c faq abridged

8/6/2019 C FAQ Abridged

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From: [email protected] (Steve Summit)Newsgroups: comp.lang.c,comp.lang.c.moderated,comp.answers,news.answersSubject: comp.lang.c Answers (Abridged) to Frequently Asked Questions(FAQ)Followup-To: posterDate: 3 Jul 2004 23:27:44 GMT

Organization: better late than neverLines: 2014Message-ID: Reply-To: [email protected]: Short answers for those who don't care to read the longerexplanationsX-Last-Modified: July 3, 2004X-Archive-Name: C-faq/abridgedX-Version: 4.0X-URL: http://www.eskimo.com/~scs/C-faq/top.html

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[Last modified July 3, 2004 by scs.]

This article is Copyright 1990-2004 by Steve Summit. Content from thebook _C Programming FAQs: Frequently Asked Questions_ is made availablehere by permission of the author and the publisher as a service to thecommunity. It is intended to complement the use of the published textand is protected by international copyright laws. The on-line contentmay be accessed freely for personal use but may not be republishedwithout permission.

This article contains minimal answers to the comp.lang.c frequently-asked questions list. More detailed explanations and references can befound in the long version (see question 20.40 for availability, or ftpto rtfm.mit.edu, or send the mail message "help" to [email protected]). Or, see the web version athttp://www.eskimo.com/~scs/C-faq/top.html , or the book _C ProgrammingFAQs: Frequently Asked Questions_ (Addison-Wesley, 1996, ISBN0-201-84519-9).

Section 1. Declarations and Initializations

1.1: How should I decide which integer type to use?

A: If you might need large values (tens of thousands), use long.Otherwise, if space is very important, use short. Otherwise,use int.

1.4: What should the 64-bit type be on a machine that can support it?

A: C99 specifies long long.


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1.7: What's the best way to declare and define global variables?

A: The best arrangement is to place each definition in somerelevant .c file, with an external declaration in a header file.

1.11: What does extern mean in a function declaration?

A: Nothing, really; the keyword extern is optional here.

1.12: What's the auto keyword good for?

A: Nothing.

1.14: I can't seem to define a linked list node which contains apointer to itself.

A: Structures in C can certainly contain pointers to themselves;

the discussion and example in section 6.5 of K&R make thisclear. Problems arise if an attempt is made to define (and use)a typedef in the midst of such a declaration; avoid this.

1.21: How do I declare an array of N pointers to functions returningpointers to functions returning pointers to char?

A: char *(*(*a[N])())();Using a chain of typedefs, or the cdecl program, makes thesedeclarations easier.

1.25: My compiler is complaining about an invalid redeclaration of afunction, but I only define it once.

A: Calling an undeclared function declares it implicitly asreturning int.

1.25b: What's the right declaration for main()?

A: See questions 11.12a through 11.15.

1.30: What am I allowed to assume about the initial values ofvariables which are not explicitly initialized?

A: Uninitialized variables with "static" duration start out as 0,

as if the programmer had initialized them. Variables with"automatic" duration, and dynamically-allocated memory, startout containing garbage (with the exception of calloc).

1.31: Why can't I initialize a local array with a string?

A: Perhaps you have a pre-ANSI compiler.

1.31b: What's wrong with "char *p = malloc(10);" ?


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A: Function calls are not allowed in initializers for global orstatic variables.

1.32: What is the difference between char a[] = "string"; andchar *p = "string"; ?

A: The first declares an initialized and modifiable array; thesecond declares a pointer initialized to a not-necessarily-modifiable constant string.

1.34: How do I initialize a pointer to a function?

A: Use something like "extern int func(); int (*fp)() = func;" .

Section 2. Structures, Unions, and Enumerations

2.1: What's the difference between struct x1 { ... }; andtypedef struct { ... } x2; ?

A: The first structure is named by a tag, the second by a typedefname.

2.2: Why doesn't "struct x { ... }; x thestruct;" work?

A: C is not C++.

2.3: Can a structure contain a pointer to itself?

A: See question 1.14.

2.4: How can I implement opaque (abstract) data types in C?

A: One good way is to use structure pointers which point tostructure types which are not publicly defined.

2.4b: Is there a good way of simulating OOP-style inheritance in C?

A: There are some clumsy ways, but nothing like C++.

2.6: I came across some code that declared a structure with the lastmember an array of one element, and then did some tricky

allocation to make it act like the array had several elements.Is this legal or portable?

A: An official interpretation has deemed that it is not strictlyconforming with the C Standard.

2.8: Is there a way to compare structures automatically?

A: No.


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2.10: Can I pass constant values to functions which accept structurearguments?

A: In C99 you can use "compound literals".

2.11: How can I read/write structures from/to data files?

A: It is relatively straightforward to use fread and fwrite.

2.12: How can I turn off structure padding?

A: There is no standard method.

2.13: Why does sizeof report a larger size than I expect for astructure type?

A: The alignment of arrays of structures must be preserved.

2.14: How can I determine the byte offset of a field within astructure?

A: ANSI C defines the offsetof() macro in .

2.15: How can I access structure fields by name at run time?

A: Build a table of names and offsets, using the offsetof() macro.

2.18: I have a program which works correctly, but dumps core after itfinishes. Why?

A: Check to see if main() is misdeclared, perhaps because apreceding structure type declaration is missing its trailingsemicolon, causing main() to be declared as returning astructure. See also questions 10.9 and 16.4.

2.20: Can I initialize unions?

A: In the original ANSI C, only the first-named member; in C99,using "designated initializers", yes, any member.

2.22: What's the difference between an enumeration and a set ofpreprocessor #defines?

