C Faq - Part 2

1. NULL POINTERS [CLICK FOR ANSWERS]

1. What is this infamous null pointer, anyway?
2. How do I "get" a null pointer in my programs?
3. What is NULL and how is it #defined?
4. How should NULL be #defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?
5. If NULL were defined as "((char *)0)," wouldn't that make function calls which pass an uncast NULL work?
6. I use the preprocessor macro "#define Nullptr(type) (type *)0" to help me build null pointers of the correct type.
7. Is the abbreviated pointer comparison "if(p)" to test for non-null pointers valid? What if the internal representation for null pointers is nonzero?
8. If "NULL" and "0" are equivalent, which should I use?
9. 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 null pointers?
10. I'm confused. NULL is guaranteed to be 0, but the null pointer is not?
11. Why is there so much confusion surrounding null pointers? Why do these questions come up so often?
12. I'm still confused. I just can't understand all this null pointer stuff.
13. Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to be represented internally by zeroes?
14. Seriously, have any actual machines really used nonzero null pointers?
15. What does a run-time "null pointer assignment" error mean?

2. ARRAYS AND POINTERS [CLICK FOR ANSWERS]

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?
2. But I heard that char a[] was identical to char *a.
3. So what is meant by the "equivalence of pointers and arrays" in C?
4. Why are array and pointer declarations interchangeable as function formal parameters?
5. How can an array be an lvalue, if you can't assign to it?
6. Why doesn't sizeof properly report the size of an array which is a parameter to a function?
7. Someone explained to me that arrays were really just constant pointers.
8. What is the real difference between arrays and pointers?
9. I came across some "joke" code containing the "expression" 5["abcdef"].  How can this be legal C?
10. My compiler complained when I passed a two-dimensional array to a routine expecting a pointer to a pointer.
11. How do I write functions which accept 2-dimensional arrays when the "width" is not known at compile time?
12. How do I declare a pointer to an array?
13. What's the difference between array and &array?
14. How can I dynamically allocate a multidimensional array?
15. How can I use statically- and dynamically-allocated multidimensional arrays interchangeably when passing them to functions?
16. Can I simulate a non-0-based array with a pointer?
17. I passed a pointer to a function which initialized it, but the pointer in the caller was unchanged.
18. 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?
19. Can I use a void ** pointer to pass a generic pointer to a function by reference?

3. MEMORY ALLOCATION [CLICK FOR ANSWERS]

1. Why doesn't the code "char *answer; gets(answer);" work?
2. I can't get strcat to work. I tried "char *s1 = "Hello, ", *s2 = "world!", *s3 = strcat(s1, s2);" but I got strange results.
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?
4. I have a function that is supposed to return a string, but when it returns to its caller, the returned string is garbage.
5. Why does some code carefully cast the values returned by malloc to the pointer type being allocated?
6. You can't use dynamically-allocated memory after you free it, can you?
7. How does free() know how many bytes to free?
8. So can I query the malloc package to find out how big an allocated block is?
9. When I free a dynamically-allocated structure containing pointers, do I have to free each subsidiary pointer first?
10. Why doesn't my program's memory usage go down when I free memory?
11. Must I free allocated memory before the program exits?
12. Is it legal to pass a null pointer as the first argument to realloc()?
13. Is it safe to use calloc's zero-fill guarantee for pointer and floating-point values?
14. What is alloca and why is its use discouraged?

4. EXPRESSIONS [CLICK FOR ANSWERS]

1. Why doesn't the code "a[i] = i++;" work?
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?
3. How could the code "int i = 2; i = i++;" ever give 4?
4. I just tried some allegedly-undefined code on an ANSI-conforming compiler, and got the results I expected.
5. Don't precedence and parentheses dictate order of evaluation?
6. But what about the &&, ||, and comma operators?
7. If I'm not using the value of the expression, should I use i++ or ++i to increment a variable?
8. Why doesn't the code "int a = 1000, b = 1000; long int c = a * b;" work?

