you can always use a bona-fide function, which can take a variable number of arguments in a well-defined way. See questions 15.4 and 15.5. (If you needed a macro replacement, try using a function plus a non-function-like macro, e.g. #define printf myprintf .) References: C9X Sec. 6.8.3, Sec. 6.8.3.1. Section 11. ANSI/ISO Standard C 11.1: What is the "ANSI C Standard?" A: In 1983, the American National Standards Institute (ANSI) commissioned a committee, X3J11, to standardize the C language. After a long, arduous process, including several widespread public reviews, the committee's work was finally ratified as ANS X3.159-1989 on December 14, 1989, and published in the spring of 1990. For the most part, ANSI C standardized existing practice, with a few additions from C++ (most notably function prototypes) and support for multinational character sets (including the controversial trigraph sequences). The ANSI C standard also formalized the C run-time library support routines. A year or so later, the Standard was adopted as an international standard, ISO/IEC 9899:1990, and this ISO Standard replaced the earlier X3.159 even within the United States (where it was known as ANSI/ISO 9899-1990 [1992]). As an ISO Standard, it is subject to ongoing revision through the release of Technical Corrigenda and Normative Addenda. In 1994, Technical Corrigendum 1 (TC1) amended the Standard in about 40 places, most of them minor corrections or clarifications, and Normative Addendum 1 (NA1) added about 50 pages of new material, mostly specifying new library functions for internationalization. In 1995, TC2 added a few more minor corrections. Most recently, a major revision of the Standard, "C99", has been completed and adopted. Several versions of the Standard, including C99 and the original ANSI Standard, have included a "Rationale," explaining many of its decisions, and discussing a number of subtle points, including several of those covered here. 11.2: How can I get a copy of the Standard? A: An electronic (PDF) copy is available on-line, for US$18, from www.ansi.org. Paper copies are available in the United States from American National Standards Institute 11 W. 42nd St., 13th floor New York, NY 10036 USA (+1) 212 642 4900 and Global Engineering Documents 15 Inverness Way E Englewood, CO 80112 USA (+1) 303 397 2715 (800) 854 7179 (U.S. & Canada) In other countries, contact the appropriate national standards body, or ISO in Geneva at: ISO Sales Case Postale 56 CH-1211 Geneve 20 Switzerland (or see URL http://www.iso.ch or check the comp.std.internat FAQ list, Standards.Faq). The mistitled _Annotated ANSI C Standard_, with annotations by Herbert Schildt, contains most of the text of ISO 9899; it is published by Osborne/McGraw-Hill, ISBN 0-07-881952-0, and sells in the U.S. for approximately $40. It has been suggested that the price differential between this work and the official standard reflects the value of the annotations: they are plagued by numerous errors and omissions, and a few pages of the Standard itself are missing. Many people on the net recommend ignoring the annotations entirely. A review of the annotations ("annotated annotations") by Clive Feather can be found on the web at http://www.lysator.liu.se/c/schildt.html . The text of the original ANSI Rationale can be obtained by anonymous ftp from ftp.uu.net (see question 18.16) in directory doc/standards/ansi/X3.159-1989, and is also available on the web at http://www.lysator.liu.se/c/rat/title.html . That Rationale has also been printed by Silicon Press, ISBN 0-929306-07-4. Public review drafts of C9X were available from ISO/IEC JTC1/SC22/WG14's web site, http://www.dkuug.dk/JTC1/SC22/WG14/ . See also question 11.2b below. 11.2b: Where can I get information about updates to the Standard? A: You can find information (including C9X drafts) at the web sites http://www.lysator.liu.se/c/index.html, http://www.dkuug.dk/JTC1/SC22/WG14/, and http://www.dmk.com/ . 11.3: My ANSI compiler complains about a mismatch when it sees extern int func(float); int func(x) float x; { ... A: You have mixed the new-style prototype declaration "extern int func(float);" with the old-style definition "int func(x) float x;". It is usually possible to mix the two styles (see question 11.4), but not in this case. Old C (and ANSI C, in the absence of prototypes, and in variable-length argument lists; see question 15.2) "widens" certain arguments when they are passed to functions. floats are promoted to double, and characters and short integers are promoted to int. (For old-style function definitions, the values are automatically converted back to the corresponding narrower types within the body of the called function, if they are declared that way there.) This problem can be fixed either by using new-style syntax consistently in the definition: int func(float x) { ... } or by changing the new-style prototype declaration to match the old-style definition: extern int func(double); (In this case, it would be clearest to change the old-style definition to use double as well, if possible.) It is arguably much safer to avoid "narrow" (char, short int, and float) function arguments and return types altogether. See also question 1.25. References: K&R1 Sec. A7.1 p. 186; K&R2 Sec. A7.3.2 p. 202; ISO Sec. 6.3.2.2, Sec. 6.5.4.3; Rationale Sec. 3.3.2.2, Sec. 3.5.4.3; H&S Sec. 9.2 pp. 265-7, Sec. 9.4 pp. 272-3. 11.4: Can you mix old-style and new-style function syntax? A: Doing so is legal, but requires a certain amount of care (see especially question 11.3). Modern practice, however, is to use the prototyped form in both declarations and definitions. (The old-style syntax is marked as obsolescent, so official support for it may be removed some day.) References: ISO Sec. 6.7.1, Sec. 6.9.5; H&S Sec. 9.2.2 pp. 265-7, Sec. 9.2.5 pp. 269-70. 11.5: Why does the declaration extern int f(struct x *p); give me an obscure warning message about "struct x declared inside parameter list"? A: In a quirk of C's normal block scoping rules, a structure declared (or even mentioned) for the first time within a prototype cannot be compatible with other structures declared in the same source file (it goes out of scope at the end of the prototype). To resolve the problem, precede the prototype with the vacuous- looking declaration struct x; which places an (incomplete) declaration of struct x at file scope, so that all following declarations involving struct x can at least be sure they're referring to the same struct x. References: ISO Sec. 6.1.2.1, Sec. 6.1.2.6, Sec. 6.5.2.3. 11.8: I don't understand why I can't use const values in initializers and array dimensions, as in const int n = 5; int a[n]; A: The const qualifier really means "read-only"; an object so qualified is a run-time object which cannot (normally) be assigned to. The value of a const-qualified object is therefore *not* a constant expression in the full sense of the term. (C is unlike C++ in this regard.) When you need a true compile- time constant, use a preprocessor #define (or perhaps an enum). References: ISO Sec. 6.4; H&S Secs. 7.11.2,7.11.3 pp. 226-7. 11.8b: If you can't modify string literals, why aren't they defined as being arrays of const characters? A: One reason is that so very much code contains lines like char *p = "Hello, world!"; which are not necessarily incorrect. These lines would suffer the diagnostic messages, but it's really any later attempt to modify what p points to which would be problems. See also question 1.32. 11.9: What's the difference between "const char *p" and "char * const p"? A: "const char *p" (which can also be written "char const *p") declares a pointer to a constant character (you can't change any pointed-to characters); "char * const p" declares a constant pointer to a (variable) character (i.e. you can't change the pointer). Read these "inside out" to understand them; see also question 1.21. References: ISO Sec. 6.5.4.1; Rationale Sec. 3.5.4.1; H&S Sec. 4.4.4 p. 81. 11.10: Why can't I pass a char ** to a function which expects a const char **? A: You can use a pointer-to-T (for any type T) where a pointer-to- const-T is expected. However, the rule (an explicit exception) which permits slight mismatches in qualified pointer types is not applied recursively, but only at the top level. If you must assign or pass pointers which have qualifier mismatches at other than the first level of indirection, you must use explicit casts (e.g. (const char **) in this case), although as always, the need for such a cast may indicate a deeper problem which the cast doesn't really fix. References: ISO Sec. 6.1.2.6, Sec. 6.3.16.1, Sec. 6.5.3; H&S Sec. 7.9.1 pp. 221-2. 11.12a: What's the correct declaration of main()? A: Either int main(), int main(void), or int main(int argc, char *argv[]) (with alternate spellings of argc and *argv[] obviously allowed). See also questions 11.12b to 11.15 below. References: ISO Sec. 5.1.2.2.1, Sec. G.5.1; H&S Sec. 20.1 p. 416; CT&P Sec. 3.10 pp. 50-51. 11.12b: Can I declare main() as void, to shut off these annoying "main returns no value" messages? A: No. main() must be declared as returning an int, and as taking either zero or two arguments, of the appropriate types. If you're calling exit() but still getting warnings, you may have to insert a redundant return statement (or use some kind of "not reached" directive, if available). Declaring a function as void does not merely shut off or rearrange warnings: it may also result in a different function call/return sequence, incompatible with what the caller (in main's case, the C run-time startup code) expects. (Note that this discussion of main() pertains only to "hosted" implementations; none of it applies to "freestanding" implementations, which may not even have main(). However, freestanding implementations are comparatively rare, and if you're using one, you probably know it. If you've never heard of the distinction, you're probably using a hosted implementation, and the above rules apply.) References: ISO Sec. 5.1.2.2.1, Sec. G.5.1; H&S Sec. 20.1 p. 416; CT&P Sec. 3.10 pp. 50-51. 11.13: But what about main's third argument, envp? A: It's a non-standard (though common) extension. If you really need to access the environment in ways beyond what the standard getenv() function provides, though, the global variable environ is probably a better avenue (though it's equally non-standard). References: ISO Sec. G.5.1; H&S Sec. 20.1 pp. 416-7. 11.14a: I believe that declaring void main() can't fail, since I'm calling exit() instead of returning, and anyway my operating system ignores a program's exit/return status. A: It doesn't matter whether main() returns or not, or whether anyone looks at the status; the problem is that when main() is misdeclared, its caller (the runtime startup code) may not even be able to *call* it correctly (due to the potential clash of calling conventions; see question 11.12b). Your operating system may ignore the exit status, and void main() may work for you, but it is not portable and not correct. 11.14b: So what could go wrong? Are there really any systems where void main() doesn't work? A: It has been reported that programs using void main() and compiled using BC++ 4.5 can crash. Some compilers (including DEC C V4.1 and gcc with certain warnings enabled) will complain about void main(). 11.15: The book I've been using, _C Programing for the Compleat Idiot_, always uses void main(). A: Perhaps its author counts himself among the target audience. Many books unaccountably use void main() in examples, and assert that it's correct. They're wrong. 11.16: Is exit(status) truly equivalent to returning the same status from main()? A: Yes and no. The Standard says that they are equivalent. However, a return from main() cannot be expected to work if data local to main() might be needed during cleanup; see also question 16.4. A few very old, nonconforming systems may once have had problems with one or the other form. (Finally, the two forms are obviously not equivalent in a recursive call to main().) References: K&R2 Sec. 7.6 pp. 163-4; ISO Sec. 5.1.2.2.3. 11.17: I'm trying to use the ANSI "stringizing" preprocessing operator `#' to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value. A: You can use something like the following two-step procedure to force a macro to be expanded as well as stringized: #define Str(x) #x #define Xstr(x) Str(x) #define OP plus char *opname = Xstr(OP); This code sets opname to "plus" rather than "OP". An equivalent circumlocution is necessary with the token-pasting operator ## when the values (rather than the names) of two macros are to be concatenated. References: ISO Sec. 6.8.3.2, Sec. 6.8.3.5. 11.18: What does the message "warning: macro replacement within a string literal" mean? A: Some pre-ANSI compilers/preprocessors interpreted macro definitions like #define TRACE(var, fmt) printf("TRACE: var = fmt\n", var) such that invocations like TRACE(i, %d); were expanded as printf("TRACE: i = %d\n", i); In other words, macro parameters were expanded even inside string literals and character constants. Macro expansion is *not* defined in this way by K&R or by Standard C. When you do want to turn macro arguments into strings, you can use the new # preprocessing operator, along with string literal concatenation (another new ANSI feature): #define TRACE(var, fmt) \ printf("TRACE: " #var " = " #fmt "\n", var) See also question 11.17 above. References: H&S Sec. 3.3.8 p. 51. 11.19: I'm getting strange syntax errors inside lines I've #ifdeffed out. A: Under ANSI C, the text inside a "turned off" #if, #ifdef, or #ifndef must still consist of "valid preprocessing tokens." This means that the characters " and ' must each be paired just as in real C code, and the pairs mustn't cross line boundaries. (Note particularly that an apostrophe within a contracted word looks like the beginning of a character constant.) Therefore, natural-language comments and pseudocode should always be written between the "official" comment delimiters /* and */. (But see question 20.20, and also 10.25.) References: ISO Sec. 5.1.1.2, Sec. 6.1; H&S Sec. 3.2 p. 40. 11.20: What are #pragmas and what are they good for? A: The #pragma directive provides a single, well-defined "escape hatch" which can be used for all sorts of (nonportable) implementation-specific controls and extensions: source listing control, structure packing, warning suppression (like lint's old /* NOTREACHED */ comments), etc. References: ISO Sec. 6.8.6; H&S Sec. 3.7 p. 61. 11.21: What does "#pragma once" mean? I found it in some header files. A: It is an extension implemented by some preprocessors to help make header files idempotent; it is equivalent to the #ifndef trick mentioned in question 10.7, though less portable. 11.22: Is char a[3] = "abc"; legal? What does it mean? A: It is legal in ANSI C (and perhaps in a few pre-ANSI systems), though useful only in rare circumstances. It declares an array of size three, initialized with the three characters 'a', 'b', and 'c', *without* the usual terminating '\0' character. The array is therefore not a true C string and cannot be used with strcpy, printf %s, etc. Most of the time, you should let the compiler count the initializers when initializing arrays (in the case of the initializer "abc", of course, the computed size will be 4). References: ISO Sec. 6.5.7; H&S Sec. 4.6.4 p. 98. 11.24: Why can't I perform arithmetic on a void * pointer? A: The compiler doesn't know the size of the pointed-to objects. Before performing arithmetic, convert the pointer either to char * or to the pointer type you're trying to manipulate (but see also question 4.5). References: ISO Sec. 6.1.2.5, Sec. 6.3.6; H&S Sec. 7.6.2 p. 204. 11.25: What's the difference between memcpy() and memmove()? A: memmove() offers guaranteed behavior if the source and destination arguments overlap. memcpy() makes no such guarantee, and may therefore be more efficiently implementable. When in doubt, it's safer to use memmove(). References: K&R2 Sec. B3 p. 250; ISO Sec. 7.11.2.1, Sec. 7.11.2.2; Rationale Sec. 4.11.2; H&S Sec. 14.3 pp. 341-2; PCS Sec. 11 pp. 165-6. 11.26: What should malloc(0) do? Return a null pointer or a pointer to 0 bytes? A: The ANSI/ISO Standard says that it may do either; the behavior is implementation-defined (see question 11.33). References: ISO Sec. 7.10.3; PCS Sec. 16.1 p. 386. 11.27: Why does the ANSI Standard place limits on the length and case- significance of external identifiers? A: The problem is linkers which are under control of neither the ANSI/ISO Standard nor the C compiler developers on the systems which have them. The limitation is only that identifiers be *significant* in some initial sequence of characters, not that they be restricted to that many characters in total length. (The limitation was to six characters in the original ANSI Standard, but has been relaxed to 31 in C99.) References: ISO Sec. 6.1.2, Sec. 6.9.1; Rationale Sec. 3.1.2; C9X Sec. 6.1.2; H&S Sec. 2.5 pp. 22-3. 11.29: My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors. A: Perhaps it is a pre-ANSI compiler, unable to accept function prototypes and the like. See also questions 1.31, 10.9, 11.30, and 16.1b. 11.30: Why are some ANSI/ISO Standard library functions showing up as undefined, even though I've got an ANSI compiler? A: It's possible to have a compiler available which accepts ANSI syntax, but not to have ANSI-compatible header files or run-time libraries installed. (In fact, this situation is rather common when using a non-vendor-supplied compiler such as gcc.) See also questions 11.29, 13.25, and 13.26. 11.31: Does anyone have a tool for converting old-style C programs to ANSI C, or vice versa, or for automatically generating prototypes? A: Two programs, protoize and unprotoize, convert back and forth between prototyped and "old style" function definitions and declarations. (These programs do *not* handle full-blown translation between "Classic" C and ANSI C.) These programs are part of the FSF's GNU C compiler distribution; see question 18.3. The unproto program (/pub/unix/unproto5.shar.Z on ftp.win.tue.nl) is a filter which sits between the preprocessor and the next compiler pass, converting most of ANSI C to traditional C on-the-fly. The GNU GhostScript package comes with a little program called ansi2knr. Before converting ANSI C back to old-style, beware that such a conversion cannot always be made both safely and automatically. ANSI C introduces new features and complexities not found in K&R C. You'll especially need to be careful of prototyped function calls; you'll probably need to insert explicit casts. See also questions 11.3 and 11.29. Several prototype generators exist, many as modifications to lint. A program called CPROTO was posted to comp.sources.misc in March, 1992. There is another program called "cextract." Many vendors supply simple utilities like these with their compilers. See also question 18.16. (But be careful when generating prototypes for old functions with "narrow" parameters; see question 11.3.) 11.32: Why won't the Frobozz Magic C Compiler, which claims to be ANSI compliant, accept this code? I know that the code is ANSI, because gcc accepts it. A: Many compilers support a few non-Standard extensions, gcc more so than most. Are you sure that the code being rejected doesn't rely on such an extension? It is usually a bad idea to perform experiments with a particular compiler to determine properties of a language; the applicable standard may permit variations, or the compiler may be wrong. See also question 11.35. 