A: There is little difference. The C Standard states thatenumerations are compatible with integral types.

2.24: Is there an easy way to print enumeration values symbolically?

A: No.


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Section 3. Expressions

3.1: Why doesn't the code "a[i] = i++;" work?

A: The variable i is both modified and separately referenced in thesame expression.

3.2: Under my compiler, the code "int i = 7;printf("%d\n", i++ * i++);" prints 49. Regardless of the orderof evaluation, shouldn't it print 56?

A: The operations implied by the postincrement and postdecrementoperators ++ and -- are performed at some time after theoperand's former values are yielded and before the end of theexpression, but not necessarily immediately after, or beforeother parts of the expression are evaluated.

3.3: What should the code "int i = 3; i = i++;" do?

A: The expression is undefined.

3.3b: Here's a slick expression: "a ^= b ^= a ^= b". It swaps a and bwithout using a temporary.

A: Not portably; its behavior is undefined.

3.4: Don't precedence and parentheses dictate order of evaluation?

A: Operator precedence and explicit parentheses impose only apartial ordering on the evaluation of an expression, which doesnot generally include the order of side effects.

3.5: But what about the && and || operators?

A: There is a special exception for these operators: left-to-rightevaluation is guaranteed.

3.8: What's a "sequence point"?

A: A point (at the end of a full expression, or at the ||, &&, ?:,or comma operators, or just before a function call) at which allside effects are guaranteed to be complete.

3.9: So given a[i] = i++; we don't know which cell of a[] getswritten to, but i does get incremented by one, right?

A: Not necessarily! Once an expression or program becomesundefined, *all* aspects of it become undefined.

3.12a: What's the difference between ++i and i++?

A: ++i adds one to i and "returns" the incremented value; i++


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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 areentirely equivalent when only their side effect is needed.

3.14: Why doesn't the code "int a = 1000, b = 1000;long int c = a * b;" work?

A: You must manually cast one of the operands to (long).

3.16: Can I use ?: on the left-hand side of an assignment expression?

A: No.

Section 4. Pointers

4.2: What's wrong with "char *p; *p = malloc(10);"?

A: The pointer you declared is p, not *p.

4.3: Does *p++ increment p, or what it points to?

A: *p++ increments p. To increment the value pointed to by p, use(*p)++ .

4.5: I want to use a char * pointer to step over some ints. Whydoesn't "((int *)p)++;" work?

A: In C, a cast operator is a conversion operator, and bydefinition it yields an rvalue, which cannot be assigned to, orincremented with ++.

4.8: I have a function which accepts, and is supposed to initialize,a pointer, but the pointer in the caller remains unchanged.

A: The called function probably altered only the passed copy of thepointer.

4.9: Can I use a void ** pointer as a parameter so that a functioncan accept a generic pointer by reference?

A: Not portably.

4.10: I have a function which accepts a pointer to an int. How can Ipass a constant like 5 to it?

A: In C99, you can use a "compound literal". Otherwise, declare a


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temporary variable.

4.11: Does C even have "pass by reference"?

A: Not really, though it can be simulated.

4.12: I've seen different syntax used for calling functions viapointers.

A: The extra parentheses and explicit * are now officiallyoptional, although some older implementations require them.

4.15: How do I convert an int to a char *?

A: See question 13.1, 8.6, or 19.25, depending on what you'retrying to do.

Section 5. Null Pointers

5.1: What is this infamous null pointer, anyway?

A: For each pointer type, there is a special value -- the "nullpointer" -- which is distinguishable from all other pointervalues and which is not the address of any object or function.

5.2: How do I get a null pointer in my programs?

A: A constant 0 in a pointer context is converted into a nullpointer at compile time. A "pointer context" is aninitialization, assignment, or comparison with one side avariable or expression of pointer type, and (in ANSI standard C)a function argument which has a prototype in scope declaring aparameter as being of pointer type. In other contexts (functionarguments without prototypes, or in the variable part ofvariadic function calls) a constant 0 with an appropriateexplicit cast is required.

5.3: Is the abbreviated pointer comparison "if(p)" to test for non-null pointers valid?

A: Yes. The construction "if(p)" works, regardless of the internalrepresentation of null pointers, because the compiler

essentially rewrites it as "if(p != 0)" and goes on to convert 0into the correct null pointer.

5.4: What is NULL and how is it defined?

A: NULL is simply a preprocessor macro, defined as a null pointerconstant, typically 0 or ((void *)0), which is used (as astylistic convention, in preference to unadorned 0's) togenerate null pointers.


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5.5: How should NULL be defined on a machine which uses a nonzero bitpattern as the internal representation of a null pointer?

A: The same as on any other machine: as 0. (The compiler makes thetranslation, upon seeing a 0, not the preprocessor; see also

question 5.4.)

5.6: If NULL were defined as "((char *)0)," wouldn't that makefunction calls which pass an uncast NULL work?

A: Not in the most general case. (A cast might still required totell the compiler which kind of null pointer is required, sinceit may be different from (char *)0.)

5.9: If NULL and 0 are equivalent as null pointer constants, whichshould I use?

A: Either; the distinction is entirely stylistic.

5.10: But wouldn't it be better to use NULL, in case the value of NULLchanges?

A: No. NULL is a constant zero, so a constant zero is equallysufficient.

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 valid, does not buy much.

5.13: This is strange. NULL is guaranteed to be 0, but the nullpointer is not?

A: A "null pointer" is a language concept whose particular internalvalue does not matter. A null pointer is requested in sourcecode with the character "0". "NULL" is a preprocessor macro,which is always #defined as 0 (or ((void *)0)).

5.14: Why is there so much confusion surrounding null pointers?