5. ANSI C [CLICK FOR ANSWERS]

1. What is the "ANSI C Standard?"
2. How can I get a copy of the Standard?
3. Does anyone have a tool for converting old-style C programs to ANSI C, or for automatically generating prototypes?
4. How do I keep the ANSI "stringizing" preprocessing operator from stringizing the macro's name rather than its value?
5. Why can't I use const values in initializers and array dimensions?
6. What's the difference between "char const *p" and "char * const p"?
7. Why can't I pass a char ** to a function which expects a const char **?
8. My ANSI compiler complains about a mismatch when it sees
   
   extern int func(float);
   
   int func(x)
   float x;
   {...
9. Can you mix old-style and new-style function syntax?
10. Why does the declaration "extern f(struct x {int s;} *p);" give me a warning message?
11. I'm getting strange syntax errors inside code which I've #ifdeffed out.
12. Can I declare main as void, to shut off these annoying "main returns no value" messages?
13. Is exit(status) truly equivalent to returning status from main?
14. Why does the ANSI Standard not guarantee more than six monocase characters of external identifiersignificance?
15. What is the difference between memcpy and memmove?
16. My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors.
17. Why are some ANSI/ISO Standard library routines showing up as undefined, even though I've got an ANSI compiler?
18. Why won't frobozz-cc, which claims to be ANSI compliant, accept this code?
19. Why can't I perform arithmetic on a void * pointer?
20. Is char a[3] = "abc"; legal?
21. What are #pragmas and what are they good for?
22. What does "#pragma once" mean?
23. What's the difference between implementation-defined, unspecified, and undefined behavior?

6. C PREPROCESSOR [CLICK FOR ANSWERS]

1. How can I write a generic macro to swap two values?
2. 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.
3. What's the best way to write a multi-statement cpp macro?
4. Is it acceptable for one header file to #include another?
5. Does the sizeof operator work in preprocessor #if directives?
6. How can I use a preprocessor #if expression to detect endianness?
7. I've got this tricky processing I want to do at compile time and I can't figure out a way to get cpp to do it.
8. How can I preprocess some code to remove selected conditional compilations, without preprocessing everything?
9. How can I list all of the pre#defined identifiers?
10. How can I write a cpp macro which takes a variable number of arguments?

7. VARIABLE-LENGTH ARGUMENT LISTS [CLICK FOR ANSWERS]

1. How can I write a function that takes a variable number of arguments?
2. 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?
3. How can I discover how many arguments a function was actually called with?
4. I can't get the va_arg macro to pull in an argument of type pointer-to-function.
5. 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)?
6. How can I call a function with an argument list built up at run time?

8. BOOLEAN EXPRESSIONS AND VARIABLES [CLICK FOR ANSWERS]

1. What is the right type to use for boolean values in C? Why isn't it a standard type? Should #defines or enums be used for the true and false values?
2. What if a built-in boolean or relational operator "returns" something other than 1?

9. STRUCTS, ENUMS, AND UNIONS [CLICK FOR ANSWERS]

1. What is the difference between an enum and a series of preprocessor #defines?
2. I heard that structures could be assigned to variables and passed to and from functions, but K&R I says not.
3. How does struct passing and returning work?
4. I have a program which works correctly, but dumps core after it finishes. Why?
5. Why can't you compare structs?
6. How can I read/write structs from/to data files?
7. I came across some code that declared a structure with the last member an array of one element, and then did some tricky allocation to make the array act like it had several elements.  Is this legal and/or portable?
8. How can I determine the byte offset of a field within a structure?
9. How can I access structure fields by name at run time?
10. Why does sizeof report a larger size than I expect for a structure type, as if there was padding at the end?
11. How can I turn off structure padding?
12. Can I initialize unions?
13. Can I pass constant values to routines which accept struct arguments?