11.33: People seem to make a point of distinguishing between implementation-defined, unspecified, and undefined behavior. What's the difference? A: Briefly: implementation-defined means that an implementation must choose some behavior and document it. Unspecified means that an implementation should choose some behavior, but need not document it. Undefined means that absolutely anything might happen. In no case does the Standard impose requirements; in the first two cases it occasionally suggests (and may require a choice from among) a small set of likely behaviors. Note that since the Standard imposes *no* requirements on the behavior of a compiler faced with an instance of undefined behavior, the compiler can do absolutely anything. In particular, there is no guarantee that the rest of the program will perform normally. It's perilous to think that you can tolerate undefined behavior in a program; see question 3.2 for a relatively simple example. If you're interested in writing portable code, you can ignore the distinctions, as you'll usually want to avoid code that depends on any of the three behaviors. See also questions 3.9, and 11.34. (A fourth defined class of not-quite-precisely-defined behavior, without the same stigma attached to it, is "locale-specific".) References: ISO Sec. 3.10, Sec. 3.16, Sec. 3.17; Rationale Sec. 1.6. 11.33b: What does it really mean for a program to be "legal" or "valid" or "conforming"? A: Simply stated, the Standard talks about three kinds of conformance: conforming programs, strictly conforming programs, and conforming implementations. A "conforming program" is one that is accepted by a conforming implementation. A "strictly conforming program" is one that uses the language exactly as specified in the Standard, and that does not depend on any implementation-defined, unspecified, or undefined behavior. A "conforming implementation" is one that does everything the Standard says it's supposed to. References: ISO Sec. ; Rationale Sec. 1.7. 11.34: I'm appalled that the ANSI Standard leaves so many issues undefined. Isn't a Standard's whole job to standardize these things? A: It has always been a characteristic of C that certain constructs behaved in whatever way a particular compiler or a particular piece of hardware chose to implement them. This deliberate imprecision often allows compilers to generate more efficient code for common cases, without having to burden all programs with extra code to assure well-defined behavior of cases deemed to be less reasonable. Therefore, the Standard is simply codifying existing practice. A programming language standard can be thought of as a treaty between the language user and the compiler implementor. Parts of that treaty consist of features which the compiler implementor agrees to provide, and which the user may assume will be available. Other parts, however, consist of rules which the user agrees to follow and which the implementor may assume will be followed. As long as both sides uphold their guarantees, programs have a fighting chance of working correctly. If *either* side reneges on any of its commitments, nothing is guaranteed to work. See also question 11.35. References: Rationale Sec. 1.1. 11.35: People keep saying that the behavior of i = i++ is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected. A: A compiler may do anything it likes when faced with undefined behavior (and, within limits, with implementation-defined and unspecified behavior), including doing what you expect. It's unwise to depend on it, though. See also questions 7.3b, 11.32, 11.33, and 11.34. Section 12. Stdio 12.1: What's wrong with this code? char c; while((c = getchar()) != EOF) ... A: For one thing, the variable to hold getchar's return value must be an int. getchar() can return all possible character values, as well as EOF. By squeezing getchar's return value into a char, either a normal character might be misinterpreted as EOF, or the EOF might be altered (particularly if type char is unsigned) and so never seen. References: K&R1 Sec. 1.5 p. 14; K&R2 Sec. 1.5.1 p. 16; ISO Sec. 6.1.2.5, Sec. 7.9.1, Sec. 7.9.7.5; H&S Sec. 5.1.3 p. 116, Sec. 15.1, Sec. 15.6; CT&P Sec. 5.1 p. 70; PCS Sec. 11 p. 157. 12.1b: I have a simple little program that reads characters until EOF, but how do I actually *enter* that "EOF" value from the keyboard? A: It turns out that the value of EOF as seen within your C program has essentially nothing to do with the keystroke combination you might use to signal end-of-file from the keyboard. Depending on your operating system, you indicate end-of-file from the keyboard using various keystroke combinations, usually either control-D or control-Z. 12.2: Why does the code while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); } copy the last line twice? A: In C, end-of-file is only indicated *after* an input routine has tried to read, and failed. (In other words, C's I/O is not like Pascal's.) Usually, you should just check the return value of the input routine -- fgets(), for example, returns NULL on end- of-file. In virtually all cases, there's no need to use feof() at all. References: K&R2 Sec. 7.6 p. 164; ISO Sec. 7.9.3, Sec. 7.9.7.1, Sec. 7.9.10.2; H&S Sec. 15.14 p. 382. 12.4: My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program. A: It's best to use an explicit fflush(stdout) whenever output should definitely be visible (and especially if the text does not end with \n). Several mechanisms attempt to perform the fflush() for you, at the "right time," but they tend to apply only when stdout is an interactive terminal. (See also question 12.24.) References: ISO Sec. 7.9.5.2. 12.5: How can I read one character at a time, without waiting for the RETURN key? A: See question 19.1. 12.6: How can I print a '%' character in a printf format string? I tried \%, but it didn't work. A: Simply double the percent sign: %% . \% can't work, because the backslash \ is the *compiler's* escape character, while here our problem is that the % is essentially printf's escape character. See also question 19.17. References: K&R1 Sec. 7.3 p. 147; K&R2 Sec. 7.2 p. 154; ISO Sec. 7.9.6.1. 12.9: Someone told me it was wrong to use %lf with printf(). How can printf() use %f for type double, if scanf() requires %lf? A: It's true that printf's %f specifier works with both float and double arguments. Due to the "default argument promotions" (which apply in variable-length argument lists such as printf's, whether or not prototypes are in scope), values of type float are promoted to double, and printf() therefore sees only doubles. (printf() does accept %Lf, for long double.) See also questions 12.13 and 15.2. References: K&R1 Sec. 7.3 pp. 145-47, Sec. 7.4 pp. 147-50; K&R2 Sec. 7.2 pp. 153-44, Sec. 7.4 pp. 157-59; ISO Sec. 7.9.6.1, Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64, Sec. 15.11 pp. 366-78; CT&P Sec. A.1 pp. 121-33. 12.9b: What printf format should I use for a typedef like size_t when I don't know whether it's long or some other type? A: Use a cast to convert the value to a known, conservatively- sized type, then use the printf format matching that type. For example, to print the size of a type, you might use printf("%lu", (unsigned long)sizeof(thetype)); 12.10: How can I implement a variable field width with printf? That is, instead of %8d, I want the width to be specified at run time. A: printf("%*d", width, x) will do just what you want. See also question 12.15. References: K&R1 Sec. 7.3; K&R2 Sec. 7.2; ISO Sec. 7.9.6.1; H&S Sec. 15.11.6; CT&P Sec. A.1. 12.11: How can I print numbers with commas separating the thousands? What about currency formatted numbers? A: The functions in <locale.h> begin to provide some support for these operations, but there is no standard function for doing either task. (The only thing printf() does in response to a custom locale setting is to change its decimal-point character.) References: ISO Sec. 7.4; H&S Sec. 11.6 pp. 301-4. 12.12: Why doesn't the call scanf("%d", i) work? A: The arguments you pass to scanf() must always be pointers. To fix the fragment above, change it to scanf("%d", &i) . 12.12b: Why *does* the call char s[30]; scanf("%s", s); work (without the &)? A: You always need a *pointer*; you don't necessarily need an explicit &. When you pass an array to scanf(), you do not need the &, because arrays are always passed to functions as pointers, whether you use & or not. See questions 6.3 and 6.4. 12.13: Why doesn't this code: double d; scanf("%f", &d); work? A: Unlike printf(), scanf() uses %lf for values of type double, and %f for float. See also question 12.9. 12.15: How can I specify a variable width in a scanf() format string? A: You can't; an asterisk in a scanf() format string means to suppress assignment. You may be able to use ANSI stringizing and string concatenation to accomplish about the same thing, or you can construct the scanf format string at run time. 12.17: When I read numbers from the keyboard with scanf "%d\n", it seems to hang until I type one extra line of input. A: Perhaps surprisingly, \n in a scanf format string does *not* mean to expect a newline, but rather to read and discard characters as long as each is a whitespace character. See also question 12.20. References: K&R2 Sec. B1.3 pp. 245-6; ISO Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64. 12.18a: I'm reading a number with scanf %d and then a string with gets(), but the compiler seems to be skipping the call to gets()! A: scanf %d won't consume a trailing newline. If the input number is immediately followed by a newline, that newline will immediately satisfy the gets(). As a general rule, you shouldn't try to interlace calls to scanf() with calls to gets() (or any other input routines); scanf's peculiar treatment of newlines almost always leads to trouble. Either use scanf() to read everything or nothing. See also questions 12.20 and 12.23. References: ISO Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64. 12.19: I figured I could use scanf() more safely if I checked its return value to make sure that the user typed the numeric values I expect, but sometimes it seems to go into an infinite loop. A: When scanf() is attempting to convert numbers, any non-numeric characters it encounters terminate the conversion *and are left on the input stream*. Therefore, unless some other steps are taken, unexpected non-numeric input "jams" scanf() again and again: scanf() never gets past the bad character(s) to encounter later, valid data. If the user types a character like `x' in response to a numeric scanf format such as %d or %f, code that simply re-prompts and retries the same scanf() call will immediately reencounter the same `x'. See also question 12.20. References: ISO Sec. 7.9.6.2; H&S Sec. 15.8 pp. 357-64. 12.20: Why does everyone say not to use scanf()? What should I use instead? A: scanf() has a number of problems -- see questions 12.17, 12.18a, and 12.19. Also, its %s format has the same problem that gets() has (see question 12.23) -- it's hard to guarantee that the receiving buffer won't overflow. More generally, scanf() is designed for relatively structured, formatted input (its name is in fact derived from "scan formatted"). If you pay attention, it will tell you whether it succeeded or failed, but it can tell you only approximately where it failed, and not at all how or why. It's nearly impossible to do decent error recovery with scanf(); usually it's far easier to read entire lines (with fgets() or the like), then interpret them, either using sscanf() or some other techniques. (Functions like strtol(), strtok(), and atoi() are often useful; see also question 13.6.) If you do use any scanf variant, be sure to check the return value to make sure that the expected number of items were found. Also, if you use %s, be sure to guard against buffer overflow. References: K&R2 Sec. 7.4 p. 159. 12.21: How can I tell how much destination buffer space I'll need for an arbitrary sprintf call? How can I avoid overflowing the destination buffer with sprintf()? A: When the format string being used with sprintf() is known and relatively simple, you can sometimes predict a buffer size in an ad-hoc way. If the format consists of one or two %s's, you can count the fixed characters in the format string yourself (or let sizeof count them for you) and add in the result of calling strlen() on the string(s) to be inserted. For integers, the number of characters produced by %d is no more than ((sizeof(int) * CHAR_BIT + 2) / 3 + 1) /* +1 for '-' */ (CHAR_BIT is in <limits.h>), though this computation may be over-conservative. (It computes the number of characters required for a base-8 representation of a number; a base-10 expansion is guaranteed to take as much room or less.) When the format string is more complicated, or is not even known until run time, predicting the buffer size becomes as difficult as reimplementing sprintf(), and correspondingly error-prone (and inadvisable). A last-ditch technique which is sometimes suggested is to use fprintf() to print the same text to a bit bucket or temporary file, and then to look at fprintf's return value or the size of the file (but see question 19.12, and worry about write errors). If there's any chance that the buffer might not be big enough, you won't want to call sprintf() without some guarantee that the buffer will not overflow and overwrite some other part of memory. If the format string is known, you can limit %s expansion by using %.Ns for some N, or %.*s (see also question 12.10). To avoid the overflow problem, you can use a length-limited version of sprintf(), namely snprintf(). It is used like this: snprintf(buf, bufsize, "You typed \"%s\"", answer); snprintf() has been available in several stdio libraries (including GNU and 4.4bsd) for several years. It has finally been standardized in C99. As an extra, added bonus, the C99 snprintf() provides a way to predict the size required for an arbitrary sprintf() call. C99's snprintf() returns the number of characters it would have placed in the buffer, and it may be called with a buffer size of 0. Therefore, the call nch = snprintf(NULL, 0, fmtstring, /* other arguments */ ); predicts the number of characters required for the fully- formatted string. Yet another option is the (nonstandard) asprintf() function, present in various C libraries including bsd's and GNU's, which formats to (and returns a pointer to) a malloc'ed buffer, like this: char *buf; asprintf(&buf, "%d = %s", 42, "forty-two"); /* now buf points to malloc'ed space containing formatted string */ References: C9X Sec. 7.13.6.6. 12.23: Why does everyone say not to use gets()? A: Unlike fgets(), gets() cannot be told the size of the buffer it's to read into, so it cannot be prevented from overflowing that buffer. The Standard fgets() function is a vast improvement over gets(), although it's not perfect, either. (If long lines are a real possibility, their proper handling must be carefully considered.) See question 7.1 for a code fragment illustrating the replacement of gets() with fgets(). References: Rationale Sec. 4.9.7.2; H&S Sec. 15.7 p. 356. 12.24: Why does errno contain ENOTTY after a call to printf()? A: Many implementations of the stdio package adjust their behavior slightly if stdout is a terminal. To make the determination, these implementations perform some operation which happens to fail (with ENOTTY) if stdout is not a terminal. Although the output operation goes on to complete successfully, errno still contains ENOTTY. (Note that it is only meaningful for a program to inspect the contents of errno after an error has been reported; errno is not guaranteed to be 0 otherwise.) References: ISO Sec. 7.1.4, Sec. 7.9.10.3; CT&P Sec. 5.4 p. 73; PCS Sec. 14 p. 254. 12.25: What's the difference between fgetpos/fsetpos and ftell/fseek? What are fgetpos() and fsetpos() good for? A: ftell() and fseek() use type long int to represent offsets (positions) in a file, and may therefore be limited to offsets of about 2 billion (2**31-1). The newer fgetpos() and fsetpos() functions, on the other hand, use a special typedef, fpos_t, to represent the offsets. The type behind this typedef, if chosen appropriately, can represent arbitrarily large offsets, so fgetpos() and fsetpos() can be used with arbitrarily huge files. fgetpos() and fsetpos() also record the state associated with multibyte streams. See also question 1.4. References: K&R2 Sec. B1.6 p. 248; ISO Sec. 7.9.1, Secs. 7.9.9.1,7.9.9.3; H&S Sec. 15.5 p. 252. 12.26a: How can I flush pending input so that a user's typeahead isn't read at the next prompt? Will fflush(stdin) work? A: fflush() is defined only for output streams. Since its definition of "flush" is to complete the writing of buffered characters (not to discard them), discarding unread input would not be an analogous meaning for fflush on input streams. See also question 12.26b. References: ISO Sec. 7.9.5.2; H&S Sec. 15.2. 12.26b: If fflush() won't work, what can I use to flush input? A: It depends on what you're trying to do. If you're trying to get rid of an unread newline or other unexpected input after calling scanf() (see questions 12.18a-12.19), you really need to rewrite or replace the call to scanf() (see question 12.20). Alternatively, you can consume the rest of a partially-read line with a simple code fragment like while((c = getchar()) != '\n' && c != EOF) /* discard */ ; (You may also be able to use the curses flushinp() function.) There is no standard way to discard unread characters from a stdio input stream, nor would such a way necessarily be sufficient, since unread characters can also accumulate in other, OS-level input buffers. If you're trying to actively discard typed-ahead input (perhaps in anticipation of issuing a critical prompt), you'll have to use a system-specific technique; see questions 19.1 and 19.2. References: ISO Sec. 7.9.5.2; H&S Sec. 15.2. 12.27: fopen() is failing for certain pathnames. A: See questions 19.17 and 19.17b. 12.30: I'm trying to update a file in place, by using fopen mode "r+", reading a certain string, and writing back a modified string, but it's not working. A: Be sure to call fseek before you write, both to seek back to the beginning of the string you're trying to overwrite, and because an fseek or fflush is always required between reading and writing in the read/write "+" modes. Also, remember that you can only overwrite characters with the same number of replacement characters, and that overwriting in text mode may truncate the file at that point, and that you may have to preserve line lengths. See also question 19.14. References: ISO Sec. 7.9.5.3. 12.33: How can I redirect stdin or stdout to a file from within a program? A: Use freopen() (but see question 12.34 below). References: ISO Sec. 7.9.5.4; H&S Sec. 15.2. 12.34: Once I've used freopen(), how can I get the original stdout (or stdin) back? A: There isn't a good way. If you need to switch back, the best solution is not to have used freopen() in the first place. Try using your own explicit output (or input) stream variable, which you can reassign at will, while leaving the original stdout (or stdin) undisturbed. It may be possible, in a nonportable way, to save away information about a stream before calling freopen(), such that the original stream can later be restored. One way is to use a system-specific call such as dup() or dup2(), if available. Another is to copy or inspect the contents of the FILE structure, but this is exceedingly nonportable and unreliable. 12.36b: How can I arrange to have output go two places at once, e.g. to the screen and to a file? A: You can't do this directly, but you could write your own printf variant which printed everything twice. Here is a simple example: #include <stdio.h> #include <stdarg.h> void f2printf(FILE *fp1, FILE *fp2, char *fmt, ...) { va_list argp; va_start(argp, fmt); vfprintf(fp1, fmt, argp); va_end(argp); va_start(argp, fmt); vfprintf(fp2, fmt, argp); va_end(argp); } where f2printf() is just like fprintf() except that you give it two file pointers and it prints to both of them. See also question 15.5. 12.38: How can I read a binary data file properly? I'm occasionally seeing 0x0a and 0x0d values getting garbled, and I seem to hit EOF prematurely if the data contains the value 0x1a. A: When you're reading a binary data file, you should specify "rb" mode when calling fopen(), to make sure that text file translations do not occur. Similarly, when writing binary data files, use "wb". Note that the text/binary distinction is made when you open the file: once a file is open, it doesn't matter which I/O calls you use on it. See also question 20.5. References: ISO Sec. 7.9.5.3; H&S Sec. 15.2.1 p. 348. Section 13. Library Functions 13.1: How can I convert numbers to strings (the opposite of atoi)? Is there an itoa() function? A: Just use sprintf(). (Don't worry that sprintf() may be overkill, potentially wasting run time or code space; it works well in practice.) See the examples in the answer to question 7.5a; see also questions 8.6 and 12.21. You can obviously use sprintf() to convert long or floating- point numbers to strings as well (using %ld or %f). References: K&R1 Sec. 3.6 p. 60; K&R2 Sec. 3.6 p. 64. 