A: 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 tosuggest that the value could change some day, or on some weirdmachine.

5.15: I'm confused. I just can't understand all this null pointerstuff.

A: A simple rule is, "Always use `0' or `NULL' for null pointers,and always cast them when they are used as arguments in function


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calls."

5.16: Given all the confusion surrounding null pointers, wouldn't itbe easier simply to require them to be represented internally byzeroes?

A: Such a requirement would accomplish little.

5.17: Seriously, have any actual machines really used nonzero nullpointers?

A: Machines manufactured by Prime, Honeywell-Bull, and CDC, as wellas Symbolics Lisp Machines, have done so.

5.20: What does a run-time "null pointer assignment" error mean?

A: It means that you've written, via a null pointer, to an invalidlocation. (See also question 16.8.)

Section 6. Arrays and Pointers

6.1: I had the definition char a[6] in one source file, and inanother I declared extern char *a. Why didn't it work?

A: The declaration extern char *a simply does not match the actualdefinition. Use extern char a[].

6.2: But I heard that char a[] was identical to char *a.

A: Not at all. Arrays are not pointers. A reference like x[3]generates different code depending on whether x is an array or apointer.

6.3: So what is meant by the "equivalence of pointers and arrays" inC?

A: An lvalue of type array-of-T which appears in an expressiondecays into a pointer to its first element; the type of theresultant pointer is pointer-to-T. So for an array a andpointer p, you can say "p = a;" and then p[3] and a[3] willaccess the same element.

6.4: Why are array and pointer declarations interchangeable asfunction formal parameters?

A: It's supposed to be a convenience.

6.7: How can an array be an lvalue, if you can't assign to it?

A: An array is not a "modifiable lvalue."


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6.8: What is the real difference between arrays and pointers?

A: Arrays automatically allocate space which is fixed in size andlocation; pointers are dynamic.

6.9: Someone explained to me that arrays were really just constant

pointers.

A: An array name is "constant" in that it cannot be assigned to,but an array is *not* a pointer.

6.11: I came across some "joke" code containing the "expression"5["abcdef"] . How can this be legal C?

A: Yes, array subscripting is commutative in C. The arraysubscripting operation a[e] is defined as being identical to*((a)+(e)).

6.12: What's the difference between array and &array?

A: The type.

6.13: How do I declare a pointer to an array?

A: Usually, you don't want to. Consider using a pointer to one ofthe array's elements instead.

6.14: How can I set an array's size at run time?

A: It's straightforward to use malloc() and a pointer.

6.15: How can I declare local arrays of a size matching a passed-inarray?

A: If you don't have a C99 compiler, you can't.

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." See the full list for code samples.

6.17: Can I simulate a non-0-based array with a pointer?

A: Not if the pointer points outside of the block of memory it isintended to access.

6.18: My compiler complained when I passed a two-dimensional array toa function expecting a pointer to a pointer.

A: The rule by which arrays decay into pointers is *not* appliedrecursively. An array of arrays (i.e. a two-dimensional array


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in C) decays into a pointer to an array, not a pointer to apointer.

6.19: How do I write functions which accept two-dimensional arrayswhen the width is not known at compile time?

A: It's not always particularly easy.

6.20: How can I use statically- and dynamically-allocatedmultidimensional arrays interchangeably when passing them tofunctions?

A: There is no single perfect method, but see the full list forsome ideas.

6.21: Why doesn't sizeof properly report the size of an array which isa parameter to a function?

A: The sizeof operator reports the size of the pointer parameterwhich the function actually receives.

Section 7. Memory Allocation

7.1: Why doesn't the code "char *answer; gets(answer);" work?

A: The pointer variable answer has not been set to point to anyvalid storage. The simplest way to correct this fragment is touse a local array, instead of a pointer.

7.2: I can't get strcat() to work. I tried "char *s3 =strcat(s1, s2);" but I got strange results.

A: Again, the main problem here is that space for the concatenatedresult is not properly allocated.

7.3: But the man page for strcat() says that it takes two char *'s asarguments. How am I supposed to know to allocate things?

A: In general, when using pointers you *always* have to considermemory allocation, if only to make sure that the compiler isdoing it for you.

7.3b: I just tried the code "char *p; strcpy(p, "abc");" and itworked. Why didn't it crash?

A: You got "lucky".

7.3c: How much memory does a pointer variable allocate?

A: Only enough memory to hold the pointer itself, not any memoryfor the pointer to point to.


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A: Under the segmented architecture of PC compatibles, it can bedifficult to use more than 640K with any degree of transparency.See also question 19.23.

7.19: My program is crashing, apparently somewhere down inside malloc.

A: Make sure you aren't using more memory than you malloc'ed,especially for strings (which need strlen(str) + 1 bytes).

7.20: You can't use dynamically-allocated memory after you free it,can you?

A: No. Some early documentation implied otherwise, but the claimis no longer valid.

7.21: Why isn't a pointer null after calling free()?

A: C's pass-by-value semantics mean that called functions can never

permanently change the values of their arguments.

7.22: When I call malloc() to allocate memory for a local pointer, doI have to explicitly free() it?

A: Yes.

7.23: When I free a dynamically-allocated structure containingpointers, do I also have to free each subsidiary pointer?

A: Yes.

7.24: Must I free allocated memory before the program exits?

A: You shouldn't have to.

7.25: Why doesn't my program's memory usage go down when I freememory?

A: Most implementations of malloc/free do not return freed memoryto the operating system.

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.

7.27: So can I query the malloc package to find out how big anallocated block is?

A: Not portably.

7.30: Is it legal to pass a null pointer as the first argument torealloc()?


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A: ANSI C sanctions this usage, although several earlierimplementations do not support it.