10. DECLARATIONS [CLICK FOR ANSWERS]

1. How do you decide which integer type to use?
2. What should the 64-bit type on new, 64-bit machines be?
3. I can't seem to define a linked list node which contains a pointer to itself.
4. How do I declare an array of N pointers to functions returning pointers to functions returning pointersto characters?
5. How can I declare a function that returns a pointer to a function of its own type?
6. My compiler is complaining about an invalid redeclaration of a function, but I only define it once and call it once.
7. What's the best way to declare and define global variables?
8. What does extern mean in a function declaration?
9. How do I initialize a pointer to a function?
10. I've seen different methods used for calling through pointers to functions.
11. What's the auto keyword good for?

11. STDIO [CLICK FOR ANSWERS]

1. What's wrong with the code "char c; while((c = getchar()) != EOF)..." ?
2. How can I print a '%' character with printf?
3. Why doesn't the code scanf("%d", i); work?
4. Why doesn't the code double d; scanf("%f", &d); work?
5. Why won't the code "while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?
6. Why does everyone say not to use gets()?
7. Why does errno contain ENOTTY after a call to printf?
8. My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program.
9. When I read from the keyboard with scanf, it seems to hang until I type one extra line of input.
10. I'm trying to update a file in place, by using fopen mode "r+", but it's not working.
11. How can I read one character at a time, without waiting for the RETURN key?
12. Will fflush(stdin) flush unread characters from the standard input stream?
13. How can I redirect stdin or stdout from within a program?
14. Once I've used freopen, how can I get the original stdout back?
15. How can I recover the file name given an open file descriptor?

12. LIBRARY SUBROUTINES [CLICK FOR ANSWERS]

1. Why does strncpy not always write a '\0'?
2. I'm trying to sort an array of strings with qsort, using strcmp as the comparison function, but it's not working.
3. Now I'm trying to sort an array of structures with qsort. My comparison routine 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?
4. How can I convert numbers to strings?
5. How can I get the time of day in a C program?
6. How can I convert a struct tm or a string into a time_t?
7. How can I perform calendar manipulations?
8. I need a random number generator.
9. How can I get random integers in a certain range?
10. Each time I run my program, I get the same sequence of numbers back from rand().
11. I need a random true/false value, so I'm taking rand() % 2, but it's just alternating 0, 1, 0, 1, 0...
12. I'm trying to port this old program. Why do I get "undefined external" errors for some library routines?
13. I get errors due to library routines being undefined even though I #include the right header files.
14. I'm still getting errors due to library routines being undefined, even though I'm requesting the right libraries.
15. I need some code to do regular expression matching.
16. How can I split up a string into whitespace-separated arguments?

13. LINT [CLICK FOR ANSWERS]

1. I just typed in this program, and it's acting strangely. Can you see anything wrong with it?
2. How can I shut off the "warning: possible pointer alignment problem" message lint gives me for each call to malloc?
3. Where can I get an ANSI-compatible lint?

14. STYLE [CLICK FOR ANSWERS]

1. Is the code "if(!strcmp(s1, s2))" good style?
2. What's the best style for code layout in C?
3. Where can I get the "Indian Hill Style Guide" and other coding standards?

15. FLOATING POINT [CLICK FOR ANSWERS]

1. My floating-point calculations are acting strangely and giving me different answers on different machines.
2. I keep getting "undefined: _sin" compilation errors.
3. Where is C's exponentiation operator?
4. How do I round numbers?
5. How do I test for IEEE NaN and other special values?
6. I'm having trouble with a Turbo C program which crashes and says something like "floating point formats not linked."

16. SYSTEM DEPENDENCIES [CLICK FOR ANSWERS]

1. How can I read a single character from the keyboard without waiting for a newline?
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?
3. How can I clear the screen?
4. How do I read the mouse?
5. How can my program discover the complete pathname to the executable file from which it was invoked?
6. How can a process change an environment variable in its caller?
7. How can I check whether a file exists?
8. How can I find out the size of a file, prior to reading it in?
9. How can a file be shortened in-place without completely clearing or rewriting it?
10. How can I implement a delay, or time a user's response, with sub-second resolution?
11. How can I read in an object file and jump to routines in it?
12. How can I invoke an operating system command from within a program?
13. How can I invoke an operating system command and trap its output?
14. How can I read a directory in a C program?
15. How can I do serial ("comm") port I/O?