13.2: Why does strncpy() not always place a '\0' terminator in the destination string? A: strncpy() was first designed to handle a now-obsolete data structure, the fixed-length, not-necessarily-\0-terminated "string." (A related quirk of strncpy's is that it pads short strings with multiple \0's, out to the specified length.) strncpy() is admittedly a bit cumbersome to use in other contexts, since you must often append a '\0' to the destination string by hand. You can get around the problem by using strncat() instead of strncpy(): if the destination string starts out empty (that is, if you do *dest = '\0' first), strncat() does what you probably wanted strncpy() to do. Another possibility is sprintf(dest, "%.*s", n, source) . When arbitrary bytes (as opposed to strings) are being copied, memcpy() is usually a more appropriate function to use than strncpy(). 13.5: Why do some versions of toupper() act strangely if given an upper-case letter? Why does some code call islower() before toupper()? A: Older versions of toupper() and tolower() did not always work correctly on arguments which did not need converting (i.e. on digits or punctuation or letters already of the desired case). In ANSI/ISO Standard C, these functions are guaranteed to work appropriately on all character arguments. References: ISO Sec. 7.3.2; H&S Sec. 12.9 pp. 320-1; PCS p. 182. 13.6: How can I split up a string into whitespace-separated fields? How can I duplicate the process by which main() is handed argc and argv? A: The only Standard function available for this kind of "tokenizing" is strtok(), although it can be tricky to use and it may not do everything you want it to. (For instance, it does not handle quoting.) References: K&R2 Sec. B3 p. 250; ISO Sec. 7.11.5.8; H&S Sec. 13.7 pp. 333-4; PCS p. 178. 13.7: I need some code to do regular expression and wildcard matching. A: Make sure you recognize the difference between classic regular expressions (variants of which are used in such Unix utilities as ed and grep), and filename wildcards (variants of which are used by most operating systems). There are a number of packages available for matching regular expressions. Most packages use a pair of functions, one for "compiling" the regular expression, and one for "executing" it (i.e. matching strings against it). Look for header files named <regex.h> or <regexp.h>, and functions called regcmp/regex, regcomp/regexec, or re_comp/re_exec. (These functions may exist in a separate regexp library.) A popular, freely- redistributable regexp package by Henry Spencer is available from ftp.cs.toronto.edu in pub/regexp.shar.Z or in several other archives. The GNU project has a package called rx. See also question 18.16. Filename wildcard matching (sometimes called "globbing") is done in a variety of ways on different systems. On Unix, wildcards are automatically expanded by the shell before a process is invoked, so programs rarely have to worry about them explicitly. Under MS-DOS compilers, there is often a special object file which can be linked in to a program to expand wildcards while argv is being built. Several systems (including MS-DOS and VMS) provide system services for listing or opening files specified by wildcards. Check your compiler/library documentation. See also questions 19.20 and 20.3. 13.8: I'm trying to sort an array of strings with qsort(), using strcmp() as the comparison function, but it's not working. A: By "array of strings" you probably mean "array of pointers to char." The arguments to qsort's comparison function are pointers to the objects being sorted, in this case, pointers to pointers to char. strcmp(), however, accepts simple pointers to char. Therefore, strcmp() can't be used directly. Write an intermediate comparison function like this: /* compare strings via pointers */ int pstrcmp(const void *p1, const void *p2) { return strcmp(*(char * const *)p1, *(char * const *)p2); } The comparison function's arguments are expressed as "generic pointers," const void *. They are converted back to what they "really are" (pointers to pointers to char) and dereferenced, yielding char *'s which can be passed to strcmp(). (Don't be misled by the discussion in K&R2 Sec. 5.11 pp. 119-20, which is not discussing the Standard library's qsort). References: ISO Sec. 7.10.5.2; H&S Sec. 20.5 p. 419. 13.9: Now I'm trying to sort an array of structures with qsort(). My comparison function takes pointers to structures, but the compiler complains that the function is of the wrong type for qsort(). How can I cast the function pointer to shut off the warning? A: The conversions must be in the comparison function, which must be declared as accepting "generic pointers" (const void *) as discussed in question 13.8 above. The comparison function might look like int mystructcmp(const void *p1, const void *p2) { const struct mystruct *sp1 = p1; const struct mystruct *sp2 = p2; /* now compare sp1->whatever and sp2-> ... */ (The conversions from generic pointers to struct mystruct pointers happen in the initializations sp1 = p1 and sp2 = p2; the compiler performs the conversions implicitly since p1 and p2 are void pointers.) If, on the other hand, you're sorting pointers to structures, you'll need indirection, as in question 13.8: sp1 = *(struct mystruct * const *)p1 . In general, it is a bad idea to insert casts just to "shut the compiler up." Compiler warnings are usually trying to tell you something, and unless you really know what you're doing, you ignore or muzzle them at your peril. See also question 4.9. References: ISO Sec. 7.10.5.2; H&S Sec. 20.5 p. 419. 13.10: How can I sort a linked list? A: Sometimes it's easier to keep the list in order as you build it (or perhaps to use a tree instead). Algorithms like insertion sort and merge sort lend themselves ideally to use with linked lists. If you want to use a standard library function, you can allocate a temporary array of pointers, fill it in with pointers to all your list nodes, call qsort(), and finally rebuild the list pointers based on the sorted array. References: Knuth Sec. 5.2.1 pp. 80-102, Sec. 5.2.4 pp. 159-168; Sedgewick Sec. 8 pp. 98-100, Sec. 12 pp. 163-175. 13.11: How can I sort more data than will fit in memory? A: You want an "external sort," which you can read about in Knuth, Volume 3. The basic idea is to sort the data in chunks (as much as will fit in memory at one time), write each sorted chunk to a temporary file, and then merge the files. Your operating system may provide a general-purpose sort utility, and if so, you can try invoking it from within your program: see questions 19.27 and 19.30. References: Knuth Sec. 5.4 pp. 247-378; Sedgewick Sec. 13 pp. 177-187. 13.12: How can I get the current date or time of day in a C program? A: Just use the time(), ctime(), localtime() and/or strftime() functions. Here is a simple example: #include <stdio.h> #include <time.h> int main() { time_t now; time(&now); printf("It's %s", ctime(&now)); return 0; } If you need control over the format, use strftime(). If you need sub-second resolution, see question 19.37. References: K&R2 Sec. B10 pp. 255-7; ISO Sec. 7.12; H&S Sec. 18. 13.13: I know that the library function localtime() will convert a time_t into a broken-down struct tm, and that ctime() will convert a time_t to a printable string. How can I perform the inverse operations of converting a struct tm or a string into a time_t? A: ANSI C specifies a library function, mktime(), which converts a struct tm to a time_t. Converting a string to a time_t is harder, because of the wide variety of date and time formats which might be encountered. Some systems provide a strptime() function, which is basically the inverse of strftime(). Other popular functions are partime() (widely distributed with the RCS package) and getdate() (and a few others, from the C news distribution). See question 18.16. References: K&R2 Sec. B10 p. 256; ISO Sec. 7.12.2.3; H&S Sec. 18.4 pp. 401-2. 13.14: How can I add N days to a date? How can I find the difference between two dates? A: The ANSI/ISO Standard C mktime() and difftime() functions provide some (limited) support for both problems. mktime() accepts non-normalized dates, so it is straightforward to take a filled-in struct tm, add or subtract from the tm_mday field, and call mktime() to normalize the year, month, and day fields (and incidentally convert to a time_t value). difftime() computes the difference, in seconds, between two time_t values; mktime() can be used to compute time_t values for two dates to be subtracted. However, these solutions are guaranteed to work correctly only for dates in the range which can be represented as time_t's. (For conservatively-sized time_t, that range is often -- but not always -- from 1970 to approximately 2037; note however that there are time_t representations other than as specified by Unix and Posix.) The tm_mday field is an int, so day offsets of more than 32,736 or so may cause overflow. Note also that at daylight saving time changeovers, local days are not 24 hours long (so don't assume that division by 86400 will be exact). Another approach to both problems, which will work over a much wider range of dates, is to use "Julian day numbers". Code for handling Julian day numbers can be found in the Snippets collection (see question 18.15c), the Simtel/Oakland archives (file JULCAL10.ZIP, see question 18.16), and the "Date conversions" article mentioned in the References. See also questions 13.13, 20.31, and 20.32. References: K&R2 Sec. B10 p. 256; ISO Secs. 7.12.2.2,7.12.2.3; H&S Secs. 18.4,18.5 pp. 401-2; David Burki, "Date Conversions". 13.15: I need a random number generator. A: The Standard C library has one: rand(). The implementation on your system may not be perfect, but writing a better one isn't necessarily easy, either. If you do find yourself needing to implement your own random number generator, there is plenty of literature out there; see the References below or the sci.math.num-analysis FAQ list. There are also any number of packages on the net: old standbys are r250, RANLIB, and FSULTRA (see question 18.16), and there is much recent work by Marsaglia, and Matumoto and Nishimura (the "Mersenne Twister"), and some code collected by Don Knuth on his web pages. References: K&R2 Sec. 2.7 p. 46, Sec. 7.8.7 p. 168; ISO Sec. 7.10.2.1; H&S Sec. 17.7 p. 393; PCS Sec. 11 p. 172; Knuth Vol. 2 Chap. 3 pp. 1-177; Park and Miller, "Random Number Generators: Good Ones are Hard to Find". 13.16: How can I get random integers in a certain range? A: The obvious way, rand() % N /* POOR */ (which tries to return numbers from 0 to N-1) is poor, because the low-order bits of many random number generators are distressingly *non*-random. (See question 13.18.) A better method is something like (int)((double)rand() / ((double)RAND_MAX + 1) * N) If you'd rather not use floating point, another method is rand() / (RAND_MAX / N + 1) Both methods obviously require knowing RAND_MAX (which ANSI #defines in <stdlib.h>), and assume that N is much less than RAND_MAX. (Note, by the way, that RAND_MAX is a *constant* telling you what the fixed range of the C library rand() function is. You cannot set RAND_MAX to some other value, and there is no way of requesting that rand() return numbers in some other range.) If you're starting with a random number generator which returns floating-point values between 0 and 1, all you have to do to get integers from 0 to N-1 is multiply the output of that generator by N. References: K&R2 Sec. 7.8.7 p. 168; PCS Sec. 11 p. 172. 13.17: Each time I run my program, I get the same sequence of numbers back from rand(). A: You can call srand() to seed the pseudo-random number generator with a truly random (or at least variable) initial value, such as the time of day. Here is a simple example: #include <stdlib.h> #include <time.h> srand((unsigned int)time((time_t *)NULL)); (Unfortunately, this code isn't perfect -- among other things, the time_t returned by time() might be a floating-point type, hence not portably convertible to unsigned int without the possibility of overflow. See also question 19.37.) Note also that it's rarely useful to call srand() more than once during a run of a program; in particular, don't try calling srand() before each call to rand(), in an attempt to get "really random" numbers. References: K&R2 Sec. 7.8.7 p. 168; ISO Sec. 7.10.2.2; H&S Sec. 17.7 p. 393. 13.18: I need a random true/false value, so I'm just taking rand() % 2, but it's alternating 0, 1, 0, 1, 0... A: Poor pseudorandom number generators (such as the ones unfortunately supplied with some systems) are not very random in the low-order bits. Try using the higher-order bits: see question 13.16. References: Knuth Sec. 3.2.1.1 pp. 12-14. 13.20: How can I generate random numbers with a normal or Gaussian distribution? A: Here is one method, recommended by Knuth and due originally to Marsaglia: #include <stdlib.h> #include <math.h> double gaussrand() { static double V1, V2, S; static int phase = 0; double X; if(phase == 0) { do { double U1 = (double)rand() / RAND_MAX; double U2 = (double)rand() / RAND_MAX; V1 = 2 * U1 - 1; V2 = 2 * U2 - 1; S = V1 * V1 + V2 * V2; } while(S >= 1 || S == 0); X = V1 * sqrt(-2 * log(S) / S); } else X = V2 * sqrt(-2 * log(S) / S); phase = 1 - phase; return X; } See the extended versions of this list (see question 20.40) for other ideas. References: Knuth Sec. 3.4.1 p. 117; Marsaglia and Bray, "A Convenient Method for Generating Normal Variables"; Press et al., _Numerical Recipes in C_ Sec. 7.2 pp. 288-290. 13.25: I keep getting errors due to library functions being undefined, but I'm #including all the right header files. A: In general, a header file contains only external declarations. In some cases (especially if the functions are nonstandard) obtaining the actual *definitions* may require explicitly asking for the correct libraries to be searched when you link the program. (#including the header doesn't do that.) See also questions 10.11, 11.30, 13.26, 14.3, and 19.40. 13.26: I'm still getting errors due to library functions being undefined, even though I'm explicitly requesting the right libraries while linking. A: Many linkers make one pass over the list of object files and libraries you specify, and extract from libraries only those modules which satisfy references which have so far come up as undefined. Therefore, the order in which libraries are listed with respect to object files (and each other) is significant; usually, you want to search the libraries last. (For example, under Unix, put any -l options towards the end of the command line.) See also question 13.28. 13.28: What does it mean when the linker says that _end is undefined? A: That message is a quirk of the old Unix linkers. You get an error about _end being undefined only when other symbols are undefined, too -- fix the others, and the error about _end will disappear. (See also questions 13.25 and 13.26.) 13.29: My compiler is complaining that printf is undefined! How can this be? A: Allegedly, there are C compilers for Microsoft Windows which do not support printf(). It may be possible to convince such a compiler that what you are writing is a "console application" meaning that it will open a "console window" in which printf() is supported. Section 14. Floating Point 14.1: When I set a float variable to, say, 3.1, why is printf printing it as 3.0999999? A: Most computers use base 2 for floating-point numbers as well as for integers. Although 0.1 is a nice, polite-looking fraction in base 10, its base-2 representation is an infinitely-repeating fraction (0.0001100110011...), so exact decimal fractions such as 3.1 cannot be represented exactly in binary. Depending on how carefully your compiler's binary/decimal conversion routines (such as those used by printf) have been written, you may see discrepancies when numbers not exactly representable in base 2 are assigned or read in and then printed (i.e. converted from base 10 to base 2 and back again). See also question 14.6. 14.2: I'm trying to take some square roots, but I'm getting crazy numbers. A: Make sure that you have #included <math.h>, and correctly declared other functions returning double. (Another library function to be careful with is atof(), which is declared in <stdlib.h>.) See also question 14.3 below. References: CT&P Sec. 4.5 pp. 65-6. 14.3: I'm trying to do some simple trig, and I am #including <math.h>, but I keep getting "undefined: sin" compilation errors. A: Make sure you're actually linking with the math library. For instance, due to a longstanding bug in Unix and Linux systems, you usually need to use an explicit -lm flag, at the *end* of the command line, when compiling/linking. See also questions 13.25, 13.26, and 14.2. 14.4a: My floating-point calculations are acting strangely and giving me different answers on different machines. A: First, see question 14.2 above. If the problem isn't that simple, recall that digital computers usually use floating-point formats which provide a close but by no means exact simulation of real number arithmetic. Underflow, cumulative precision loss, and other anomalies are often troublesome. Don't assume that floating-point results will be exact, and especially don't assume that floating-point values can be compared for equality. (Don't throw haphazard "fuzz factors" in, either; see question 14.5.) These problems are no worse for C than they are for any other computer language. Certain aspects of floating-point are usually defined as "however the processor does them" (see also question 11.34), otherwise a compiler for a machine without the "right" model would have to do prohibitively expensive emulations. This article cannot begin to list the pitfalls associated with, and workarounds appropriate for, floating-point work. A good numerical programming text should cover the basics; see also the references below. References: Kernighan and Plauger, _The Elements of Programming Style_ Sec. 6 pp. 115-8; Knuth, Volume 2 chapter 4; David Goldberg, "What Every Computer Scientist Should Know about Floating-Point Arithmetic". 14.5: What's a good way to check for "close enough" floating-point equality? A: Since the absolute accuracy of floating point values varies, by definition, with their magnitude, the best way of comparing two floating point values is to use an accuracy threshold which is relative to the magnitude of the numbers being compared. Rather than double a, b; ... if(a == b) /* WRONG */ use something like #include <math.h> if(fabs(a - b) <= epsilon * fabs(a)) where epsilon is a value chosen to set the degree of "closeness" (and where you know that a will not be zero). References: Knuth Sec. 4.2.2 pp. 217-8. 14.6: How do I round numbers? A: The simplest and most straightforward way is with code like (int)(x + 0.5) This technique won't work properly for negative numbers, though (for which you could use something like (int)(x < 0 ? x - 0.5 : x + 0.5)). 14.7: Why doesn't C have an exponentiation operator? A: Because few processors have an exponentiation instruction. C has a pow() function, declared in <math.h>, although explicit multiplication is usually better for small positive integral exponents. References: ISO Sec. 7.5.5.1; H&S Sec. 17.6 p. 393. 14.8: The predefined constant M_PI seems to be missing from my machine's copy of <math.h>. A: That constant (which is apparently supposed to be the value of pi, accurate to the machine's precision), is not standard. If you need pi, you'll have to define it yourself, or compute it with 4*atan(1.0) or acos(-1.0). References: PCS Sec. 13 p. 237. 