7.31: What's the difference between calloc() and malloc()?

A: calloc() takes two arguments, and initializes the allocatedmemory to all-bits-0.

7.32: What is alloca() and why is its use discouraged?

A: alloca() allocates memory which is automatically freed when thefunction which called alloca() returns. alloca() cannot bewritten portably, is difficult to implement on machines withouta stack, and fails under certain conditions if implementedsimply.

Section 8. Characters and Strings

8.1: Why doesn't "strcat(string, '!');" work?

A: strcat() concatenates *strings*, not characters.

8.2: Why won't the test if(string == "value") correctly comparestring against the value?

A: It's comparing pointers. To compare two strings, use strcmp().

8.3: Why can't I assign strings to character arrays?

A: Strings are arrays, and you can't assign arrays directly. Usestrcpy() instead.

8.6: How can I get the numeric (character set) value corresponding toa character?

A: In C, if you have the character, you have its value.

8.9: Why is sizeof('a') not 1?

A: Character constants in C are of type int.

Section 9. Boolean Expressions and Variables

9.1: What is the right type to use for Boolean values in C?

A: There's no one right answer; see the full list for somediscussion.

9.2: What if a built-in logical or relational operator "returns"


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something other than 1?

A: When a Boolean value is generated by a built-in operator, it isguaranteed to be 1 or 0. (This is *not* true for some libraryroutines such as isalpha.)

9.3: Is if(p), where p is a pointer, valid?

A: Yes. See question 5.3.

Section 10. C Preprocessor

10.2: I've got some cute preprocessor macros that let me write C codethat looks more like Pascal. What do y'all think?

A: Bleah.

10.3: How can I write a generic macro to swap two values?

A: There is no good answer to this question. The best all-aroundsolution is probably to forget about using a macro.

10.4: What's the best way to write a multi-statement macro?

A: #define Func() do {stmt1; stmt2; ... } while(0) /* (no trailing ;)*/

10.6: What are .h files and what should I put in them?

A: Header files (also called ".h files") should generally containcommon declarations and macro, structure, and typedefdefinitions, but not variable or function definitions.

10.7: Is it acceptable for one header file to #include another?

A: It's a question of style, and thus receives considerable debate.

10.8a: What's the difference between #include and #include "" ?

A: Roughly speaking, the syntax is for Standard headers and ""is for project headers.

10.8b: What are the complete rules for header file searching?

A: The exact behavior is implementation-defined; see the full listfor some discussion.

10.9: I'm getting strange syntax errors on the very first declarationin a file, but it looks fine.

A: Perhaps there's a missing semicolon at the end of the last


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declaration in the last header file you're #including.

10.10b: I'm #including the header file for a function, but the linkerkeeps saying it's undefined.


10.11: Where can I get a copy of a missing header file?

A: Contact your vendor, or see question 18.16 or the full list.

10.12: How can I construct preprocessor #if expressions whichcompare

strings?

A: You can't do it directly; try #defining several manifestconstants and implementing conditionals on those.

10.13: Does the sizeof operator work in preprocessor #if directives?

A: No.

10.14: Can I use an #ifdef in a #define line, to define somethingtwo

different ways?

A: No.

10.15: Is there anything like an #ifdef for typedefs?

A: Unfortunately, no.

10.16: How can I use a preprocessor #if expression to detectendianness?

A: You probably can't.

10.18: How can I preprocess some code to remove selected conditionalcompilations, without preprocessing everything?

A: Look for a program called unifdef, rmifdef, or scpp.

10.19: How can I list all of the predefined identifiers?

A: If the compiler documentation is unhelpful, try extractingprintable strings from the compiler or preprocessor executable.

10.20: I have some old code that tries to construct identifiers witha

macro like "#define Paste(a, b) a/**/b", but it doesn't work anymore.


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A: Try the ANSI token-pasting operator ##.

10.22: What does the message "warning: macro replacement within astring literal" mean?


10.23-4: I'm having trouble using macro arguments inside stringliterals, 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'tfigure out a way to do it.

A: Consider writing your own little special-purpose preprocessingtool, instead.

10.26: How can I write a macro which takes a variable number ofarguments?

A: Here is one popular trick. Note that the parentheses aroundprintf's argument list are in the macro call, not thedefinition.

#define DEBUG(args) (printf("DEBUG: "), printf args)

if(n != 0) DEBUG(("n is %d\n", n));

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 to standardize the C language. Theirwork was ratified as ANS X3.159-1989, and has since been adoptedas ISO/IEC 9899:1990, and later amended.

11.2: How can I get a copy of the Standard?

A: Copies are available electronically from ansi.com, from ANSI inNew York, or from Global Engineering Documents in Englewood, CO,

or from any national standards body, or from ISO in Geneva, orrepublished within one or more books. See the unabridged listfor details.

11.2b: Where can I get information about updates to the Standard?

A: See the full list for pointers.

11.3: My ANSI compiler is complaining about prototype mismatches for


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A: No.

11.13: But what about main's third argument, envp?

A: It's a non-standard (though common) extension.

11.14a: I believe that declaring void main() can't fail, since I'mcalling exit() instead of returning.

A: It doesn't matter whether main() returns or not, the problem isthat its caller may not even be able to *call* it correctly.

11.14b: Are there systems where void main() doesn't work?

A: Yes.

11.15: The book I've been using always uses void main().

A: It's wrong.

11.16: Is exit(status) truly equivalent to returning the same statusfrom main()?

A: Yes and no. (See the full list for details.)

11.17: How do I get the ANSI "stringizing" preprocessing operator`#'

to stringize the macro's value instead of its name?