17. MISCELLANEOUS [CLICK FOR ANSWERS]

1. What can I safely assume about the initial values of variables which are not explicitly initialized?
2. What's wrong with
   f() { char a[] = "Hello, world!"; }
   ?
3. How can I write data files which can be read on other machines with different data formats?
4. How can I insert or delete a line from the middle of a file?
5. How can I return several values from a function?
6. How can I call a function, given its name as a string?
7. I seem to be missing the system header file <sgtty.h>. Can someone send me a copy?
8. How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions from C?
9. Does anyone know of a program for converting Pascal or FORTRAN to C?
10. Can I use a C++ compiler to compile C code?
11. I'm looking for C development tools (cross-reference generators, code beautifiers, etc.).
12. Where can I get copies of all these public-domain programs?
13. When will the next Obfuscated C Code Contest be held? How can I get a copy of the previous winning entries?
14. Why don't C comments nest? Are they legal inside quoted strings?
15. How can I get the ASCII value corresponding to a character?
16. How can I implement sets and/or arrays of bits?
17. What is the most efficient way to count the number of bits which are set in a value?
18. How can I make this code more efficient?
19. Are pointers really faster than arrays? How much do function calls slow things down?
20. Why does the code "char *p = "Hello, world!"; p[0] = tolower(p[0]);" crash?
21. This program crashes before it even runs!
22. What does "Segmentation violation" mean?
23. My program is crashing, apparently somewhere down inside malloc.
24. Does anyone have a C compiler test suite I can use?
25. Where can I get a YACC grammar for C?
26. I need code to parse and evaluate expressions.
27. I need to compare two strings for close, but not necessarily exact, equality.
28. How can I find the day of the week given the date?
29. Will 2000 be a leap year?
30. How do you pronounce "char"?
31. What's a good book for learning C?
32. Are there any C tutorials on the net?
33. Where can I get extra copies of this list?

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Section 1. Null Pointers

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1.1: WHAT IS THIS INFAMOUS NULL POINTER, ANYWAY?

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 not the address of any object or function.  That is, the address-of operator & will never yield a null pointer, nor will a successful call tomalloc.  (malloc returns 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; an uninitialized pointer might point anywhere.  See also questions 3.1, 3.13, and 17.1.

As mentioned in the definition 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 it can make the distinction if necessary (see below).

References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S Sec. 5.3 p. 91; ANSI Sec. 3.2.2.3 p. 38.

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

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)

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.  For instance, the Unix system call "execl" takes a variable-length, null-pointer-terminated list of character pointer arguments.  To generate a null pointer in a function call context, an explicit cast is typically required, to force the 0 to be in a pointer context:

               execl("/bin/sh", "sh", "-c", "ls", (char *)0);

If the (char *) cast 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 unadorned 0's.  Function prototypes cannot provide the types for variable arguments in variable-length argument lists, however, so explicit casts are still required for those arguments.  It is safest always to cast null pointer function arguments, to guard against varargs functions or those without prototypes, to allow interim use of non-ANSI compilers, and to demonstrate that you know what you are doing.  (Incidentally, it's also a simpler rule to remember.)

Summary:

Unadorned 0 okay:

• initialization
• assignment
• comparison
• function call, prototype in scope, fixed argument

Explicit cast required:

• function call, no prototype in scope
• variable argument in varargs function call

References: K&R I Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R II Sec. A7.10 p. 207, Sec. A7.17 p. 209; H&S Sec. 4.6.3 p. 72; ANSI Sec. 3.2.2.3 .

1.3: What is NULL and how is it #defined?

As a matter of style, many people prefer not to have unadorned 0's scattered throughout their programs. For this reason, the preprocessor macro NULL is #defined (by <stdio.h> or <stddef.h>), with value 0 (or(void *)0, about which more later).  A programmer who wishes to make explicit the distinction between 0 the integer and 0 the null pointer can then use NULL whenever a null pointer is required.  This is a stylistic convention only; the preprocessor turns NULL back to 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 1.2 above applies for NULL as well as 0.)