14.9: How do I test for IEEE NaN and other special values? A: Many systems with high-quality IEEE floating-point implementations provide facilities (e.g. predefined constants, and functions like isnan(), either as nonstandard extensions in <math.h> or perhaps in <ieee.h> or <nan.h>) to deal with these values cleanly, and work is being done to formally standardize such facilities. A crude but usually effective test for NaN is exemplified by #define isnan(x) ((x) != (x)) although non-IEEE-aware compilers may optimize the test away. C99 provides isnan(), fpclassify(), and several other classification routines. Another possibility is to format the value in question using sprintf(): on many systems it generates strings like "NaN" and "Inf" which you could compare for in a pinch. See also question 19.39. References: C9X Sec. 7.7.3. 14.11: What's a good way to implement complex numbers in C? A: It is straightforward to define a simple structure and some arithmetic functions to manipulate them. C99 supports complex as a standard type. See also questions 2.10 and 14.12. References: C9X Sec. 6.1.2.5, Sec. 7.8. 14.12: I'm looking for some code to do: Fast Fourier Transforms (FFT's) matrix arithmetic (multiplication, inversion, etc.) complex arithmetic A: Ajay Shah has prepared a nice index of free numerical software which has been archived pretty widely; one URL is ftp://ftp.math.psu.edu/pub/FAQ/numcomp-free-c . See also questions 18.9b, 18.13, 18.15c, and 18.16. 14.13: I'm having trouble with a Turbo C program which crashes and says something like "floating point formats not linked." A: Some compilers for small machines, including Turbo C (and Ritchie's original PDP-11 compiler), leave out certain floating point support if it looks like it will not be needed. In particular, the non-floating-point versions of printf() and scanf() save space by not including code to handle %e, %f, and %g. It happens that Borland's heuristics for determining whether the program uses floating point are insufficient, and the programmer must sometimes insert a dummy call to a floating-point library function (such as sqrt(); any will do) to force loading of floating-point support. (See the comp.os.msdos.programmer FAQ list for more information.) Section 15. Variable-Length Argument Lists 15.1: I heard that you have to #include <stdio.h> before calling printf(). Why? A: So that a proper prototype for printf() will be in scope. A compiler may use a different calling sequence for functions which accept variable-length argument lists. (It might do so if calls using variable-length argument lists were less efficient than those using fixed-length.) Therefore, a prototype (indicating, using the ellipsis notation "...", that the argument list is of variable length) must be in scope whenever a varargs function is called, so that the compiler knows to use the varargs calling mechanism. References: ISO Sec. 6.3.2.2, Sec. 7.1.7; Rationale Sec. 3.3.2.2, Sec. 4.1.6; H&S Sec. 9.2.4 pp. 268-9, Sec. 9.6 pp. 275-6. 15.2: How can %f be used for both float and double arguments in printf()? Aren't they different types? A: In the variable-length part of a variable-length argument list, the "default argument promotions" apply: types char and short int are promoted to int, and float is promoted to double. (These are the same promotions that apply to function calls without a prototype in scope, also known as "old style" function calls; see question 11.3.) Therefore, printf's %f format always sees a double. (Similarly, %c always sees an int, as does %hd.) See also questions 12.9 and 12.13. References: ISO Sec. 6.3.2.2; H&S Sec. 6.3.5 p. 177, Sec. 9.4 pp. 272-3. 15.3: I had a frustrating problem which turned out to be caused by the line printf("%d", n); where n was actually a long int. I thought that ANSI function prototypes were supposed to guard against argument type mismatches like this. A: When a function accepts a variable number of arguments, its prototype does not (and cannot) provide any information about the number and types of those variable arguments. Therefore, the usual protections do *not* apply in the variable-length part of variable-length argument lists: the compiler cannot perform implicit conversions or (in general) warn about mismatches. See also questions 5.2, 11.3, 12.9, and 15.2. 15.4: How can I write a function that takes a variable number of arguments? A: Use the facilities of the <stdarg.h> header. Here is a function which concatenates an arbitrary number of strings into malloc'ed memory: #include <stdlib.h> /* for malloc, NULL, size_t */ #include <stdarg.h> /* for va_ stuff */ #include <string.h> /* for strcat et al. */ char *vstrcat(const char *first, ...) { size_t len; char *retbuf; va_list argp; char *p; if(first == NULL) return NULL; len = strlen(first); va_start(argp, first); while((p = va_arg(argp, char *)) != NULL) len += strlen(p); va_end(argp); retbuf = malloc(len + 1); /* +1 for trailing \0 */ if(retbuf == NULL) return NULL; /* error */ (void)strcpy(retbuf, first); va_start(argp, first); /* restart; 2nd scan */ while((p = va_arg(argp, char *)) != NULL) (void)strcat(retbuf, p); va_end(argp); return retbuf; } Usage is something like char *str = vstrcat("Hello, ", "world!", (char *)NULL); Note the cast on the last argument; see questions 5.2 and 15.3. (Also note that the caller must free the returned, malloc'ed storage.) References: K&R2 Sec. 7.3 p. 155, Sec. B7 p. 254; ISO Sec. 7.8; Rationale Sec. 4.8; H&S Sec. 11.4 pp. 296-9; CT&P Sec. A.3 pp. 139-141; PCS Sec. 11 pp. 184-5, Sec. 13 p. 242. 15.5: How can I write a function that takes a format string and a variable number of arguments, like printf(), and passes them to printf() to do most of the work? A: Use vprintf(), vfprintf(), or vsprintf(). Here is an error() function which prints an error message, preceded by the string "error: " and terminated with a newline: #include <stdio.h> #include <stdarg.h> void error(const char *fmt, ...) { va_list argp; fprintf(stderr, "error: "); va_start(argp, fmt); vfprintf(stderr, fmt, argp); va_end(argp); fprintf(stderr, "\n"); } References: K&R2 Sec. 8.3 p. 174, Sec. B1.2 p. 245; ISO Secs. 7.9.6.7,7.9.6.8,7.9.6.9; H&S Sec. 15.12 pp. 379-80; PCS Sec. 11 pp. 186-7. 15.6: How can I write a function analogous to scanf(), that calls scanf() to do most of the work? A: C99 (but *not* any earlier C Standard) supports vscanf(), vfscanf(), and vsscanf(). References: C9X Secs. 7.3.6.12-14. 15.8: How can I discover how many arguments a function was actually called with? A: This information is not available to a portable program. Some old systems provided a nonstandard nargs() function, but its use was always questionable, since it typically returned the number of words passed, not the number of arguments. (Structures, long ints, and floating point values are usually passed as several words.) Any function which takes a variable number of arguments must be able to determine *from the arguments themselves* how many of them there are. printf-like functions do this by looking for formatting specifiers (%d and the like) in the format string (which is why these functions fail badly if the format string does not match the argument list). Another common technique, applicable when the arguments are all of the same type, is to use a sentinel value (often 0, -1, or an appropriately-cast null pointer) at the end of the list (see the execl() and vstrcat() examples in questions 5.2 and 15.4). Finally, if the types are predictable, you can pass an explicit count of the number of variable arguments (although it's usually a nuisance for the caller to supply). References: PCS Sec. 11 pp. 167-8. 15.9: My compiler isn't letting me declare a function int f(...) { } i.e. with no fixed arguments. A: Standard C requires at least one fixed argument, in part so that you can hand it to va_start(). See also question 15.10. References: ISO Sec. 6.5.4, Sec. 6.5.4.3, Sec. 7.8.1.1; H&S Sec. 9.2 p. 263. 15.10: I have a varargs function which accepts a float parameter. Why isn't va_arg(argp, float) working? A: In the variable-length part of variable-length argument lists, the old "default argument promotions" apply: arguments of type float are always promoted (widened) to type double, and types char and short int are promoted to int. Therefore, it is never correct to invoke va_arg(argp, float); instead you should always use va_arg(argp, double). Similarly, use va_arg(argp, int) to retrieve arguments which were originally char, short, or int. (For analogous reasons, the last "fixed" argument, as handed to va_start(), should not be widenable, either.) See also questions 11.3 and 15.2. References: ISO Sec. 6.3.2.2; Rationale Sec. 4.8.1.2; H&S Sec. 11.4 p. 297. 15.11: I can't get va_arg() to pull in an argument of type pointer-to- function. A: The type-rewriting games which the va_arg() macro typically plays are stymied by overly-complicated types such as pointer- to-function. If you use a typedef for the function pointer type, however, all will be well. See also question 1.21. References: ISO Sec. 7.8.1.2; Rationale Sec. 4.8.1.2. 15.12: How can I write a function which takes a variable number of arguments and passes them to some other function (which takes a variable number of arguments)? A: In general, you cannot. Ideally, you should provide a version of that other function which accepts a va_list pointer (analogous to vfprintf(); see question 15.5 above). If the arguments must be passed directly as actual arguments, or if you do not have the option of rewriting the second function to accept a va_list (in other words, if the second, called function must accept a variable number of arguments, not a va_list), no portable solution is possible. (The problem could perhaps be solved by resorting to machine-specific assembly language; see also question 15.13 below.) 15.13: How can I call a function with an argument list built up at run time? A: There is no guaranteed or portable way to do this. If you're curious, ask this list's editor, who has a few wacky ideas you could try... Instead of an actual argument list, you might consider passing an array of generic (void *) pointers. The called function can then step through the array, much like main() might step through argv. (Obviously this works only if you have control over all the called functions.) (See also question 19.36.) Section 16. Strange Problems 16.1b: I'm getting baffling syntax errors which make no sense at all, and it seems like large chunks of my program aren't being compiled. A: Check for unclosed comments, mismatched #if/#ifdef/#ifndef/ #else/#endif directives, and perhaps unclosed quotes; remember to check header files, too. (See also questions 2.18, 10.9, and 11.29.) 16.1c: Why isn't my procedure call working? The compiler seems to skip right over it. A: Does the code look like this? myprocedure; C has only functions, and function calls always require parenthesized argument lists, even if empty. Use myprocedure(); 16.3: This program crashes before it even runs! (When single-stepping with a debugger, it dies before the first statement in main().) A: You probably have one or more very large (kilobyte or more) local arrays. Many systems have fixed-size stacks, and even those which perform dynamic stack allocation automatically (e.g. Unix) can be confused when the stack tries to grow by a huge chunk all at once. It is often better to declare large arrays with static duration (unless of course you need a fresh set with each recursive call, in which case you could dynamically allocate them with malloc(); see also question 1.31). (See also questions 11.12b, 16.4, 16.5, and 18.4.) 16.4: I have a program that seems to run correctly, but it crashes as it's exiting, *after* the last statement in main(). What could be causing this? A: Look for a misdeclared main() (see questions 2.18, 10.9, 11.12b, and 11.14a), or local buffers passed to setbuf() or setvbuf(), or problems in cleanup functions registered by atexit(). See also questions 7.5a and 11.16. References: CT&P Sec. 5.3 pp. 72-3. 16.5: This program runs perfectly on one machine, but I get weird results on another. Stranger still, adding or removing a debugging printout changes the symptoms... A: Lots of things could be going wrong; here are a few of the more common things to check: uninitialized local variables (see also question 7.1) integer overflow, especially on 16-bit machines, especially of an intermediate result when doing things like a * b / c (see also question 3.14) undefined evaluation order (see questions 3.1 through 3.4) omitted declaration of external functions, especially those which return something other than int, or have "narrow" or variable arguments (see questions 1.25, 11.3, 14.2, and 15.1) dereferenced null pointers (see section 5) improper malloc/free use: assuming malloc'ed memory contains 0, assuming freed storage persists, freeing something twice, corrupting the malloc arena (see also questions 7.19 and 7.20) pointer problems in general (see also question 16.8) mismatch between printf() format and arguments, especially trying to print long ints using %d (see question 12.9) trying to allocate more memory than an unsigned int can count, especially on machines with limited memory (see also questions 7.16 and 19.23) array bounds problems, especially of small, temporary buffers, perhaps used for constructing strings with sprintf() (see also questions 7.1 and 12.21) invalid assumptions about the mapping of typedefs, especially size_t floating point problems (see questions 14.1 and 14.4a) anything you thought was a clever exploitation of the way you believe code is generated for your specific system Proper use of function prototypes can catch several of these problems; lint would catch several more. See also questions 16.3, 16.4, and 18.4. 16.6: Why does this code: char *p = "hello, world!"; p[0] = 'H'; crash? A: String literals are not necessarily modifiable, except (in effect) when they are used as array initializers. Try char a[] = "hello, world!"; See also question 1.32. References: ISO Sec. 6.1.4; H&S Sec. 2.7.4 pp. 31-2. 16.8: What do "Segmentation violation", "Bus error", and "General protection fault" mean? A: These generally mean that your program tried to access memory it shouldn't have, invariably as a result of stack corruption or improper pointer use. Likely causes are overflow of local ("automatic," stack-allocated) arrays; inadvertent use of null pointers (see also questions 5.2 and 5.20) or uninitialized, misaligned, or otherwise improperly allocated pointers (see questions 7.1 and 7.2); corruption of the malloc arena (see question 7.19); and mismatched function arguments, especially involving pointers; two possibilities are scanf() (see question 12.12) and fprintf() (make sure it receives its first FILE * argument). See also questions 16.3 and 16.4. Section 17. Style 17.1: What's the best style for code layout in C? A: K&R, while providing the example most often copied, also supply a good excuse for disregarding it: The position of braces is less important, although people hold passionate beliefs. We have chosen one of several popular styles. Pick a style that suits you, then use it consistently. It is more important that the layout chosen be consistent (with itself, and with nearby or common code) than that it be "perfect." If your coding environment (i.e. local custom or company policy) does not suggest a style, and you don't feel like inventing your own, just copy K&R. (The tradeoffs between various indenting and brace placement options can be exhaustively and minutely examined, but don't warrant repetition here. See also the Indian Hill Style Guide.) The elusive quality of "good style" involves much more than mere code layout details; don't spend time on formatting to the exclusion of more substantive code quality issues. See also question 10.6. References: K&R1 Sec. 1.2 p. 10; K&R2 Sec. 1.2 p. 10. 17.3: Here's a neat trick for checking whether two strings are equal: if(!strcmp(s1, s2)) Is this good style? A: It is not particularly good style, although it is a popular idiom. The test succeeds if the two strings are equal, but the use of ! ("not") suggests that it tests for inequality. Another option is to use a macro: #define Streq(s1, s2) (strcmp((s1), (s2)) == 0) See also question 17.10. 17.4: Why do some people write if(0 == x) instead of if(x == 0)? A: It's a trick to guard against the common error of writing if(x = 0) If you're in the habit of writing the constant before the ==, the compiler will complain if you accidentally type if(0 = x) Evidently it can be easier for some people to remember to reverse the test than to remember to type the doubled = sign. (Of course, the trick only helps when comparing to a constant.) References: H&S Sec. 7.6.5 pp. 209-10. 17.4b: I've seen function declarations that look like this: extern int func __((int, int)); What are those extra parentheses and underscores for? A: They're part of a trick which allows the prototype part of the function declaration to be turned off for a pre-ANSI compiler. Somewhere else is a conditional definition of the __ macro like this: #ifdef __STDC__ #define __(proto) proto #else #define __(proto) () #endif The extra parentheses in the invocation extern int func __((int, int)); are required so that the entire prototype list (perhaps containing many commas) is treated as the single argument expected by the macro. 17.5: I came across some code that puts a (void) cast before each call to printf(). Why? A: printf() does return a value, though few programs bother to check the return values from each call. Since some compilers (and lint) will warn about discarded return values, an explicit cast to (void) is a way of saying "Yes, I've decided to ignore the return value from this call, but please continue to warn me about other (perhaps inadvertently) ignored return values." It's also common to use void casts on calls to strcpy() and strcat(), since the return value is never surprising. References: K&R2 Sec. A6.7 p. 199; Rationale Sec. 3.3.4; H&S Sec. 6.2.9 p. 172, Sec. 7.13 pp. 229-30. 17.8: What is "Hungarian Notation"? Is it worthwhile? A: Hungarian Notation is a naming convention, invented by Charles Simonyi, which encodes information about a variable's type (and perhaps its intended use) in its name. It is well-loved in some circles and roundly castigated in others. Its chief advantage is that it makes a variable's type or intended use obvious from its name; its chief disadvantage is that type information is not necessarily a worthwhile thing to carry around in the name of a variable. References: Simonyi and Heller, "The Hungarian Revolution" . 17.9: Where can I get the "Indian Hill Style Guide" and other coding standards? A: Various documents are available for anonymous ftp from: Site: File or directory: ftp.cs.washington.edu pub/cstyle.tar.Z (the updated Indian Hill guide) ftp.cs.toronto.edu doc/programming (including Henry Spencer's "10 Commandments for C Programmers") ftp.cs.umd.edu pub/style-guide You may also be interested in the books _The Elements of Programming Style_, _Plum Hall Programming Guidelines_, and _C Style: Standards and Guidelines_; see the Bibliography. See also question 18.9. 17.10: Some people say that goto's are evil and that I should never use them. Isn't that a bit extreme? A: Programming style, like writing style, is somewhat of an art and cannot be codified by inflexible rules, although discussions about style often seem to center exclusively around such rules. In the case of the goto statement, it has long been observed that unfettered use of goto's quickly leads to unmaintainable spaghetti code. However, a simple, unthinking ban on the goto statement does not necessarily lead immediately to beautiful programming: an unstructured programmer is just as capable of constructing a Byzantine tangle without using any goto's (perhaps substituting oddly-nested loops and Boolean control variables, instead). Most observations or "rules" about programming style usually work better as guidelines than rules, and work much better if programmers understand what the guidelines are trying to accomplish. Blindly avoiding certain constructs or following rules without understanding them can lead to just as many problems as the rules were supposed to avert. Furthermore, many opinions on programming style are just that: opinions. It's usually futile to get dragged into "style wars," because on certain issues (such as those referred to in questions 5.3, 5.9, 9.2, and 10.7), opponents can never seem to agree, or agree to disagree, or stop arguing. Section 18. Tools and Resources [NOTE: Much of the information in this section is fairly old and may be out-of-date, especially the URLs of various allegedly publicly-available packages. Caveat lector.] 