A: You can use a two-step #definition to force a macro to beexpanded as well as stringized.

11.18: What does the message "warning: macro replacement within astring literal" mean?

A: Some pre-ANSI compilers/preprocessors expanded macro parameterseven inside string literals and character constants.

11.19: I'm getting strange syntax errors inside lines I've #ifdeffedout.

A: Under ANSI C, #ifdeffed-out text must still consist of "validpreprocessing tokens." This means that there must be no

newlines inside quotes, and no unterminated comments or quotes(i.e. no single apostrophes).

11.20: What are #pragmas ?

A: The #pragma directive provides a single, well-defined "escapehatch" which can be used for extensions.

11.21: What does "#pragma once" mean?


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A: It is an extension implemented by some preprocessors to helpmake header files idempotent.

11.22: Is char a[3] = "abc"; legal?

A: Yes, in ANSI C.

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.

11.25: What's the difference between memcpy() and memmove()?

A: memmove() offers guaranteed behavior if the source anddestination arguments overlap.

11.26: What should malloc(0) do?

A: The behavior is implementation-defined.

11.27: Why does the ANSI Standard place limits on the length andcase-

significance of external identifiers?

A: The problem is older linkers which cannot be forced (by merewords in a Standard) to upgrade.

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.

11.30: Why are some ANSI/ISO Standard library functions showing upas

undefined, even though I've got an ANSI compiler?

A: Perhaps you don't have ANSI-compatible headers and libraries.

11.31: Does anyone have a tool for converting old-style C programsto

ANSI C, or for automatically generating prototypes?

A: See the full list for details.

11.32: Why won't frobozz-cc, which claims to be ANSI compliant,accept

this code?

A: Are you sure that the code being rejected doesn't rely on somenon-Standard extension?


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11.33: What's the difference between implementation-defined,unspecified, and undefined behavior?

A: If you're writing portable code, ignore the distinctions.Otherwise, see the full list.

11.33b: What does it really mean for a program to be "conforming"?

A: The Standard talks about three kinds of conformance: conformingprograms, strictly conforming programs, and conformingimplementations. (See the full list for definitions.)

11.34: I'm appalled that the ANSI Standard leaves so many issuesundefined.

A: In most of these cases, the Standard is simply codifyingexisting practice.

11.35: I just tried some allegedly-undefined code on an ANSI-conformingcompiler, and got the results I expected.

A: A compiler may do anything it likes when faced with undefinedbehavior, including doing what you expect.

Section 12. Stdio

12.1: What's wrong with the code "char c; while((c = getchar()) !=EOF) ..."?

A: The variable to hold getchar's return value must be an int.

12.1b: How do I enter EOF from the keyboard?

A: Depending on your operating system, usually either control-D orcontrol-Z.

12.2: Why won't the code "while(!feof(infp)) {fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?

A: EOF is only indicated *after* an input routine fails.

12.4: My program's prompts and intermediate output don't always showup on the screen.

A: It's best to use an explicit fflush(stdout) whenever outputshould definitely be visible.

12.5: How can I read one character at a time, without waiting for theRETURN key?


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12.6: How can I print a '%' character with printf?

A: "%%".

12.9: How can printf() use %f for type double, if scanf() requires%lf?

A: C's "default argument promotions" mean that values of type floatare promoted to double.

12.9b: What printf format should I use for a typedef when I don'tknow

the underlying type?

A: Use a cast to convert the value to a known type, then use theprintf format matching that type.

12.10: How can I implement a variable field width with printf?

A: Use printf("%*d", width, x).

12.11: How can I print numbers with commas separating the thousands?

A: There is no standard function (but see ).

12.12: Why doesn't the call scanf("%d", i) work?

A: The arguments you pass to scanf() must always be pointers.

12.12b: Why *does* the call "char s[30]; scanf("%s", s);" work?

A: What scanf() needs is pointers, and arrays are always passed tofunctions as pointers. See question 6.3.

12.13: Why doesn't the code "double d; scanf("%f", &d);" work?

A: Unlike printf(), scanf() uses %lf for double, and %f for float.

12.15: How can I specify a variable width in a scanf() formatstring?

A: You can't.

12.17: When I read numbers from the keyboard with scanf "%d\n", itseems to hang until I type one extra line of input.

A: Try using "%d" instead of "%d\n".

12.18a: I'm reading a number with scanf %d and then a string withgets(), but the compiler seems to be skipping the call to


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gets()!

A: scanf() and gets() do not work well together.

12.19: I'm re-prompting the user if scanf() fails, but sometimes itseems to go into an infinite loop.

A: scanf() tends to "jam" on bad input since it does not discardit.

12.20: Why does everyone say not to use scanf()? What should I useinstead?

A: scanf() has a number of problems. Usually, it's easier to readentire lines and then interpret them.

12.21: How can I tell how much destination buffer space I'll needfor

an arbitrary sprintf call? How can I avoid overflowing thedestination buffer with sprintf()?

A: Use the new snprintf() function, if you can.

12.23: Why does everyone say not to use gets()?

A: It cannot be prevented from overflowing the input buffer.

12.24: Why does errno contain ENOTTY after a call to printf()?

A: Don't worry about it. It is only meaningful for a program toinspect the contents of errno after an error has been reported.

12.25: What's the difference between fgetpos/fsetpos andftell/fseek?

A: fgetpos() and fsetpos() use a special typedef which may allowthem to work with larger files than ftell() and fseek().

12.26a: Will fflush(stdin) flush unread characters from the standardinput stream?

A: No.

12.26b: If fflush() won't work, what can I use to flush input?

A: It depends on what you're trying to do; see the full list fordetails. (But first see question 12.20.)

12.27: fopen() is failing for certain pathnames.