NULL should only be used for pointers; see question 1.8.

References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S Sec. 13.1 p. 283; ANSI Sec. 4.1.5 p. 99, Sec. 3.2.2.3 p. 38, Rationale Sec. 4.1.5 p. 74.

1.4: How should NULL be #defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?

Programmers should never need to know the internal representation(s) of null pointers, because they are normally taken care of by the compiler.  If a machine uses a nonzero bit pattern for null pointers, it is the compiler's responsibility to generate it when the programmer requests, by writing "0" or "NULL," a null pointer.  Therefore, #defining NULL as 0 on a machine for which internal null pointers are nonzero is as valid as on any other, because the compiler must (and can) still generate the machine's correct null pointers in response to unadorned 0's seen in pointer contexts.

1.5: If NULL were defined as follows:

               #define NULL ((char *)0)

wouldn't that make function calls which pass an uncastNULL work?

Not in general.  The problem is that there are machines which use different internal representations for pointers to different types of data.  The suggested #definition would make uncast NULLarguments to functions expecting pointers to characters to work correctly, but pointer arguments to other types would still be problematical, and legal constructions such as

               FILE *fp = NULL;

could fail.

Nevertheless, ANSI C allows the alternate

               #define NULL ((void *)0)

definition for NULL.  Besides 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 1.8).

References: ANSI Rationale Sec. 4.1.5 p. 74.

1.6: I use the preprocessor macro

               #define Nullptr(type) (type *)0

to help me build null pointers of the correct type.

This trick, though popular in some circles, does not buy much. It is not needed in assignments and comparisons; see question 1.2.  It does not even save keystrokes. Its use suggests to the reader that the author is shaky on the subject of null pointers, and requires the reader to check the #definition of the macro, its invocations, and all other pointer usages much more carefully.  See also question 8.1.

1.7: Is the abbreviated pointer comparison "if(p)" to test for non- null pointers valid? What if the internal representation for null pointers is nonzero?

When C requires the boolean value of an expression (in the if, while, for, and do statements, and with the&&, ||, !, and ?: operators), a false value is produced 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 a null pointer, and use the correct value. There is no trickery involved here; compilers do work this way, and generate identical code for both statements.  The internal representation of a pointer does not matter.

The boolean negation operator, !, can be described as follows:

               !expr   is essentially equivalent to    expr?0:1

It is left as an exercise for the reader to show 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.

See also question 8.2.

References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91; ANSI Secs. 3.3.3.3, 3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, and 3.6.5 .

1.8: If "NULL" and "0" are equivalent, which should I use?

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 #definition, and prefer to use unadorned "0" instead.  There is no one right answer. C programmers must understand that "NULL" and "0" are interchangeable and that an uncast "0" is perfectly acceptable in initialization, assignment, and comparison contexts.  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.  (ANSI allows the #definition of NULL to be (void *)0, which will not work 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&R II Sec. 5.4 p. 102.

1.9: But wouldn't it be better to use NULL (rather than 0) in case the value of NULLchanges, perhaps on a machine with nonzero null pointers?

No.  Although symbolic constants are often used in place of numbers because the numbers might change, this is notthe 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.

1.10: I'm confused.  NULL is guaranteed to be 0, but the null pointer is not?

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 1.1.  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 source code syntax for null pointers, which is the single character "0".  It is often hidden behind...
4. The NULL macro, which is #defined to be "0" or "(void *)0".  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 an empty string ("").  The term "null string" can be confusing in C (and should perhaps be avoided), because it involves a null ('\0') character, but not a null pointer, which brings us full circle...

This article always uses the phrase "null pointer" (in lower case) for sense 1, the character "0" for sense 3, and the capitalized word "NULL" for sense 4.