18.1: I need: A: Look for programs (see also question 18.16) named: a C cross-reference cflow, cxref, calls, cscope, generator xscope, or ixfw a C beautifier/pretty- cb, indent, GNU indent, or printer vgrind a revision control or CVS, RCS, or SCCS configuration management tool a C source obfuscator obfus, shroud, or opqcp (shrouder) a "make" dependency makedepend, or try cc -M or generator cpp -M tools to compute code ccount, Metre, lcount, or csize; metrics there is also a package sold by McCabe and Associates a C lines-of-source this can be done very crudely counter with the standard Unix utility wc, and somewhat better with grep -c ";" a C declaration aid check volume 14 of (cdecl) comp.sources.unix (see question 18.16) and K&R2 a prototype generator see question 11.31 a tool to track down see question 18.2 malloc problems a "selective" C see question 10.18 preprocessor language translation see questions 11.31 and 20.26 tools C verifiers (lint) see question 18.7 a C compiler! see question 18.3 (This list of tools is by no means complete; if you know of tools not mentioned, you're welcome to contact this list's maintainer.) Other lists of tools, and discussion about them, can be found in the Usenet newsgroups comp.compilers and comp.software-eng. See also questions 18.3 and 18.16. 18.2: How can I track down these pesky malloc problems? A: A number of debugging packages exist to help track down malloc problems; one popular one is Conor P. Cahill's "dbmalloc", posted to comp.sources.misc in 1992, volume 32. Others are "leak", available in volume 27 of the comp.sources.unix archives; JMalloc.c and JMalloc.h in the "Snippets" collection; MEMDEBUG from ftp.crpht.lu in pub/sources/memdebug ; and Electric Fence. See also question 18.16. A number of commercial debugging tools exist, and can be invaluable in tracking down malloc-related and other stubborn problems: CodeCenter (formerly Saber-C) from Centerline Software (http://www.centerline.com/). Insight (now Insure?), from ParaSoft Corporation (http://www.parasoft.com/). Purify, from Rational Software (http://www- 306.ibm.com/software/rational/, formerly Pure Software, now part of IBM). ZeroFault, from The ZeroFault Group, http://www.zerofault.com/. 18.3: What's a free or cheap C compiler I can use? A: A popular and high-quality free C compiler is the FSF's GNU C compiler, or gcc; see the gcc home page at http://gcc.gnu.org/. An MS-DOS port, djgpp, is also available; see the djgpp home page at http://www.delorie.com/djgpp/. As far as I know, there are versions of gcc for Macs and Windows machines, too. Another popular compiler is lcc, described at http://www.cs.virginia.edu/~lcc-win32/ and http://www.cs.princeton.edu/software/lcc/. A very inexpensive MS-DOS compiler is Power C from Mix Software, 1132 Commerce Drive, Richardson, TX 75801, USA, 214-783-6001. A shareware MS-DOS C compiler is available from ftp.hitech.com.au/hitech/pacific. Registration is optional for non-commercial use. Archives associated with the comp.compilers newsgroup contain a great deal of information about available compilers, interpreters, grammars, etc. (for many languages). The comp.compilers archives at http://compilers.iecc.com/ include an FAQ list and a catalog of free compilers. See also question 18.16. 18.4: I just typed in this program, and it's acting strangely. Can you see anything wrong with it? A: See if you can run lint first (perhaps with the -a, -c, -h, -p or other options). Many C compilers are really only half- compilers, electing not to diagnose numerous source code difficulties which would not actively preclude code generation. See also questions 16.5, 16.8, and 18.7. References: Ian Darwin, _Checking C Programs with lint_ . 18.7: Where can I get an ANSI-compatible lint? A: Products called PC-Lint and FlexeLint are available from Gimpel Software at http://www.gimpel.com/. The Unix System V release 4 lint is ANSI-compatible, and is available separately (bundled with other C tools) from UNIX Support Labs or from System V resellers. Another ANSI-compatible lint (which can also perform higher- level formal verification) is Splint (formerly lclint) at http://lclint.cs.virginia.edu/. In the absence of lint, many modern compilers do attempt to diagnose almost as many problems as lint does. (Many netters recommend gcc -Wall -pedantic .) 18.8: Don't ANSI function prototypes render lint obsolete? A: Not really. First of all, prototypes work only if they are present and correct; an inadvertently incorrect prototype is worse than useless. Secondly, lint checks consistency across multiple source files, and checks data declarations as well as functions. Finally, an independent program like lint will probably always be more scrupulous at enforcing compatible, portable coding practices than will any particular, implementation-specific, feature- and extension-laden compiler. If you do want to use function prototypes instead of lint for cross-file consistency checking, make sure that you set the prototypes up correctly in header files. See questions 1.7 and 10.6. 18.9: Are there any C tutorials or other resources on the net? A: There are several of them: Tom Torfs has a nice tutorial at http://cprog.tomsweb.net . "Notes for C programmers," by Christopher Sawtell, are available by ftp from svr-ftp.eng.cam.ac.uk in misc/sawtell_C.shar and garbo.uwasa.fi in pc/c-lang/c- lesson.zip, or on the web at http://www.fi.uib.no/Fysisk/Teori/KURS/OTHER/newzealand.html . Tim Love's "C for Programmers" is available by ftp from svr- ftp.eng.cam.ac.uk in the misc directory. An html version is at http://www-h.eng.cam.ac.uk/help/tpl/languages/C/teaching_C/ teaching_C.html . The Coronado Enterprises C tutorials are available on Simtel mirrors in pub/msdos/c or on the web at http://www.coronadoenterprises.com/tutorials/c/index.html . There is a web-based course by Steve Holmes at http://www.strath.ac.uk/IT/Docs/Ccourse/ . Martin Brown has C course material on the web at http://www-isis.ecs.soton.ac.uk/computing/c/Welcome.html . On some Unix machines you can try typing "learn c" at the shell prompt (but the lessons may be quite dated). Finally, the author of this FAQ list once taught a couple of C classes and has placed their notes on the web; they are at http://www.eskimo.com/~scs/cclass/cclass.html . [Disclaimer: I have not reviewed many of these tutorials, and I gather that they tend to contain errors. With the exception of the one with my name on it, I can't vouch for any of them. Also, this sort of information rapidly becomes out-of-date; these addresses may not work by the time you read this and try them.] Several of these tutorials, plus a great deal of other information about C, are accessible via the web at http://www.lysator.liu.se/c/index.html . Vinit Carpenter maintains a list of resources for learning C and C++; it is posted to comp.lang.c and comp.lang.c++, and archived where this FAQ list is (see question 20.40), or on the web at http://www.cyberdiem.com/vin/learn.html . See also questions 18.9b, 18.10, and 18.15c. 18.9b: Where can I find some good code examples to study and learn from? A: Here are a couple of links to explore: ftp://garbo.uwasa.fi/pc/c-lang/00index.txt http://www.eskimo.com/~scs/src/ (Beware, though, that there is all too much truly bletcherous code out there, too. Don't "learn" from bad code that it's the best anyone can do; you can do better.) See also questions 18.9, 18.13, 18.15c, and 18.16. 18.10: What's a good book for learning C? What about advanced books and references? A: There are far too many books on C to list here; it's impossible to rate them all. Many people believe that the best one was also the first: _The C Programming Language_, by Kernighan and Ritchie ("K&R," now in its second edition). Opinions vary on K&R's suitability as an initial programming text: many of us did learn C from it, and learned it well; some, however, feel that it is a bit too clinical as a first tutorial for those without much programming background. Several sets of annotations and errata are available on the net, see e.g. http://www.csd.uwo.ca/~jamie/.Refs/.Footnotes/C-annotes.html , http://www.eskimo.com/~scs/cclass/cclass.html , and http://cm.bell-labs.com/cm/cs/cbook/2ediffs.html . Many comp.lang.c regulars recommend _C: A Modern Approach_, by K.N. King. An excellent reference manual is _C: A Reference Manual_, by Samuel P. Harbison and Guy L. Steele, now in its fourth edition. Though not suitable for learning C from scratch, this FAQ list has been published in book form; see the Bibliography. The Association of C and C++ Users (ACCU) maintains a comprehensive set of bibliographic reviews of C/C++ titles at http://www.accu.org/bookreviews/public/. See also question 18.9 above. 18.13: Where can I find the sources of the standard C libraries? A: The GNU project has a complete implementation at http://www.gnu.org/software/libc/. Another source (though not public domain) is _The Standard C Library_, by P.J. Plauger (see the Bibliography). See also questions 18.9b, 18.15c, and 18.16. 18.13b: Is there an on-line C reference manual? A: Two possibilities are http://www.cs.man.ac.uk/standard_c/_index.html and http://www.dinkumware.com/htm_cl/index.html . 18.13c: Where can I get a copy of the ANSI/ISO C Standard? A: See question 11.2. 18.14: I need code to parse and evaluate expressions. A: Two available packages are "defunc," posted to comp.sources.misc in December, 1993 (V41 i32,33), to alt.sources in January, 1994, and available from sunsite.unc.edu in pub/packages/development/libraries/defunc-1.3.tar.Z, and "parse," at lamont.ldgo.columbia.edu. Other options include the S-Lang interpreter, available via anonymous ftp from amy.tch.harvard.edu in pub/slang, and the shareware Cmm ("C- minus-minus" or "C minus the hard stuff"). See also questions 18.16 and 20.6. There is also some parsing/evaluation code in _Software Solutions in C_ (chapter 12, pp. 235-55). 18.15: Where can I get a BNF or YACC grammar for C? A: The definitive grammar is of course the one in the ANSI standard; see question 11.2. Another grammar by Jim Roskind is available at ftp.eskimo.com in u/s/scs/roskind_grammar.Z . A fleshed-out, working instance of the ANSI C90 grammar (due to Jeff Lee) is on ftp.uu.net (see question 18.16) in usenet/net.sources/ansi.c.grammar.Z (including a companion lexer). The FSF's GNU C compiler contains a grammar, as does the appendix to K&R2. The comp.compilers archives contain more information about grammars; see question 18.3. References: K&R1 Sec. A18 pp. 214-219; K&R2 Sec. A13 pp. 234-239; ISO Sec. B.2; H&S pp. 423-435 Appendix B. 18.15b: Does anyone have a C compiler test suite I can use? A: Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one; other packages are Ronald Guilmette's RoadTest(tm) Compiler Test Suites (ftp to netcom.com, pub/rfg/roadtest/announce.txt for information) and Nullstone's Automated Compiler Performance Analysis Tool (see http://www.nullstone.com). The FSF's GNU C (gcc) distribution includes a c-torture-test which checks a number of common problems with compilers. Kahan's paranoia test, found in netlib/paranoia on netlib.att.com, strenuously tests a C implementation's floating point capabilities. 18.15c: Where are some collections of useful code fragments and examples? A: Bob Stout's popular "SNIPPETS" collection is available from ftp.brokersys.com in directory pub/snippets or on the web at http://www.brokersys.com/snippets/ . Lars Wirzenius's "publib" library is available from ftp.funet.fi in directory pub/languages/C/Publib/. See also questions 14.12, 18.9, 18.9b, 18.13, and 18.16. 18.15d: I need code for performing multiple precision arithmetic. A: Some popular packages are the "quad" functions within the BSD Unix libc sources (ftp.uu.net, /systems/unix/bsd-sources/.../ src/lib/libc/quad/*), the GNU MP library "libmp", the MIRACL package (see http://indigo.ie/~mscott/ ), the "calc" program by David Bell and Landon Curt Noll, and the old Unix libmp.a. See also questions 14.12 and 18.16. References: Schumacher, ed., _Software Solutions in C_ Sec. 17 pp. 343-454. 18.16: Where and how can I get copies of all these freely distributable programs? A: As the number of available programs, the number of publicly accessible archive sites, and the number of people trying to access them all grow, this question becomes both easier and more difficult to answer. There are a number of large, public-spirited archive sites out there, such as ftp.uu.net, archive.umich.edu, oak.oakland.edu, sumex-aim.stanford.edu, and wuarchive.wustl.edu, which have huge amounts of software and other information all freely available. For the FSF's GNU project, the central distribution site is prep.ai.mit.edu . These well-known sites tend to be extremely busy and hard to reach, but there are also numerous "mirror" sites which try to spread the load around. On the connected Internet, the traditional way to retrieve files from an archive site is with anonymous ftp. For those without ftp access, there are also several ftp-by-mail servers in operation. More and more, the world-wide web (WWW) is being used to announce, index, and even transfer large data files. There are probably yet newer access methods, too. Those are some of the easy parts of the question to answer. The hard part is in the details -- this article cannot begin to track or list all of the available archive sites or all of the various ways of accessing them. If you have access to the net at all, you probably have access to more up-to-date information about active sites and useful access methods than this FAQ list does. The other easy-and-hard aspect of the question, of course, is simply *finding* which site has what you're looking for. There is a tremendous amount of work going on in this area, and there are probably new indexing services springing up every day. One of the first was "archie", and of course there are a number of high-profile commercial net indexing and searching services such as Alta Vista, Excite, and Yahoo. If you have access to Usenet, see the regular postings in the comp.sources.unix and comp.sources.misc newsgroups, which describe the archiving policies for those groups and how to access their archives, two of which are ftp://gatekeeper.dec.com/pub/usenet/comp.sources.unix/ and ftp://ftp.uu.net/usenet/comp.sources.unix/. The comp.archives newsgroup contains numerous announcements of anonymous ftp availability of various items. Finally, the newsgroup comp.sources.wanted is generally a more appropriate place to post queries for source availability, but check *its* FAQ list, "How to find sources," before posting there. See also questions 14.12, 18.9b, 18.13, and 18.15c. Section 19. System Dependencies 19.1: How can I read a single character from the keyboard without waiting for the RETURN key? How can I stop characters from being echoed on the screen as they're typed? A: Alas, there is no standard or portable way to do these things in C. Concepts such as screens and keyboards are not even mentioned in the Standard, which deals only with simple I/O "streams" of characters. At some level, interactive keyboard input is usually collected and presented to the requesting program a line at a time. This gives the operating system a chance to support input line editing (backspace/delete/rubout, etc.) in a consistent way, without requiring that it be built into every program. Only when the user is satisfied and presses the RETURN key (or equivalent) is the line made available to the calling program. Even if the calling program appears to be reading input a character at a time (with getchar() or the like), the first call blocks until the user has typed an entire line, at which point potentially many characters become available and many character requests (e.g. getchar() calls) are satisfied in quick succession. When a program wants to read each character immediately as it arrives, its course of action will depend on where in the input stream the line collection is happening and how it can be disabled. Under some systems (e.g. MS-DOS, VMS in some modes), a program can use a different or modified set of OS-level input calls to bypass line-at-a-time input processing. Under other systems (e.g. Unix, VMS in other modes), the part of the operating system responsible for serial input (often called the "terminal driver") must be placed in a mode which turns off line-at-a-time processing, after which all calls to the usual input routines (e.g. read(), getchar(), etc.) will return characters immediately. Finally, a few systems (particularly older, batch-oriented mainframes) perform input processing in peripheral processors which cannot be told to do anything other than line-at-a-time input. Therefore, when you need to do character-at-a-time input (or disable keyboard echo, which is an analogous problem), you will have to use a technique specific to the system you're using, assuming it provides one. Since comp.lang.c is oriented towards those topics that the C language has defined support for, you will usually get better answers to other questions by referring to a system-specific newsgroup such as comp.unix.questions or comp.os.msdos.programmer, and to the FAQ lists for these groups. Note that the answers may differ even across variants of otherwise similar systems (e.g. across different variants of Unix); bear in mind when answering system-specific questions that the answer that applies to your system may not apply to everyone else's. However, since these questions are frequently asked here, here are brief answers for some common situations. Some versions of curses have functions called cbreak(), noecho(), and getch() which do what you want. If you're specifically trying to read a short password without echo, you might try getpass(). Under Unix, you can use ioctl() to play with the terminal driver modes (CBREAK or RAW under "classic" versions; ICANON, c_cc[VMIN] and c_cc[VTIME] under System V or POSIX systems; ECHO under all versions), or in a pinch, system() and the stty command. (For more information, see <sgtty.h> and tty(4) under classic versions, <termio.h> and termio(4) under System V, or <termios.h> and termios(4) under POSIX.) Under MS-DOS, use getch() or getche(), or the corresponding BIOS interrupts. Under VMS, try the Screen Management (SMG$) routines, or curses, or issue low-level $QIO's with the IO$_READVBLK function code (and perhaps IO$M_NOECHO, and others) to ask for one character at a time. (It's also possible to set character-at-a-time or "pass through" modes in the VMS terminal driver.) Under other operating systems, you're on your own. (As an aside, note that simply using setbuf() or setvbuf() to set stdin to unbuffered will *not* generally serve to allow character-at-a-time input.) If you're trying to write a portable program, a good approach is to define your own suite of three functions to (1) set the terminal driver or input system into character-at-a-time mode (if necessary), (2) get characters, and (3) return the terminal driver to its initial state when the program is finished. (Ideally, such a set of functions might be part of the C Standard, some day.) The extended versions of this FAQ list (see question 20.40) contain examples of such functions for several popular systems. See also question 19.2. References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1; POSIX Sec. 7. 