A: See questions 19.17 and 19.17b.


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12.30: I'm trying to update a file in place, by using fopen mode"r+",

but it's not working.

A: Be sure to call fseek between reading and writing.

12.33: How can I redirect stdin or stdout from within a program?

A: Use freopen().

12.34: Once I've used freopen(), how can I get the original streamback?

A: There isn't a good way. Try avoiding freopen.

12.36b: How can I arrange to have output go two places at once?

A: You could write your own printf variant which printed everything

twice. See question 15.5.

12.38: How can I read a binary data file properly?

A: Be sure to specify "rb" mode when calling fopen().

Section 13. Library Functions

13.1: How can I convert numbers to strings?

A: Just use sprintf().

13.2: Why does strncpy() not always write a '\0'?

A: For mildly-interesting historical reasons.

13.5: Why do some versions of toupper() act strangely if given anupper-case letter?

A: Older versions of toupper() and tolower() did not always work asexpected in this regard.

13.6: How can I split up a string into whitespace-separated fields?

A: Try strtok().

13.7: I need some code to do regular expression and wildcard matching.

A: regexp libraries abound; see the full list for details.

13.8: I'm trying to sort an array of strings with qsort(), usingstrcmp() as the comparison function, but it's not working.


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A: Make sure you're actually linking with the math library.

14.4a: My floating-point calculations are acting strangely andgiving

me different answers on different machines.

A: First, see question 14.2 above. If the problem isn't thatsimple, see the full list for a brief explanation, or any goodprogramming book for a better one.

14.5: What's a good way to check for "close enough" floating-pointequality?

A: The best way is to use an accuracy threshold which is relativeto the magnitude of the numbers being compared.

14.6: How do I round numbers?

A: For positive numbers, try (int)(x + 0.5) .

14.7: Where is C's exponentiation operator?

A: Try using the pow() function.

14.8: The predefined constant M_PI seems to be missing from .

A: That constant is not standard.

14.9: How do I test for IEEE NaN and other special values?

A: There is not yet a portable way, but see the full list forideas.

14.11: What's a good way to implement complex numbers in C?

A: It is straightforward to define a simple structure and somearithmetic functions to manipulate them.

14.12: I'm looking for some mathematical library code.

A: See Ajay Shah's index of free numerical software atftp://ftp.math.psu.edu/pub/FAQ/numcomp-free-c .

14.13: I'm having trouble with a Turbo C program which crashes andsays

something like "floating point formats not linked."

A: You may have to insert a dummy call to a floating-point libraryfunction to force loading of floating-point support.

Section 15. Variable-Length Argument Lists


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15.1: I heard that you have to #include before callingprintf(). Why?

A: So that a proper prototype for printf() will be in scope.

15.2: How can %f be used for both float and double arguments inprintf()?

A: In variable-length argument lists, types char and short int arepromoted to int, and float is promoted to double.

15.3: Why don't function prototypes guard against mismatches inprintf's arguments?

A: Function prototypes do not provide any information about thenumber and types of variable arguments.

15.4: How can I write a function that takes a variable number ofarguments?

A: Use the header.

15.5: How can I write a function that takes a format string and avariable number of arguments, like printf(), and passes them toprintf() to do most of the work?

A: Use vprintf(), vfprintf(), or vsprintf().

15.6: How can I write a function analogous to scanf(), that callsscanf() to do most of the work?

A: C99 supports vscanf().

15.8: How can I discover how many arguments a function was actuallycalled with?

A: Any function which takes a variable number of arguments must beable to determine *from the arguments' values* how many of themthere are.

15.9: My compiler isn't letting me declare a function that accepts*only* variable arguments.

A: Standard C requires at least one fixed argument.

15.10: Why isn't "va_arg(argp, float)" working?

A: Because the "default argument promotions" apply in variable-length argument lists, you should always useva_arg(argp, double).


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15.11: I can't get va_arg() to pull in an argument of type pointer-to-

function.

A: Use a typedef.

15.12: How can I write a function which takes a variable number ofarguments and passes them to some other function ?

A: In general, you cannot.

15.13: How can I call a function with an argument list built up atrun

time?

A: You can't.

Section 16. Strange Problems

16.1b: I'm getting baffling syntax errors which make no sense atall,

and it seems like large chunks of my program aren't beingcompiled.

A: Check for unclosed comments or mismatched preprocessingdirectives.

16.1c: Why isn't my procedure call working?

A: Function calls always require parenthesized argument lists.

16.3: This program crashes before it even runs!

A: Look for very large, local arrays.(See also questions 11.12b, 16.4, 16.5, and 18.4.)

16.4: I have a program that seems to run correctly, but then crashesas it's exiting.

A: See the full list for ideas.

16.5: This program runs perfectly on one machine, but I get weird

results on another.

A: See the full list for a brief list of possibilities.

16.6: Why does the code "char *p = "hello, world!"; p[0] = 'H';"crash?

A: String literals are not modifiable, except (in effect) when theyare used as array initializers.


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Section 18. Tools and Resources

18.1: I'm looking for C development tools (cross-reference generators,code beautifiers, etc.).

A: See the full list for a few names.

18.2: How can I track down these pesky malloc problems?

A: See the full list for a list of tools.

18.3: What's a free or cheap C compiler I can use?

A: See the full list for a brief catalog.

18.4: I just typed in this program, and it's acting strangely. Canyou see anything wrong with it?

A: See if you can run lint first.

18.7: Where can I get an ANSI-compatible lint?

A: See the unabridged list for two commercial products.

18.8: Don't ANSI function prototypes render lint obsolete?

A: Not really. A good compiler may match most of lint'sdiagnostics; few provide all.