1.11: Why is there so much confusion surrounding null pointers?  Why do these questions come up so often?

C programmers traditionally like to know 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) suggests that the value might change later, 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 above) is often overlooked.

One good way to wade out of the confusion is to imagine that C had a keyword (perhaps "nil", like Pascal) with which null pointers were requested.  The compiler could either turn "nil" into the correct type of null pointer, when it could determine the type from the source code, or complain when it could not. Now, in fact, in C the keyword for a null pointer 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, the code may not work.

1.12: I'm still confused.  I just can't understand all this null pointer stuff.

Follow these two simple rules:

1. When you want to refer to a null pointer 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, or with the internal representation of null pointers (which you shouldn't need to know), or with ANSI C refinements. Understand questions 1.1, 1.2, and 1.3, and consider 1.8 and 1.11, and you'll do fine.

1.13: Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to be represented internally by zeroes?

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 this 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 3.13).  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 rather than "0," as mentioned in question 1.11, the urge to assume an internal zero representation would not even arise.)

1.14: Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?

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.

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, 80*86 processors (PC's) may use 16 bit data pointers and 32 bit function pointers, or vice versa.

The old HP 3000 series computers use a different addressing scheme for byte addresses than for word addresses; void and char pointers therefore have a different representation than an int (structure, etc.) pointer to the same address would have.

1.15: What does a run-time "null pointer assignment" error mean? How do I track it down?

This message, which occurs only under MS-DOS (see, therefore, section 16) means that you've written, via an uninitialized and/or null pointer, to location zero.

A debugger will usually let you set a data breakpoint on location 0.  Alternately, you could write a bit of code to copy 20 or so bytes from location 0 into another buffer, and periodically check that it hasn't changed.

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Section 2. Arrays and Pointers

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

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: CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5.

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

Not at all. (What you heard has to do with formal parameters to functions; see question 2.4.)  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.  The pointer is to be known by the name "p," and can point to any char (or contiguous array of chars) anywhere.

As usual, a picture is worth a thousand words.  The statements

               char a[] = "hello";

               char *p = "world";

would result in data structures which could be represented like this:

                  +---+---+---+---+---+---+

               a: | h | e | l | l | o |\0 |

                  +---+---+---+---+---+---+

                  +-----+     +---+---+---+---+---+---+

               p: |  *======> | w | o | r | l | d |\0 |

                  +-----+     +---+---+---+---+---+---+

It is important 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 the example above, both a[3] and p[3] happen to be the character 'l', but the compiler gets there differently.  (See also questions 17.19 and 17.20.)

2.3: So what is meant by the "equivalence of pointers and arrays" in C?

Much of the confusion surrounding pointers in C can be traced to a misunderstanding of this statement. Saying that arrays and pointers are "equivalent" neither means that they are identical nor even interchangeable.

"Equivalence" refers to the following key definition:

An lvalue [see question 2.5] 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.

(The exceptions are when the array is the operand of a sizeof or &operator, or is a literal string initializer for a character array.)

As a consequence of this definition, there is no apparent difference in the behavior of the "array subscripting" operator [] as it applies to arrays and pointers.  In an expression of the form a[i], the array reference "a" 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 2.2).  In either case, the expression x[i] (where x is an array or a pointer) is, by definition, identical to *((x)+(i)).

References: K&R I Sec. 5.3 pp. 93-6; K&R II Sec. 5.3 p. 99; H&S Sec. 5.4.1 p. 93; ANSI Sec. 3.2.2.1, Sec. 3.3.2.1, Sec. 3.3.6 .

2.4: Then why are array and pointer declarations interchangeable as function formal parameters?

Since arrays decay immediately into pointers, an array is never actually passed to a function.  As a convenience, any parameter declarations which "look like" arrays, e.g.

               f(a)

               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:

               f(a)

               char *a;

This conversion holds only within function formal parameter declarations, nowhere else.  If this conversion bothers you, avoid it; many people have concluded that the confusion it causes outweighs the small advantage of having the declaration "look like" the call and/or the uses within the function.