19.2: How can I find out if there are characters available for reading (and if so, how many)? Alternatively, how can I do a read that will not block if there are no characters available? A: These, too, are entirely operating-system-specific. Some versions of curses have a nodelay() function. Depending on your system, you may also be able to use "nonblocking I/O", or a system call named "select" or "poll", or the FIONREAD ioctl, or c_cc[VTIME], or kbhit(), or rdchk(), or the O_NDELAY option to open() or fcntl(). See also question 19.1. 19.3: How can I display a percentage-done indication that updates itself in place, or show one of those "twirling baton" progress indicators? A: These simple things, at least, you can do fairly portably. Printing the character '\r' will usually give you a carriage return without a line feed, so that you can overwrite the current line. The character '\b' is a backspace, and will usually move the cursor one position to the left. (But remember to call fflush(), too.) References: ISO Sec. 5.2.2. 19.4: How can I clear the screen? How can I print text in color? How can I move the cursor to a specific x, y position? A: Such things depend on the terminal type (or display) you're using. You will have to use a library such as termcap, terminfo, or curses, or some system-specific routines, to perform these operations. On MS-DOS systems, two functions to look for are clrscr() and gotoxy(). For clearing the screen, a halfway portable solution is to print a form-feed character ('\f'), which will cause some displays to clear. Even more portable (albeit even more gunky) might be to print enough newlines to scroll everything away. As a last resort, you could use system() (see question 19.27) to invoke an operating system clear-screen command. References: PCS Sec. 5.1.4 pp. 54-60, Sec. 5.1.5 pp. 60-62. 19.5: How do I read the arrow keys? What about function keys? A: Terminfo, some versions of termcap, and some versions of curses have support for these non-ASCII keys. Typically, a special key sends a multicharacter sequence (usually beginning with ESC, '\033'); parsing these can be tricky. (curses will do the parsing for you, if you call keypad() first.) Under MS-DOS, if you receive a character with value 0 (*not* '0'!) while reading the keyboard, it's a flag indicating that the next character read will be a code indicating a special key. See any DOS programming guide for lists of keyboard scan codes. (Very briefly: the up, left, right, and down arrow keys are 72, 75, 77, and 80, and the function keys are 59 through 68.) References: PCS Sec. 5.1.4 pp. 56-7. 19.6: How do I read the mouse? A: Consult your system documentation, or ask on an appropriate system-specific newsgroup (but check its FAQ list first). Mouse handling is completely different under the X window system, MS- DOS, the Macintosh, and probably every other system. References: PCS Sec. 5.5 pp. 78-80. 19.7: How can I do serial ("comm") port I/O? A: It's system-dependent. Under Unix, you typically open, read, and write a device file in /dev, and use the facilities of the terminal driver to adjust its characteristics. (See also questions 19.1 and 19.2.) Under MS-DOS, you can use the predefined stream stdaux, or a special file like COM1, or some primitive BIOS interrupts, or (if you require decent performance) any number of interrupt-driven serial I/O packages. Several netters recommend the book _C Programmer's Guide to Serial Communications_, by Joe Campbell. 19.8: How can I direct output to the printer? A: Under Unix, either use popen() (see question 19.30) to write to the lp or lpr program, or perhaps open a special file like /dev/lp. Under MS-DOS, write to the (nonstandard) predefined stdio stream stdprn, or open the special files PRN or LPT1. Under some circumstances, another (and perhaps the only) possibility is to use a window manager's screen-capture function, and print the resulting bitmap. References: PCS Sec. 5.3 pp. 72-74. 19.9: How do I send escape sequences to control a terminal or other device? A: If you can figure out how to send characters to the device at all (see question 19.8 above), it's easy enough to send escape sequences. In ASCII, the ESC code is 033 (27 decimal), so code like fprintf(ofd, "\033[J"); sends the sequence ESC [ J . 19.9b: How can I access an I/O board directly? A: In general, there are two ways to do this: use system-specific functions such as "inport" and "outport" (if the device is accessed via an "I/O port"), or use contrived pointer variables to access "memory-mapped I/O" device locations. See question 19.25. 19.10: How can I do graphics? A: Once upon a time, Unix had a fairly nice little set of device- independent plot functions described in plot(3) and plot(5). The GNU libplot library, written by Robert Maier, maintains the same spirit and supports many modern plot devices; see http://www.gnu.org/software/plotutils/plotutils.html . A modern, platform-independent graphics library (which also supports 3D graphics and animation) is OpenGL. Other graphics standards which may be of interest are GKS and PHIGS. If you're programming for MS-DOS, you'll probably want to use libraries conforming to the VESA or BGI standards. If you're trying to talk to a particular plotter, making it draw is usually a matter of sending it the appropriate escape sequences; see also question 19.9. The vendor may supply a C- callable library, or you may be able to find one on the net. If you're programming for a particular window system (Macintosh, X windows, Microsoft Windows), you will use its facilities; see the relevant documentation or newsgroup or FAQ list. References: PCS Sec. 5.4 pp. 75-77. 19.10b: How can I display GIF and JPEG images? A: It will depend on your display environment, which may already provide these functions. Reference JPEG software is at http://www.ijg.org/files/ . 19.11: How can I check whether a file exists? I want to warn the user if a requested input file is missing. A: It's surprisingly difficult to make this determination reliably and portably. Any test you make can be invalidated if the file is created or deleted (i.e. by some other process) between the time you make the test and the time you try to open the file. Three possible test functions are stat(), access(), and fopen(). (To make an approximate test using fopen(), just open for reading and close immediately, although failure does not necessarily indicate nonexistence.) Of these, only fopen() is widely portable, and access(), where it exists, must be used carefully if the program uses the Unix set-UID feature. Rather than trying to predict in advance whether an operation such as opening a file will succeed, it's often better to try it, check the return value, and complain if it fails. (Obviously, this approach won't work if you're trying to avoid overwriting an existing file, unless you've got something like the O_EXCL file opening option available, which does just what you want in this case.) References: PCS Sec. 12 pp. 189,213; POSIX Sec. 5.3.1, Sec. 5.6.2, Sec. 5.6.3. 19.12: How can I find out the size of a file, prior to reading it in? A: If the "size of a file" is the number of characters you'll be able to read from it in C, it can be difficult or impossible to determine this number exactly. Under Unix, the stat() call will give you an exact answer. Several other systems supply a Unix-like stat() which will give an approximate answer. You can fseek() to the end and then use ftell(), or maybe try fstat(), but these tend to have the same sorts of problems: fstat() is not portable, and generally tells you the same thing stat() tells you; ftell() is not guaranteed to return a byte count except for binary files (but, strictly speaking, binary files don't necessarily support fseek to SEEK_END at all). Some systems provide functions called filesize() or filelength(), but these are obviously not portable, either. Are you sure you have to determine the file's size in advance? Since the most accurate way of determining the size of a file as a C program will see it is to open the file and read it, perhaps you can rearrange the code to learn the size as it reads. References: ISO Sec. 7.9.9.4; H&S Sec. 15.5.1; PCS Sec. 12 p. 213; POSIX Sec. 5.6.2. 19.12b: How can I find the modification date and time of a file? A: The Unix and POSIX function is stat(), which several other systems supply as well. (See also question 19.12.) 19.13: How can a file be shortened in-place without completely clearing or rewriting it? A: BSD systems provide ftruncate(), several others supply chsize(), and a few may provide a (possibly undocumented) fcntl option F_FREESP. Under MS-DOS, you can sometimes use write(fd, "", 0). However, there is no portable solution, nor a way to delete blocks at the beginning. See also question 19.14. 19.14: How can I insert or delete a line (or record) in the middle of a file? A: Short of rewriting the file, you probably can't. The usual solution is simply to rewrite the file. (Instead of deleting records, you might consider simply marking them as deleted, to avoid rewriting.) Another possibility, of course, is to use a database instead of a flat file. See also questions 12.30 and 19.13. 19.15: How can I recover the file name given an open stream or file descriptor? A: This problem is, in general, insoluble. Under Unix, for instance, a scan of the entire disk (perhaps involving special permissions) would theoretically be required, and would fail if the descriptor were connected to a pipe or referred to a deleted file (and could give a misleading answer for a file with multiple links). It is best to remember the names of files yourself as you open them (perhaps with a wrapper function around fopen()). 19.16: How can I delete a file? A: The Standard C Library function is remove(). (This is therefore one of the few questions in this section for which the answer is *not* "It's system-dependent.") On older, pre-ANSI Unix systems, remove() may not exist, in which case you can try unlink(). References: K&R2 Sec. B1.1 p. 242; ISO Sec. 7.9.4.1; H&S Sec. 15.15 p. 382; PCS Sec. 12 pp. 208,220-221; POSIX Sec. 5.5.1, Sec. 8.2.4. 19.16b: How do I copy files? A: Either use system() to invoke your operating system's copy utility (see question 19.27), or open the source and destination files (using fopen() or some lower-level file-opening system call), read characters or blocks of characters from the source file, and write them to the destination file. References: K&R Sec. 1, Sec. 7. 19.17: Why can't I open a file by its explicit path? The call fopen("c:\newdir\file.dat", "r") is failing. A: The file you actually requested -- with the characters \n and \f in its name -- probably doesn't exist, and isn't what you thought you were trying to open. In character constants and string literals, the backslash \ is an escape character, giving special meaning to the character following it. In order for literal backslashes in a pathname to be passed through to fopen() (or any other function) correctly, they have to be doubled, so that the first backslash in each pair quotes the second one: fopen("c:\\newdir\\file.dat", "r") Alternatively, under MS-DOS, it turns out that forward slashes are also accepted as directory separators, so you could use fopen("c:/newdir/file.dat", "r") (Note, by the way, that header file names mentioned in preprocessor #include directives are *not* string literals, so you may not have to worry about backslashes there.) 19.17b: fopen() isn't letting me open files like "$HOME/.profile" and "~/.myrcfile". A: Under Unix, at least, environment variables like $HOME, along with the home-directory notation involving the ~ character, are expanded by the shell, and there's no mechanism to perform these expansions automatically when you call fopen(). 19.17c: How can I suppress the dreaded MS-DOS "Abort, Retry, Ignore?" message? A: Among other things, you need to intercept the DOS Critical Error Interrupt, interrupt 24H. See the comp.os.msdos.programmer FAQ list for more details. 19.18: I'm getting an error, "Too many open files". How can I increase the allowable number of simultaneously open files? A: There are typically at least two resource limitations on the number of simultaneously open files: the number of low-level "file descriptors" or "file handles" available in the operating system, and the number of FILE structures available in the stdio library. Both must be sufficient. Under MS-DOS systems, you can control the number of operating system file handles with a line in CONFIG.SYS. Some compilers come with instructions (and perhaps a source file or two) for increasing the number of stdio FILE structures. 19.20: How can I read a directory in a C program? A: See if you can use the opendir() and readdir() functions, which are part of the POSIX standard and are available on most Unix variants. Implementations also exist for MS-DOS, VMS, and other systems. (MS-DOS also has FINDFIRST and FINDNEXT routines which do essentially the same thing, and MS Windows has FindFirstFile and FindNextFile.) readdir() returns just the file names; if you need more information about the file, try calling stat(). To match filenames to some wildcard pattern, see question 13.7. References: K&R2 Sec. 8.6 pp. 179-184; PCS Sec. 13 pp. 230-1; POSIX Sec. 5.1; Schumacher, ed., _Software Solutions in C_ Sec. 8. 19.22: How can I find out how much memory is available? A: Your operating system may provide a routine which returns this information, but it's quite system-dependent. 19.23: How can I allocate arrays or structures bigger than 64K? A: A reasonable computer ought to give you transparent access to all available memory. If you're not so lucky, you'll either have to rethink your program's use of memory, or use various system-specific techniques. 64K is (still) a pretty big chunk of memory. No matter how much memory your computer has available, it's asking a lot to be able to allocate huge amounts of it contiguously. (The C Standard does not guarantee that single objects can be 32K or larger, or 64K for C99.) Often it's a good idea to use data structures which don't require that all memory be contiguous. For dynamically-allocated multidimensional arrays, you can use pointers to pointers, as illustrated in question 6.16. Instead of a large array of structures, you can use a linked list, or an array of pointers to structures. If you're using a PC-compatible (8086-based) system, and running up against a 64K or 640K limit, consider using "huge" memory model, or expanded or extended memory, or malloc variants such as halloc() or farmalloc(), or a 32-bit "flat" compiler (e.g. djgpp, see question 18.3), or some kind of a DOS extender, or another operating system. References: ISO Sec. 5.2.4.1; C9X Sec. 5.2.4.1. 19.24: What does the error message "DGROUP data allocation exceeds 64K" mean, and what can I do about it? I thought that using large model meant that I could use more than 64K of data! A: Even in large memory models, MS-DOS compilers apparently toss certain data (strings, some initialized global or static variables) into a default data segment, and it's this segment that is overflowing. Either use less global data, or, if you're already limiting yourself to reasonable amounts (and if the problem is due to something like the number of strings), you may be able to coax the compiler into not using the default data segment for so much. Some compilers place only "small" data objects in the default data segment, and give you a way (e.g. the /Gt option under Microsoft compilers) to configure the threshold for "small." 19.25: How can I access memory (a memory-mapped device, or graphics memory) located at a certain address? A: Set a pointer, of the appropriate type, to the right number (using an explicit cast to assure the compiler that you really do intend this nonportable conversion): unsigned int *magicloc = (unsigned int *)0x12345678; Then, *magicloc refers to the location you want. If the location is a memory-mapped I/O register, you will probably also want to use the volatile qualifier. (Under MS-DOS, you may find a macro like MK_FP() handy for working with segments and offsets.) References: K&R1 Sec. A14.4 p. 210; K&R2 Sec. A6.6 p. 199; ISO Sec. 6.3.4; Rationale Sec. 3.3.4; H&S Sec. 6.2.7 pp. 171-2. 19.27: How can I invoke another program (a standalone executable, or an operating system command) from within a C program? A: Use the library function system(), which does exactly that. Note that system's return value is at best the command's exit status (although even that is not guaranteed), and usually has nothing to do with the output of the command. Note also that system() accepts a single string representing the command to be invoked; if you need to build up a complex command line, you can use sprintf(). Depending on your operating system, you may also be able to use system calls such as exec or spawn (or execl, execv, spawnl, spawnv, etc.). See also question 19.30. References: K&R1 Sec. 7.9 p. 157; K&R2 Sec. 7.8.4 p. 167, Sec. B6 p. 253; ISO Sec. 7.10.4.5; H&S Sec. 19.2 p. 407; PCS Sec. 11 p. 179. 19.30: How can I invoke another program or command and trap its output? A: Unix and some other systems provide a popen() function, which sets up a stdio stream on a pipe connected to the process running a command, so that the output can be read (or the input supplied). (Also, remember to call pclose() when you're done.) If you can't use popen(), you may be able to use system(), with the output going to a file which you then open and read. If you're using Unix and popen() isn't sufficient, you can learn about pipe(), dup(), fork(), and exec(). (One thing that probably would *not* work, by the way, would be to use freopen().) References: PCS Sec. 11 p. 169. 19.31: How can my program discover the complete pathname to the executable from which it was invoked? A: argv[0] may contain all or part of the pathname, or it may contain nothing. You may be able to duplicate the command language interpreter's search path logic to locate the executable if the name in argv[0] is present but incomplete. However, there is no guaranteed solution. References: K&R1 Sec. 5.11 p. 111; K&R2 Sec. 5.10 p. 115; ISO Sec. 5.1.2.2.1; H&S Sec. 20.1 p. 416. 19.32: How can I automatically locate a program's configuration files in the same directory as the executable? A: It's hard; see also question 19.31 above. Even if you can figure out a workable way to do it, you might want to consider making the program's auxiliary (library) directory configurable, perhaps with an environment variable. (It's especially important to allow variable placement of a program's configuration files when the program will be used by several people, e.g. on a multiuser system.) 19.33: How can a process change an environment variable in its caller? A: It may or may not be possible to do so at all. Different operating systems implement global name/value functionality similar to the Unix environment in different ways. Whether the "environment" can be usefully altered by a running program, and if so, how, is system-dependent. Under Unix, a process can modify its own environment (some systems provide setenv() or putenv() functions for the purpose), and the modified environment is generally passed on to child processes, but it is *not* propagated back to the parent process. Under MS-DOS, it's possible to manipulate the master copy of the environment, but the required techniques are arcane. (See an MS-DOS FAQ list.) 19.36: How can I read in an object file and jump to locations in it? A: You want a dynamic linker or loader. It may be possible to malloc some space and read in object files, but you have to know an awful lot about object file formats, relocation, etc. Under BSD Unix, you could use system() and ld -A to do the linking for you. Many versions of SunOS and System V have the -ldl library which allows object files to be dynamically loaded. Under VMS, use LIB$FIND_IMAGE_SYMBOL. GNU has a package called "dld". See also question 15.13. 