18.9: Are there any C tutorials or other resources on the net?

A: There are several of them.

18.9b: Where can I find some good code examples to study and learnfrom?

A: See the longer versions of this list for some pointers.

18.10: What's a good book for learning C?

A: There are far too many books on C to list here; the full listcontains a few pointers.

18.13: Where can I find the sources of the standard C libraries?

A: Several possibilites are listed in the full list.

18.13b: Is there an on-line C reference manual?

A: Two possibilities arehttp://www.cs.man.ac.uk/standard_c/_index.html and


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http://www.dinkumware.com/htm_cl/index.html .

18.13c: Where can I get a copy of the ANSI/ISO C Standard?


18.14: I need code to parse and evaluate expressions.

A: Several available packages are mentioned in the full list.

18.15: Where can I get a BNF or YACC grammar for C?

A: See the ANSI Standard, or the unabridged list.

18.15b: Does anyone have a C compiler test suite I can use?

A: See the full list for several sources.

18.15c: Where are some collections of useful code fragments andexamples?

A: See the full list for a few sources.

18.15d: I need code for performing multiple precision arithmetic.

A: See the full list for a few ideas.

18.16: Where and how can I get copies of all these freelydistributable

programs?

A: See the regular postings in the comp.sources.unix andcomp.sources.misc newsgroups, or the full version of this list,for information.

Section 19. System Dependencies

19.1: How can I read a single character from the keyboard withoutwaiting for the RETURN key?

A: Alas, there is no standard or portable way to do this sort ofthing in C.

19.2: How can I find out how many characters are available forreading, or do a non-blocking read?

A: These, too, are entirely operating-system-specific.

19.3: How can I display a percentage-done indication that updatesitself in place, or show one of those "twirling baton" progressindicators?


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A: The character '\r' is a carriage return, and '\b' is abackspace.

19.4: How can I clear the screen, or print text in color, or move thecursor?

A: The only halfway-portable solution is the curses library.

19.5: How do I read the arrow keys? What about function keys?

A: Such things depend on the keyboard, operating system, andlibrary you're using.

19.6: How do I read the mouse?

A: What system are you using?

19.7: How can I do serial ("comm") port I/O?

A: It's system-dependent.

19.8: How can I direct output to the printer?


19.9: How do I send escape sequences to control a terminal or otherdevice?

A: By sending them. ESC is '\033' in ASCII.

19.9b: How can I access an I/O board directly?

A: Use inport() and outport() functions, or memory-mapped I/O (seequestion 19.25).

19.10: How can I do graphics?

A: There is no portable way.

19.10b: How can I display GIF and JPEG images?

A: It depends on your display environment.

19.11: How can I check whether a file exists?

A: You can try the access() or stat() functions. Otherwise, theonly guaranteed and portable way is to try opening the file.

19.12: How can I find out the size of a file, prior to reading itin?


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A: You might be able to get an estimate using stat() or fseek/ftell(but see the full list for caveats).

19.12b: How can I find the modification date of a file?

A: Try stat().

19.13: How can a file be shortened in-place without completelyclearing

or rewriting it?

A: There are various ways to do this, but there is no portablesolution.

19.14: How can I insert or delete a line in the middle of a file?

A: Short of rewriting the file, you probably can't.

19.15: How can I recover the file name given an open filedescriptor?

A: This problem is, in general, insoluble. It is best to rememberthe names of files yourself as you open them

19.16: How can I delete a file?

A: The Standard C Library function is remove().

19.16b: How do I copy files?

A: Open the source and destination files and copy a character orblock at a time, or see question 19.27.

19.17: What's wrong with the call fopen("c:\newdir\file.dat", "r")?

A: You probably need to double those backslashes.

19.17b: fopen() isn't letting me open files like "$HOME/.profile".

A: fopen() doesn't expand environment variables.

19.17c: How can I suppress the "Abort, Retry, Ignore?" message?

A: See the comp.os.msdos.programmer FAQ list.

19.18: How can I increase the allowable number of simultaneouslyopen

files?

A: Check your system documentation.

19.20: How can I read a directory in a C program?


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A: See if you can use the opendir() and readdir() functions.

19.22: How can I find out how much memory is available?

A: Your operating system may provide a routine which returns this

information.

19.23: How can I allocate arrays or structures bigger than 64K?

A: Some operating systems won't let you.

19.24: What does the error message "DGROUP exceeds 64K" mean?

A: It means that you have too much static data.

19.25: How can I access memory located at a certain address?

A: Set a pointer to the absolute address.

19.27: How can I invoke another program from within a C program?

A: Use system().

19.30: How can I invoke another program and trap its output?

A: Unix and some other systems provide a popen() function.

19.31: How can my program discover the complete pathname to theexecutable from which it was invoked?

A: argv[0] may contain all or part of the pathname. You may beable to duplicate the command language interpreter's search pathlogic to locate the executable.

19.32: How can I automatically locate a program's configurationfiles

in the same directory as the executable?

A: It's hard; see also question 19.31 above.

19.33: How can a process change an environment variable in itscaller?

A: If it's possible to do so at all, it's system dependent.

19.36: How can I read in an object file and jump to locations in it?

A: You want a dynamic linker or loader.

19.37: How can I implement a delay, or time a user's response,with sub-second resolution?


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A: Unfortunately, there is no portable way.

19.38: How can I trap or ignore keyboard interrupts like control-C?

A: Use signal().

19.39: How can I handle floating-point exceptions gracefully?

A: Take a look at matherr() and signal(SIGFPE).

19.40: How do I... Use sockets? Do networking? Writeclient/server

applications?

A: These questions have more to do with the networking facilitiesyou have available than they do with C.