References: K&R I Sec. 5.3 p. 95, Sec. A10.1 p. 205; K&R II Sec. 5.3 p. 100, Sec. A8.6.3 p. 218, Sec. A10.1 p. 226; H&S Sec. 5.4.3 p. 96; ANSI Sec. 3.5.4.3, Sec. 3.7.1, CT&P Sec. 3.3 pp. 33-4.

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

The ANSI C Standard defines a "modifiable lvalue," which an array is not.

References: ANSI Sec. 3.2.2.1 p. 37.

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

The sizeof operator reports the size of the pointer parameter which the function actually receives (see question 2.4).

2.7: Someone explained to me that arrays were really just constant pointers.

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 2.2 should make clear.

2.8: Practically speaking, what is the difference between arrays and pointers?

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 2.3), 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 question 2.14; see also question 17.20.)

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

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 anyexpression e and primary expression a, 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 17.13).

References: ANSI Rationale Sec. 3.3.2.1 p. 41.

2.10: My compiler complained when I passed a two-dimensional array to a routine expecting a pointer to a pointer.

The rule 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.  (The confusion is heightened by the existence of incorrect compilers, including some versions of pcc and pcc-derived lint's, which improperly accept assignments of multi-dimensional arrays to multi-level pointers.)  If you are passing a two-dimensional array to a function:

               int array[NROWS][NCOLUMNS];

               f(array);

the function's declaration should match:

               f(int a[][NCOLUMNS]) {...}

or

               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;" 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 "shape" of the array is still important, so the "column" dimension NCOLUMNS (and, for 3- or more dimensional arrays, the intervening ones) must be included.

If a function is already declared as accepting a pointer to a pointer, it is probably incorrect to pass a two-dimensional array directly to it.

References: K&R I Sec. 5.10 p. 110; K&R II Sec. 5.9 p. 113.

2.11: How do I write functions which accept 2-dimensional arrays when the "width" is not known at compile time?

It's not 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:"

               f2(aryp, nrows, ncolumns)

               int *aryp;

               int nrows, ncolumns;

               { ... ary[i][j] is really aryp[i * ncolumns + j] ... }

This function could be called with the array from question 2.10 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; the behavior of accessing (&array[0][0])[x] is not defined for x > NCOLUMNS.

gcc allows local arrays to be declared having sizes which are specified by a function's arguments, but this is a nonstandard extension.

See also question 2.15.

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

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 2.3), which is convenient; subscripting or incrementing the resultant pointer accesses the individual members of the array.  True pointers to arrays, when subscripted or incremented, step over entire arrays, and are generally only useful when operating on arrays of arrays, if at all.  (See question 2.10 above.)

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 10.4.)  If the size of the array is unknown, N can be omitted, but the resulting type, "pointer to array of unknown size," is useless.

2.13: Since array references decay to pointers, given

                int array[NROWS][NCOLUMNS];

what's the difference between array and &array?

Under ANSI/ISO Standard C, &array yields a pointer, of type pointer-to-array-of-T, to the entire array (see also question 2.12).  Under pre-ANSI C, the & in&array generally elicited a warning, and was generally ignored.  Under all C compilers, an unadorned reference to an array yields a pointer, of type pointer-to-T, to the array's first element. (See also question 2.3.)

2.14: How can I dynamically allocate a multidimensional array?

It is usually best to allocate an array of pointers, and then initialize each pointer to a dynamically-allocated "row."  Here is a two-dimensional example:

               int **array1 = (int **)malloc(nrows * sizeof(int *));

               for(i = 0; i < nrows; i++)

                               array1[i] = (int *)malloc(ncolumns * sizeof(int));

(In "real" code, of course, malloc would be declared correctly, and each return value checked.)

You can keep the array's contents contiguous, while making later reallocation of individual rows difficult, with a bit of explicit pointer arithmetic:

               int **array2 = (int **)malloc(nrows * sizeof(int *));

               array2[0] = (int *)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:array[i][j].