19.37: How can I implement a delay, or time a user's response, with sub-second resolution? A: Unfortunately, there is no portable way. Routines you might look for on your system include clock(), delay(), ftime(), gettimeofday(), msleep(), nap(), napms(), nanosleep(), setitimer(), sleep(), Sleep(), times(), and usleep(). (A function called wait(), however, is at least under Unix *not* what you want.) The select() and poll() calls (if available) can be pressed into service to implement simple delays. On MS-DOS machines, it is possible to reprogram the system timer and timer interrupts. Of these, only clock() is part of the ANSI Standard. The difference between two calls to clock() gives elapsed execution time, and may even have subsecond resolution, if CLOCKS_PER_SEC is greater than 1. However, clock() gives elapsed processor time used by the current program, which on a multitasking system may differ considerably from real time. If you're trying to implement a delay and all you have available is a time-reporting function, you can implement a CPU-intensive busy-wait, but this is only an option on a single-user, single- tasking machine, as it is terribly antisocial to any other processes. Under a multitasking operating system, be sure to use a call which puts your process to sleep for the duration, such as sleep() or select(), or pause() in conjunction with alarm() or setitimer(). For really brief delays, it's tempting to use a do-nothing loop like long int i; for(i = 0; i < 1000000; i++) ; but resist this temptation if at all possible! For one thing, your carefully-calculated delay loops will stop working properly next month when a faster processor comes out. Perhaps worse, a clever compiler may notice that the loop does nothing and optimize it away completely. References: H&S Sec. 18.1 pp. 398-9; PCS Sec. 12 pp. 197-8,215-6; POSIX Sec. 4.5.2. 19.38: How can I trap or ignore keyboard interrupts like control-C? A: The basic step is to call signal(), either as #include <signal.h> signal(SIGINT, SIG_IGN); to ignore the interrupt signal, or as extern void func(int); signal(SIGINT, func); to cause control to transfer to function func() on receipt of an interrupt signal. On a multi-tasking system such as Unix, it's best to use a slightly more involved technique: extern void func(int); if(signal(SIGINT, SIG_IGN) != SIG_IGN) signal(SIGINT, func); The test and extra call ensure that a keyboard interrupt typed in the foreground won't inadvertently interrupt a program running in the background (and it doesn't hurt to code calls to signal() this way on any system). On some systems, keyboard interrupt handling is also a function of the mode of the terminal-input subsystem; see question 19.1. On some systems, checking for keyboard interrupts is only performed when the program is reading input, and keyboard interrupt handling may therefore depend on which input routines are being called (and *whether* any input routines are active at all). On MS-DOS systems, setcbrk() or ctrlbrk() functions may also be involved. References: ISO Secs. 7.7,7.7.1; H&S Sec. 19.6 pp. 411-3; PCS Sec. 12 pp. 210-2; POSIX Secs. 3.3.1,3.3.4. 19.39: How can I handle floating-point exceptions gracefully? A: On many systems, you can define a function matherr() which will be called when there are certain floating-point errors, such as errors in the math routines in <math.h>. You may also be able to use signal() (see question 19.38 above) to catch SIGFPE. See also question 14.9. References: Rationale Sec. 4.5.1. 19.40: How do I... Use sockets? Do networking? Write client/server applications? A: All of these questions are outside of the scope of this list and have much more to do with the networking facilities which you have available than they do with C. Good books on the subject are Douglas Comer's three-volume _Internetworking with TCP/IP_ and W. R. Stevens's _UNIX Network Programming_. There is also plenty of information out on the net itself, including the "Unix Socket FAQ" at http://www.developerweb.net/sock-faq/ , and "Beej's Guide to Network Programming" at http://www.ecst.csuchico.edu/~beej/guide/net/. (One tip: depending on your OS, you may need to explicitly request the -lsocket and -lnsl libraries; see question 13.25.) 19.40b: How do I... Use BIOS calls? Write ISR's? Create TSR's? A: These are very particular to specific systems (PC compatibles running MS-DOS, most likely). You'll get much better information in a specific newsgroup such as comp.os.msdos.programmer or its FAQ list; another excellent resource is Ralf Brown's interrupt list. 19.40c: I'm trying to compile this program, but the compiler is complaining that "union REGS" is undefined, and the linker is complaining that int86() is undefined. A: Those have to do with MS-DOS interrupt programming. They don't exist on other systems. 19.40d: What are "near" and "far" pointers? A: These days, they're pretty much obsolete; they're definitely system-specific. If you really need to know, see a DOS- or Windows-specific programming reference. 19.41: But I can't use all these nonstandard, system-dependent functions, because my program has to be ANSI compatible! A: You're out of luck. Either you misunderstood your requirement, or it's an impossible one to meet. ANSI/ISO Standard C simply does not define ways of doing these things; it is a language standard, not an operating system standard. An international standard which does address many of these issues is POSIX (IEEE 1003.1, ISO/IEC 9945-1), and many operating systems (not just Unix) now have POSIX-compatible programming interfaces. It is possible, and desirable, for *most* of a program to be ANSI-compatible, deferring the system-dependent functionality to a few routines in a few files which are either heavily #ifdeffed or rewritten entirely for each system ported to. Section 20. Miscellaneous 20.1: How can I return multiple values from a function? A: Either pass pointers to several locations which the function can fill in, or have the function return a structure containing the desired values, or (in a pinch) you could theoretically use global variables. See also questions 4.8 and 7.5a. 20.3: How do I access command-line arguments? A: They are pointed to by the argv array with which main() is called. See also questions 8.2, 13.7, and 19.20. References: K&R1 Sec. 5.11 pp. 110-114; K&R2 Sec. 5.10 pp. 114-118; ISO Sec. 5.1.2.2.1; H&S Sec. 20.1 p. 416; PCS Sec. 5.6 pp. 81-2, Sec. 11 p. 159, pp. 339-40 Appendix F; Schumacher, ed., _Software Solutions in C_ Sec. 4 pp. 75-85. 20.5: How can I write data files which can be read on other machines with different word size, byte order, or floating point formats? A: The most portable solution is to use text files (usually ASCII), written with fprintf() and read with fscanf() or the like. (Similar advice also applies to network protocols.) Be skeptical of arguments which imply that text files are too big, or that reading and writing them is too slow. Not only is their efficiency frequently acceptable in practice, but the advantages of being able to interchange them easily between machines, and manipulate them with standard tools, can be overwhelming. If you must use a binary format, you can improve portability, and perhaps take advantage of prewritten I/O libraries, by making use of standardized formats such as Sun's XDR (RFC 1014), OSI's ASN.1 (referenced in CCITT X.409 and ISO 8825 "Basic Encoding Rules"), CDF, netCDF, or HDF. See also questions 2.12 and 12.38. References: PCS Sec. 6 pp. 86, 88. 20.6: If I have a char * variable pointing to the name of a function, how can I call that function? A: The most straightforward thing to do is to maintain a correspondence table of names and function pointers: int one_func(), two_func(); int red_func(), blue_func(); struct { char *name; int (*funcptr)(); } symtab[] = { "one_func", one_func, "two_func", two_func, "red_func", red_func, "blue_func", blue_func, }; Then, search the table for the name, and call via the associated function pointer. See also questions 2.15, 18.14, and 19.36. References: PCS Sec. 11 p. 168. 20.8: How can I implement sets or arrays of bits? A: Use arrays of char or int, with a few macros to access the desired bit at the proper index. Here are some simple macros to use with arrays of char: #include <limits.h> /* for CHAR_BIT */ #define BITMASK(b) (1 << ((b) % CHAR_BIT)) #define BITSLOT(b) ((b) / CHAR_BIT) #define BITSET(a, b) ((a)[BITSLOT(b)] |= BITMASK(b)) #define BITTEST(a, b) ((a)[BITSLOT(b)] & BITMASK(b)) (If you don't have <limits.h>, try using 8 for CHAR_BIT.) References: H&S Sec. 7.6.7 pp. 211-216. 20.9: How can I determine whether a machine's byte order is big-endian or little-endian? A: One way is to use a pointer: int x = 1; if(*(char *)&x == 1) printf("little-endian\n"); else printf("big-endian\n"); It's also possible to use a union. See also questions 10.16 and 20.9b. References: H&S Sec. 6.1.2 pp. 163-4. 20.9b: How do I swap bytes? A: V7 Unix had a swab() function, but it seems to have been forgotten. A problem with explicit byte-swapping code is that you have to decide whether to call it or not; see question 20.9 above. A better solution is to use functions (such as the BSD networking ntohs() et al.) which convert between the known byte order of the data and the (unknown) byte order of the machine, and to arrange for these functions to be no-ops on those machines which already match the desired byte order. If you do have to write your own byte-swapping code, the two obvious approaches are again to use pointers or unions, as in question 20.9. References: PCS Sec. 11 p. 179. 20.10: How can I convert integers to binary or hexadecimal? A: Make sure you really know what you're asking. Integers are stored internally in binary, although for most purposes it is not incorrect to think of them as being in octal, decimal, or hexadecimal, whichever is convenient. The base in which a number is expressed matters only when that number is read in from or written out to the outside world. In source code, a non-decimal base is indicated by a leading 0 or 0x (for octal or hexadecimal, respectively). During I/O, the base of a formatted number is controlled in the printf and scanf family of functions by the choice of format specifier (%d, %o, %x, etc.) and in the strtol() and strtoul() functions by the third argument. If you need to output numeric strings in arbitrary bases, you'll have to supply your own function to do so (it will essentially be the inverse of strtol). During *binary* I/O, however, the base again becomes immaterial. For more information about "binary" I/O, see question 2.11. See also questions 8.6 and 13.1. References: ISO Secs. 7.10.1.5,7.10.1.6. 20.11: Can I use base-2 constants (something like 0b101010)? Is there a printf() format for binary? A: No, on both counts. You can convert base-2 string representations to integers with strtol(). See also question 20.10. 20.12: What is the most efficient way to count the number of bits which are set in an integer? A: Many "bit-fiddling" problems like this one can be sped up and streamlined using lookup tables (but see question 20.13 below). 20.13: What's the best way of making my program efficient? A: By picking good algorithms, implementing them carefully, and making sure that your program isn't doing any extra work. For example, the most microoptimized character-copying loop in the world will be beat by code which avoids having to copy characters at all. When worrying about efficiency, it's important to keep several things in perspective. First of all, although efficiency is an enormously popular topic, it is not always as important as people tend to think it is. Most of the code in most programs is not time-critical. When code is not time-critical, it is usually more important that it be written clearly and portably than that it be written maximally efficiently. (Remember that computers are very, very fast, and that seemingly "inefficient" code may be quite efficiently compilable, and run without apparent delay.) It is notoriously difficult to predict what the "hot spots" in a program will be. When efficiency is a concern, it is important to use profiling software to determine which parts of the program deserve attention. Often, actual computation time is swamped by peripheral tasks such as I/O and memory allocation, which can be sped up by using buffering and caching techniques. Even for code that *is* time-critical, one of the least effective optimization techniques is to fuss with the coding details. Many of the "efficient coding tricks" which are frequently suggested (e.g. substituting shift operators for multiplication by powers of two) are performed automatically by even simpleminded compilers. Heavyhanded optimization attempts can make code so bulky that performance is actually degraded, and are rarely portable (i.e. they may speed things up on one machine but slow them down on another). In any case, tweaking the coding usually results in at best linear performance improvements; the big payoffs are in better algorithms. For more discussion of efficiency tradeoffs, as well as good advice on how to improve efficiency when it is important, see chapter 7 of Kernighan and Plauger's _The Elements of Programming Style_, and Jon Bentley's _Writing Efficient Programs_. 20.14: Are pointers really faster than arrays? How much do function calls slow things down? Is ++i faster than i = i + 1? A: Precise answers to these and many similar questions depend of course on the processor and compiler in use. If you simply must know, you'll have to time test programs carefully. (Often the differences are so slight that hundreds of thousands of iterations are required even to see them. Check the compiler's assembly language output, if available, to see if two purported alternatives aren't compiled identically.) For conventional machines, it is usually faster to march through large arrays with pointers rather than array subscripts, but for some processors the reverse is true. Function calls, though obviously incrementally slower than in- line code, contribute so much to modularity and code clarity that there is rarely good reason to avoid them. Before rearranging expressions such as i = i + 1, remember that you are dealing with a compiler, not a keystroke-programmable calculator. Any decent compiler will generate identical code for ++i, i += 1, and i = i + 1. The reasons for using ++i or i += 1 over i = i + 1 have to do with style, not efficiency. (See also question 3.12b.) 20.15b: People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed, but my compiler can't even replace i/=2 with a shift. A: Was i signed or unsigned? If it was signed, a shift is not equivalent (hint: think about the result if i is negative and odd), so the compiler was correct not to use it. 20.15c: How can I swap two values without using a temporary? A: The standard hoary old assembly language programmer's trick is: a ^= b; b ^= a; a ^= b; But this sort of code has little place in modern, HLL programming. Temporary variables are essentially free, and the idiomatic code using three assignments, namely int t = a; a = b; b = t; is not only clearer to the human reader, it is more likely to be recognized by the compiler and turned into the most-efficient code (e.g. perhaps even using an EXCH instruction). The latter code is obviously also amenable to use with pointers and floating-point values, unlike the XOR trick. See also questions 3.3b and 10.3. 20.17: Is there a way to switch on strings? A: Not directly. Sometimes, it's appropriate to use a separate function to map strings to integer codes, and then switch on those. Otherwise, of course, you can fall back on strcmp() and a conventional if/else chain. See also questions 10.12, 20.18, and 20.29. References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ISO Sec. 6.6.4.2; H&S Sec. 8.7 p. 248. 20.18: Is there a way to have non-constant case labels (i.e. ranges or arbitrary expressions)? A: No. The switch statement was originally designed to be quite simple for the compiler to translate, therefore case labels are limited to single, constant, integral expressions. You *can* attach several case labels to the same statement, which will let you cover a small range if you don't mind listing all cases explicitly. If you want to select on arbitrary ranges or non-constant expressions, you'll have to use an if/else chain. See also question 20.17. References: K&R1 Sec. 3.4 p. 55; K&R2 Sec. 3.4 p. 58; ISO Sec. 6.6.4.2; Rationale Sec. 3.6.4.2; H&S Sec. 8.7 p. 248. 20.19: Are the outer parentheses in return statements really optional? A: Yes. Long ago, in the early days of C, they were required, and just enough people learned C then, and wrote code which is still in circulation, that the notion that they might still be required is widespread. (As it happens, parentheses are optional with the sizeof operator, too, under certain circumstances.) References: K&R1 Sec. A18.3 p. 218; ISO Sec. 6.3.3, Sec. 6.6.6; H&S Sec. 8.9 p. 254. 20.20: Why don't C comments nest? How am I supposed to comment out code containing comments? Are comments legal inside quoted strings? A: C comments don't nest mostly because PL/I's comments, which C's are borrowed from, don't either. Therefore, it is usually better to "comment out" large sections of code, which might contain comments, with #ifdef or #if 0 (but see question 11.19). The character sequences /* and */ are not special within double- quoted strings, and do not therefore introduce comments, because a program (particularly one which is generating C code as output) might want to print them. Note also that // comments have only become legal in C as of C99. References: K&R1 Sec. A2.1 p. 179; K&R2 Sec. A2.2 p. 192; ISO Sec. 6.1.9, Annex F; Rationale Sec. 3.1.9; H&S Sec. 2.2 pp. 18-9; PCS Sec. 10 p. 130. 20.21b: Is C a great language, or what? Where else could you write something like a+++++b ? A: Well, you can't meaningfully write it in C, either. The rule for lexical analysis is that at each point during a straightforward left-to-right scan, the longest possible token is determined, without regard to whether the resulting sequence of tokens makes sense. The fragment in the question is therefore interpreted as a ++ ++ + b and cannot be parsed as a valid expression. References: K&R1 Sec. A2 p. 179; K&R2 Sec. A2.1 p. 192; ISO Sec. 6.1; H&S Sec. 2.3 pp. 19-20. 20.24: Why doesn't C have nested functions? A: It's not trivial to implement nested functions such that they have the proper access to local variables in the containing function(s), so they were deliberately left out of C as a simplification. (gcc does allow them, as an extension.) For many potential uses of nested functions (e.g. qsort comparison functions), an adequate if slightly cumbersome solution is to use an adjacent function with static declaration, communicating if necessary via a few static variables. (A cleaner solution, though unsupported by qsort(), is to pass around a pointer to a structure containing the necessary context.) 20.24b: What is assert() and when would I use it? A: It is a macro, defined in <assert.h>, for testing "assertions". An assertion essentially documents an assumption being made by the programmer, an assumption which, if violated, would indicate a serious programming error. For example, a function which was supposed to be called with a non-null pointer could write assert(p != NULL); A failed assertion terminates the program. Assertions should *not* be used to catch expected errors, such as malloc() or fopen() failures. References: K&R2 Sec. B6 pp. 253-4; ISO Sec. 7.2; H&S Sec. 19.1 p. 406. 20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions from C? (And vice versa?) A: The answer is entirely dependent on the machine and the specific calling sequences of the various compilers in use, and may not be possible at all. Read your compiler documentation very carefully; sometimes there is a "mixed-language programming guide," although the techniques for passing arguments and ensuring correct run-time startup are often arcane. For FORTRAN, more information may be found in FORT.gz by Glenn Geers, available via anonymous ftp from suphys.physics.su.oz.au in the src directory. Burkhard Burow's header file cfortran.h simplifies C/FORTRAN interfacing on many popular machines. It is available via anonymous ftp from zebra.desy.de or at http://www-zeus.desy.de/~burow . In C++, a "C" modifier in an external function declaration indicates that the function is to be called using C calling conventions. References: H&S Sec. 4.9.8 pp. 106-7. 