19.40b: How do I... Use BIOS calls? Write ISR's? Create TSR's?

A: These are very particular to a particular system.

19.40c: I'm trying to compile a program in which "union REGS" andint86() are undefined.

A: Those have to do with MS-DOS interrupt programming.

19.40d: What are "near" and "far" pointers?

A: Obsolete and unnecessary.

19.41: But I can't use all these nonstandard, system-dependentfunctions, because my program has to be ANSI compatible!

A: That's an impossible requirement. Any real program requires atleast a few services which ANSI doesn't define.

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 thedesired values.

20.3: How do I access command-line arguments?

A: Via main()'s argv parameter.

20.5: How can I write data files which can be read on other machineswith different data formats?


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A: The most portable solution is to use text files.

20.6: How can I call a function, given its name as a string?

A: The most straightforward thing to do is to maintain a

correspondence table of names and function pointers.

20.8: How can I implement sets or arrays of bits?

A: Use arrays of char or int, with a few macros to access thedesired bit at the proper index.

20.9: How can I determine whether a machine's byte order is big-endianor little-endian?

A: The usual tricks involve pointers or unions.

20.9b: How do I swap bytes?

A: You can write code using pointers or unions; see the full listfor details.

20.10: How can I convert integers to binary or hexadecimal?

A: Internally, integers are already in binary. During I/O, you maybe able to select a base.

20.11: Can I use base-2 constants (something like 0b101010)?Is there a printf() format for binary?

A: No, on both counts.

20.12: What is the most efficient way to count the number of bitswhich

are set in an integer?

A: Many "bit-fiddling" problems like this one can be sped up andstreamlined using lookup tables.

20.13: What's the best way of making my program efficient?

A: By picking good algorithms and implementing them carefully.

20.14: Are pointers really faster than arrays? How much do functioncalls slow things down?

A: Precise answers to these and many similar questions depend onthe processor and compiler in use.

20.15b: People claim that optimizing compilers are good, but minecan't


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even replace i/=2 with a shift.

A: Was i signed or unsigned?

20.15c: How can I swap two values without using a temporary?

A: The "clever" trick is a ^= b; b ^= a; a ^= b; see also question3.3b.

20.17: Is there a way to switch on strings?

A: Not directly.

20.18: Is there a way to have non-constant case labels (i.e. rangesor

arbitrary expressions)?

A: No.

20.19: Are the outer parentheses in return statements reallyoptional?

A: Yes.

20.20: Why don't C comments nest? Are they legal inside quotedstrings?

A: C comments don't nest because PL/I's comments don't either. Thecharacter sequences /* and */ are not special within double-quoted strings.

20.21b: What does a+++++b mean ?

A: Nothing. It's interpreted as "a ++ ++ + b", and cannot beparsed.

20.24: Why doesn't C have nested functions?

A: They were deliberately left out of C as a simplification.

20.24b: What is assert()?

A: It is a macro which documents an assumption being made by the

programmer; it terminates the program if the assumption isviolated.

20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP)functions

from C?

A: The answer is entirely dependent on the machine and the specificcalling sequences of the various compilers in use.


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20.26: Does anyone know of a program for converting Pascal orFORTRAN

to C?

A: Several freely distributable programs are available, namely

ptoc, p2c, and f2c. See the full list for details.

20.27: Can I use a C++ compiler to compile C code?

A: Not necessarily; C++ is not a strict superset of C.

20.28: I need to compare two strings for close, but not necessarilyexact, equality.


20.29: What is hashing?

A: A mapping of strings (or other data structures) to integers, foreasier searching.

20.31: How can I find the day of the week given the date?

A: Use mktime(), Zeller's congruence, or some code in the fulllist.

20.32: Is (year % 4 == 0) an accurate test for leap years?

A: No.

20.34: How do you write a program which produces its own source codeas

output?

A: Here's one:

char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}";main(){printf(s,34,s,34);}

20.35: What is "Duff's Device"?

A: It's a devastatingly devious way of unrolling a loop. See the

full list for details.

20.36: When will the next Obfuscated C Code Contest be held?How can I get a copy of previous winning entries?

A: See the full list, or http://www.ioccc.org/index.html .

20.37: What was the entry keyword mentioned in K&R1?


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A: It was reserved to allow functions with multiple, differently-named entry points, but it has been withdrawn.

20.38: Where does the name "C" come from, anyway?

A: C was derived from B, which was inspired by BCPL, which was a

simplification of CPL.

20.39: How do you pronounce "char"?

A: Like the English words "char," "care," or "car" (your choice).

20.39b: What do "lvalue" and "rvalue" mean?

A: An "lvalue" denotes an object that has a location; an "rvalue"is any expression that has a value.

20.40: Where can I get extra copies of this list?

A: An up-to-date copy may be obtained from ftp.eskimo.com indirectory u/s/scs/C-faq/. You can also just pull it off thenet; the unabridged version is normally posted on the first ofeach month, with an Expires: line which should keep it aroundall month. It is also posted to the newsgroups comp.answers andnews.answers. Several sites archive news.answers postings andother FAQ lists, including this one; two sites are rtfm.mit.edu(directory pub/usenet), and ftp.uu.net (directory usenet).

A hypertext version of this FAQ list is available athttp://www.eskimo.com/~scs/C-faq/top.html . An extended versionhas been published by Addison-Wesley as _C Programming FAQs:Frequently Asked Questions_ (ISBN 0-201-84519-9).

Steve [email protected]

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 asa service to the community. It is intended to complement the use of thepublished text and is protected by international copyright laws. Thecontent is made available here and may be accessed freely for personal

use but may not be republished without permission.