20.26: Does anyone know of a program for converting Pascal or FORTRAN (or LISP, Ada, awk, "Old" C, ...) to C? A: Several freely distributable programs are available: p2c A Pascal to C converter written by Dave Gillespie, posted to comp.sources.unix in March, 1990 (Volume 21); also available by anonymous ftp from csvax.cs.caltech.edu, file pub/p2c-1.20.tar.Z . ptoc Another Pascal to C converter, this one written in Pascal (comp.sources.unix, Volume 10, also patches in Volume 13?). f2c A FORTRAN to C converter jointly developed by people from Bell Labs, Bellcore, and Carnegie Mellon. To find out more about f2c, send the mail message "send index from f2c" to netlib@research.att.com or research!netlib. (It is also available via anonymous ftp on netlib.att.com, in directory netlib/f2c/.) This FAQ list's maintainer also has available a list of a few other translators. See also questions 11.31 and 18.16. 20.27: Is C++ a superset of C? Can I use a C++ compiler to compile C code? A: C++ was derived from C, and is largely based on it, but there are some legal C constructs which are not legal C++. Conversely, ANSI C inherited several features from C++, including prototypes and const, so neither language is really a subset or superset of the other; the two also define the meaning of some common constructs differently. In spite of the differences, many C programs will compile correctly in a C++ environment, and many recent compilers offer both C and C++ compilation modes. (But it's usually a bad idea to compile straight C code as if it were C++; the languages are different enough that you'll generally get poor results.) See also questions 8.9 and 20.20. References: H&S p. xviii, Sec. 1.1.5 p. 6, Sec. 2.8 pp. 36-7, Sec. 4.9 pp. 104-107. 20.28: I need a sort of an "approximate" strcmp routine, for comparing two strings for close, but not necessarily exact, equality. A: Some nice information and algorithms having to do with approximate string matching, as well as a useful bibliography, can be found in Sun Wu and Udi Manber's paper "AGREP -- A Fast Approximate Pattern-Matching Tool." Another approach involves the "soundex" algorithm, which maps similar-sounding words to the same codes. Soundex was designed for discovering similar-sounding names (for telephone directory assistance, as it happens), but it can be pressed into service for processing arbitrary words. References: Knuth Sec. 6 pp. 391-2 Volume 3; Wu and Manber, "AGREP -- A Fast Approximate Pattern-Matching Tool" . 20.29: What is hashing? A: Hashing is the process of mapping strings to integers, usually in a relatively small range. A "hash function" maps a string (or some other data structure) to a bounded number (the "hash bucket") which can more easily be used as an index in an array, or for performing repeated comparisons. (Obviously, a mapping from a potentially huge set of strings to a small set of integers will not be unique. Any algorithm using hashing therefore has to deal with the possibility of "collisions.") Many hashing functions and related algorithms have been developed; a full treatment is beyond the scope of this list. References: K&R2 Sec. 6.6; Knuth Sec. 6.4 pp. 506-549 Volume 3; Sedgewick Sec. 16 pp. 231-244. 20.31: How can I find the day of the week given the date? A: Use mktime() or localtime() (see questions 13.13 and 13.14, but beware of DST adjustments if tm_hour is 0), or Zeller's congruence (see the sci.math FAQ list), or this elegant code by Tomohiko Sakamoto: int dayofweek(int y, int m, int d) /* 0 = Sunday */ { static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4}; y -= m < 3; return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7; } See also questions 13.14 and 20.32. References: ISO Sec. 7.12.2.3. 20.32: Is (year % 4 == 0) an accurate test for leap years? (Was 2000 a leap year?) A: No, it's not accurate (and yes, 2000 was a leap year). The full expression for the present Gregorian calendar is year % 4 == 0 && (year % 100 != 0 || year % 400 == 0) See a good astronomical almanac or other reference for details. (To forestall an eternal debate: references which claim the existence of a 4000-year rule are wrong.) See also question 13.14. 20.34: Here's a good puzzle: how do you write a program which produces its own source code as output? A: It is actually quite difficult to write a self-reproducing program that is truly portable, due particularly to quoting and character set difficulties. Here is a classic example (which ought to be presented on one line, although it will fix itself the first time it's run): char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}"; main(){printf(s,34,s,34);} (This program has a few deficiencies, among other things neglecting to #include <stdio.h>, and assuming that the double- quote character " has the value 34, as it does in ASCII.) Here is an improved version, posted by James Hu: #define q(k)main(){return!puts(#k"\nq("#k")");} q(#define q(k)main(){return!puts(#k"\nq("#k")");}) 20.35: What is "Duff's Device"? A: It's a devastatingly devious way of unrolling a loop, devised by Tom Duff while he was at Lucasfilm. In its "classic" form, it was used to copy bytes, and looked like this: register n = (count + 7) / 8; /* count > 0 assumed */ switch (count % 8) { case 0: do { *to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to = *from++; case 4: *to = *from++; case 3: *to = *from++; case 2: *to = *from++; case 1: *to = *from++; } while (--n > 0); } where count bytes are to be copied from the array pointed to by from to the memory location pointed to by to (which is a memory- mapped device output register, which is why to isn't incremented). It solves the problem of handling the leftover bytes (when count isn't a multiple of 8) by interleaving a switch statement with the loop which copies bytes 8 at a time. (Believe it or not, it *is* legal to have case labels buried within blocks nested in a switch statement like this. In his announcement of the technique to C's developers and the world, Duff noted that C's switch syntax, in particular its "fall through" behavior, had long been controversial, and that "This code forms some sort of argument in that debate, but I'm not sure whether it's for or against.") 20.36: When will the next International Obfuscated C Code Contest (IOCCC) be held? How can I get a copy of the current and previous winning entries? A: The contest schedule varies over time; see http://www.ioccc.org/index.html for current details. Contest winners are usually announced at a Usenix conference, and are posted to the net sometime thereafter. Winning entries from previous years (back to 1984) are archived at ftp.uu.net (see question 18.16) under the directory pub/ioccc/; see also http://www.ioccc.org/index.html . 20.37: What was the entry keyword mentioned in K&R1? A: It was reserved to allow the possibility of having functions with multiple, differently-named entry points, a la FORTRAN. It was not, to anyone's knowledge, ever implemented (nor does anyone remember what sort of syntax might have been imagined for it). It has been withdrawn, and is not a keyword in ANSI C. (See also question 1.12.) References: K&R2 p. 259 Appendix C. 20.38: Where does the name "C" come from, anyway? A: C was derived from Ken Thompson's experimental language B, which was inspired by Martin Richards's BCPL (Basic Combined Programming Language), which was a simplification of CPL (Combined Programming Language, or perhaps Cambridge Programming Language). For a while, there was speculation that C's successor might be named P (the third letter in BCPL) instead of D, but of course the most visible descendant language today is C++. 20.39: How do you pronounce "char"? A: You can pronounce the C keyword "char" in at least three ways: like the English words "char," "care," or "car" (or maybe even "character"); the choice is arbitrary. 20.39b: What do "lvalue" and "rvalue" mean? A: Simply speaking, an "lvalue" is an expression that could appear on the left-hand sign of an assignment; you can also think of it as denoting an object that has a location. (But see question 6.7 concerning arrays.) An "rvalue" is any expression that has a value (and that can therefore appear on the right-hand sign of an assignment). 20.40: Where can I get extra copies of this list? A: An up-to-date copy may be obtained from ftp.eskimo.com in directory u/s/scs/C-faq/. You can also just pull it off the net; it is normally posted to comp.lang.c on the first of each month, with an Expires: line which should keep it around all month. A parallel, abridged version is available (and posted), as is a list of changes accompanying each significantly updated version. The various versions of this list are also posted to the newsgroups comp.answers and news.answers. Several sites archive news.answers postings and other FAQ lists, including this one; two sites are rtfm.mit.edu (directories pub/usenet/news.answers/C-faq/ and pub/usenet/comp.lang.c/) and ftp.uu.net (directory usenet/news.answers/C-faq/). If you don't have ftp access, a mailserver at rtfm.mit.edu can mail you FAQ lists: send a message containing the single word "help" to mail-server@rtfm.mit.edu . See the meta-FAQ list in news.answers for more information. A hypertext (HTML) version of this FAQ list is available on the World-Wide Web; the URL is http://www.eskimo.com/~scs/C-faq/top.html . A comprehensive site which references all Usenet FAQ lists is http://www.faqs.org/faqs/ . An extended version of this FAQ list has been published by Addison-Wesley as _C Programming FAQs: Frequently Asked Questions_ (ISBN 0-201-84519-9). An errata list is at http://www.eskimo.com/~scs/C-faq/book/Errata.html and on ftp.eskimo.com in u/s/scs/ftp/C-faq/book/Errata . Bibliography American National Standards Institute, _American National Standard for Information Systems -- Programming Language -- C_, ANSI X3.159-1989 (see question 11.2). [ANSI] American National Standards Institute, _Rationale for American National Standard for Information Systems -- Programming Language -- C_ (see question 11.2). [Rationale] Jon Bentley, _Writing Efficient Programs_, Prentice-Hall, 1982, ISBN 0-13-970244-X. David Burki, "Date Conversions," _The C Users Journal_, February 1993, pp. 29-34. Ian F. Darwin, _Checking C Programs with lint_, O'Reilly, 1988, ISBN 0-937175-30-7. David Goldberg, "What Every Computer Scientist Should Know about Floating-Point Arithmetic," _ACM Computing Surveys_, Vol. 23 #1, March, 1991, pp. 5-48. Samuel P. Harbison and Guy L. Steele, Jr., _C: A Reference Manual_, Fourth Edition, Prentice-Hall, 1995, ISBN 0-13-326224-3. [There is also a fifth edition: 2002, ISBN 0-13-089592-X.] [H&S] Mark R. Horton, _Portable C Software_, Prentice Hall, 1990, ISBN 0-13-868050-7. [PCS] Institute of Electrical and Electronics Engineers, _Portable Operating System Interface (POSIX) -- Part 1: System Application Program Interface (API) [C Language]_, IEEE Std. 1003.1, ISO/IEC 9945-1. International Organization for Standardization, ISO 9899:1990 (see question 11.2). [ISO] International Organization for Standardization, WG14/N794 Working Draft (see questions 11.1 and 11.2b). [C9X] Brian W. Kernighan and P.J. Plauger, _The Elements of Programming Style_, Second Edition, McGraw-Hill, 1978, ISBN 0-07-034207-5. Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_, Prentice-Hall, 1978, ISBN 0-13-110163-3. [K&R1] Brian W. Kernighan and Dennis M. Ritchie, _The C Programming Language_, Second Edition, Prentice Hall, 1988, ISBN 0-13-110362-8, 0-13-110370-9. (See also question 18.10.) [K&R2] Donald E. Knuth, _The Art of Computer Programming_. Volume 1: _Fundamental Algorithms_, Third Edition, Addison-Wesley, 1997, ISBN 0-201-89683-4. Volume 2: _Seminumerical Algorithms_, Third Edition, 1997, ISBN 0-201-89684-2. Volume 3: _Sorting and Searching_, Second Edition, 1998, ISBN 0-201-89685-0. [Knuth] Andrew Koenig, _C Traps and Pitfalls_, Addison-Wesley, 1989, ISBN 0-201-17928-8. [CT&P] G. Marsaglia and T.A. Bray, "A Convenient Method for Generating Normal Variables," _SIAM Review_, Vol. 6 #3, July, 1964. Stephen K. Park and Keith W. Miller, "Random Number Generators: Good Ones are Hard to Find," _Communications of the ACM_, Vol. 31 #10, October, 1988, pp. 1192-1201 (also technical correspondence August, 1989, pp. 1020-1024, and July, 1993, pp. 108-110). P.J. Plauger, _The Standard C Library_, Prentice Hall, 1992, ISBN 0-13-131509-9. Thomas Plum, _C Programming Guidelines_, Second Edition, Plum Hall, 1989, ISBN 0-911537-07-4. William H. Press, Saul A. Teukolsky, William T. Vetterling, and Brian P. Flannery, _Numerical Recipes in C_, Second Edition, Cambridge University Press, 1992, ISBN 0-521-43108-5. Dale Schumacher, Ed., _Software Solutions in C_, AP Professional, 1994, ISBN 0-12-632360-7. Robert Sedgewick, _Algorithms in C_, Addison-Wesley, 1990, ISBN 0-201-51425-7. (A new edition is being prepared; the first two volumes are ISBN 0-201-31452-5 and 0-201-31663-3.) Charles Simonyi and Martin Heller, "The Hungarian Revolution," _Byte_, August, 1991, pp. 131-138. David Straker, _C Style: Standards and Guidelines_, Prentice Hall, ISBN 0-13-116898-3. Steve Summit, _C Programming FAQs: Frequently Asked Questions_, Addison- Wesley, 1995, ISBN 0-201-84519-9. [The book version of this FAQ list; see also http://www.eskimo.com/~scs/C-faq/book/Errata.html .] Peter van der Linden, _Expert C Programming: Deep C Secrets_, Prentice Hall, 1994, ISBN 0-13-177429-8. Sun Wu and Udi Manber, "AGREP -- A Fast Approximate Pattern-Matching Tool," USENIX Conference Proceedings, Winter, 1992, pp. 153-162. There is another bibliography in the revised Indian Hill style guide (see question 17.9). See also question 18.10. Acknowledgements Thanks to Jamshid Afshar, Lauri Alanko, Michael B. Allen, David Anderson, Jens Andreasen, Tanner Andrews, Sudheer Apte, Joseph Arceneaux, Randall Atkinson, Kaleb Axon, Daniel Barker, Rick Beem, Peter Bennett, Mathias Bergqvist, Wayne Berke, Dan Bernstein, Tanmoy Bhattacharya, John Bickers, Kevin Black, Gary Blaine, Yuan Bo, Mark J. Bobak, Anthony Borla, Dave Boutcher, Alan Bowler, breadbox@muppetlabs.com, Michael Bresnahan, Walter Briscoe, Vincent Broman, Robert T. Brown, Stan Brown, John R. Buchan, Joe Buehler, Kimberley Burchett, Gordon Burditt, Scott Burkett, Eberhard Burr, Burkhard Burow, Conor P. Cahill, D'Arcy J.M. Cain, Christopher Calabrese, Ian Cargill, Vinit Carpenter, Paul Carter, Mike Chambers, Billy Chambless, C. Ron Charlton, Franklin Chen, Jonathan Chen, Raymond Chen, Richard Cheung, Avinash Chopde, Steve Clamage, Ken Corbin, Dann Corbit, Ian Cottam, Russ Cox, Jonathan Coxhead, Lee Crawford, Nick Cropper, Steve Dahmer, Jim Dalsimer, Andrew Daviel, James Davies, John E. Davis, Ken Delong, Norm Diamond, Jamie Dickson, Bob Dinse, dlynes@plenary-software, Colin Dooley, Jeff Dunlop, Ray Dunn, Stephen M. Dunn, Andrew Dunstan, Michael J. Eager, Scott Ehrlich, Arno Eigenwillig, Yoav Eilat, Dave Eisen, Joe English, Bjorn Engsig, David Evans, Andreas Fassl, Clive D.W. Feather, Dominic Feeley, Simao Ferraz, Pete Filandr, Bill Finke Jr., Chris Flatters, Rod Flores, Alexander Forst, Steve Fosdick, Jeff Francis, Ken Fuchs, Tom Gambill, Dave Gillespie, Samuel Goldstein, Willis Gooch, Tim Goodwin, Alasdair Grant, W. Wesley Groleau, Ron Guilmette, Craig Gullixson, Doug Gwyn, Michael Hafner, Zhonglin Han, Darrel Hankerson, Tony Hansen, Douglas Wilhelm Harder, Elliotte Rusty Harold, Joe Harrington, Guy Harris, John Hascall, Adrian Havill, Richard Heathfield, Des Herriott, Ger Hobbelt, Sam Hobbs, Joel Ray Holveck, Jos Horsmeier, Syed Zaeem Hosain, Blair Houghton, Phil Howard, Peter Hryczanek, James C. Hu, Chin Huang, Jason Hughes, David Hurt, Einar Indridason, Vladimir Ivanovic, Jon Jagger, Ke Jin, Kirk Johnson, David Jones, Larry Jones, Morris M. Keesan, Arjan Kenter, Bhaktha Keshavachar, James Kew, Bill Kilgore, Darrell Kindred, Lawrence Kirby, Kin-ichi Kitano, Peter Klausler, John Kleinjans, Andrew Koenig, Thomas Koenig, Adam Kolawa, Jukka Korpela, Przemyslaw Kowalczyk, Ajoy Krishnan T, Anders Kristensen, Jon Krom, Markus Kuhn, Deepak Kulkarni, Yohan Kun, B. Kurtz, Kaz Kylheku, Oliver Laumann, John Lauro, Felix Lee, Mike Lee, Timothy J. Lee, Tony Lee, Marty Leisner, Eric Lemings, Dave Lewis, Don Libes, Brian Liedtke, Philip Lijnzaad, James D. Lin, Keith Lindsay, Yen-Wei Liu, Paul Long, Patrick J. LoPresti, Christopher Lott, Tim Love, Paul Lutus, Mike McCarty, Tim McDaniel, Michael MacFaden, Allen Mcintosh, J. Scott McKellar, Kevin McMahon, Stuart MacMartin, John R. MacMillan, Robert S. Maier, Andrew Main, Bob Makowski, Evan Manning, Barry Margolin, George Marsaglia, George Matas, Brad Mears, Wayne Mery, De Mickey, Rich Miller, Roger Miller, Bill Mitchell, Mark Moraes, Darren Morby, Bernhard Muenzer, David Murphy, Walter Murray, Ralf Muschall, Ken Nakata, Todd Nathan, Taed Nelson, Pedro Zorzenon Neto, Daniel Nielsen, Landon Curt Noll, Tim Norman, Paul Nulsen, David O'Brien, Richard A. O'Keefe, Adam Kolawa, Keith Edward O'hara, James Ojaste, Max Okumoto, Hans Olsson, Thomas Otahal, Lloyd Parkes, Bob Peck, Harry Pehkonen, Andrew Phillips, Christopher Phillips, Francois Pinard, Nick Pitfield, Wayne Pollock, Polver@aol.com, Dan Pop, Don Porges, Claudio Potenza, Lutz Prechelt, Lynn Pye, Ed Price, Kevin D. Quitt, Pat Rankin, Arjun Ray, Eric S. Raymond, Christoph Regli, Peter W. Richards, James Robinson, Greg Roelofs, Eric Roode, Manfred Rosenboom, J.M. Rosenstock, Rick Rowe, Michael Rubenstein, Erkki Ruohtula, John C. Rush, John Rushford, Kadda Sahnine, Tomohiko Sakamoto, Matthew Saltzman, Rich Salz, Chip Salzenberg, Matthew Sams, Paul Sand, DaviD W. Sanderson, Frank Sandy, Christopher Sawtell, Jonas Schlein, Paul Schlyter, Doug Schmidt, Rene Schmit, Russell Schulz, Dean Schulze, Jens Schweikhardt, Chris Sears, Peter Seebach, Gisbert W. Selke, Patricia Shanahan, Girija Shanker, Clinton Sheppard, Aaron Sherman, Raymond Shwake, Nathan Sidwell, Thomas Siegel, Peter da Silva, Andrew Simmons, Joshua Simons, Ross Smith, Thad Smith, Henri Socha, Leslie J. Somos, Eric Sosman, Henry Spencer, David Spuler, Frederic Stark, James Stern, Zalman Stern, Michael Sternberg, Geoff Stevens, Alan Stokes, Bob Stout, Dan Stubbs, Tristan Styles, Richard Sullivan, Steve Sullivan, Melanie Summit, Erik Talvola, Christopher Taylor, Dave Taylor, Clarke Thatcher, Wayne Throop, Chris Torek, Steve Traugott, Brian Trial, Nikos Triantafillis, Ilya Tsindlekht, Andrew Tucker, Goran Uddeborg, Rodrigo Vanegas, Jim Van Zandt, Momchil Velikov, Wietse Venema, Tom Verhoeff, Ed Vielmetti, Larry Virden, Chris Volpe, Mark Warren, Alan Watson, Kurt Watzka, Larry Weiss, Martin Weitzel, Howard West, Tom White, Freek Wiedijk, Stephan Wilms, Tim Wilson, Dik T. Winter, Lars Wirzenius, Dave Wolverton, Mitch Wright, Conway Yee, James Youngman, Ozan S. Yigit, and Zhuo Zang, who have contributed, directly or indirectly, to this article. Thanks to the reviewers of the book-length version: Mark Brader, Vinit Carpenter, Stephen Clamage, Jutta Degener, Doug Gwyn, Karl Heuer, and Joseph Kent. Thanks to Debbie Lafferty and Tom Stone at Addison-Wesley for encouragement, and permission to cross-pollinate this list with new text from the book. Special thanks to Karl Heuer, Jutta Degener, and particularly to Mark Brader, who (to borrow a line from Steve Johnson) have goaded me beyond my inclination, and occasionally beyond my endurance, in relentless pursuit of a better FAQ list. Steve Summit scs@eskimo.com This article is Copyright 1990-2004 by Steve Summit. Content from the book _C Programming FAQs: Frequently Asked Questions_ is made available here by permission of the author and the publisher as a service to the community. It is intended to complement the use of the published text and is protected by international copyright laws. The content is made available here and may be accessed freely for personal use but may not be republished without permission. With the exception of the examples by other, cited authors (i.e. in questions 20.31 and 20.35) the C code in this article is public domain and may be used without restriction.