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 = (int *)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 can hide the explicit calculation, but invoking it then requires parentheses and commas which don't look exactly like multidimensional array subscripts.)

Finally, you can use pointers-to-arrays:

               int (*array4)[NCOLUMNS] =

                               (int (*)[NCOLUMNS])malloc(nrows * sizeof(*array4));

, but the syntax gets horrific and all but one dimension must be known at compile time.

With all of these techniques, you may of course need to remember to free the arrays (which may take several steps; see question 3.9) when they are no longer needed, and you cannot necessarily intermix the dynamically-allocated arrays with conventional, statically-allocated ones (see question 2.15 below, and also question 2.10).

2.15: How can I use statically- and dynamically-allocated multidimensional arrays interchangeably when passing them to functions?

There is no single perfect method.  Given the declarations

        int array[NROWS][NCOLUMNS];

        int **array1;

        int **array2;

        int *array3;

        int (*array4)[NCOLUMNS];

as initialized in the code fragments in questions 2.10 and 2.14, and functions declared as

        f1(int a[][NCOLUMNS], int m, int n);

        f2(int *aryp, int nrows, int ncolumns);

        f3(int **pp, int m, int n);

(see questions 2.10 and 2.11), the following calls should work as expected:

               f1(array, NROWS, NCOLUMNS);

               f1(array4, nrows, NCOLUMNS);

               f2(&array[0][0], NROWS, NCOLUMNS);

               f2(*array2, nrows, ncolumns);

               f2(array3, nrows, ncolumns);

               f2(*array4, nrows, NCOLUMNS);

               f3(array1, nrows, ncolumns);

               f3(array2, nrows, ncolumns);

The following two calls would probably work, but involve questionable casts, and work only if the dynamic ncolumns matches the static NCOLUMNS:

               f1((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);

               f1((int (*)[NCOLUMNS])array3, nrows, ncolumns);

It must again be noted that passing &array[0][0] to f2() is not strictly conforming; see question 2.11.

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 verygood understanding of arrays and pointers (and several other areas) in C.

2.16: 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.

Although this technique is attractive (and was used in old editions of the book Numerical Recipes in C), it does not conform to the C standards. 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: ANSI Sec. 3.3.6 p. 48, Rationale Sec. 3.2.2.3 p. 38; K&R II Sec. 5.3 p. 100, Sec. 5.4 pp. 102-3, Sec. A7.7 pp. 205-6.

2.17: I passed a pointer to a function which initialized it:

               ...

               int *ip;

               f(ip);

               ...

               void f(ip)

               int *ip;

               {

                               static int dummy = 5;

                               ip = &dummy;

               }

, but the pointer in the caller was unchanged.

Did the function try to initialize the pointer itself, or just what it pointed to?  Remember that arguments in C are passed by value.  The called function altered only the passed copy of the pointer.  You'll 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.

2.18: 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?

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 an anomaly in pcc- derived compilers, and an extension in gcc, that expressions such as the above are ever accepted.)  Say what you mean: use

               p = (char *)((int *)p + 1);

, or simply

               p += sizeof(int);

References: ANSI Sec. 3.3.4, Rationale Sec. 3.3.2.4 p. 43.

2.19: Can I use a void ** pointer to pass a generic pointer to a function by reference?

Not portably. There is no generic pointer-to-pointer type in C. void * acts as a generic pointer only because conversions 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 something other than a void *.

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Section 3. Memory Allocation

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3.1: Why doesn't this fragment work?

               char *answer;

               printf("Type something:\n");

               gets(answer);

               printf("You typed \"%s\"\n", answer);

The pointer variable "answer," which is handed to the gets function 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 question 17.1.)

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 <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);

Note that this example also uses fgets() instead of gets()(always a good idea; see question 11.6), allowing the size of the array to be specified, so that the end of the array will not be overwritten if the user types an overly-long line. (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.

3.2: 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.

Again, the problem is that space for the concatenated result is not properly allocated.  C does not provide an automatically- managed string type.  C compilers only allocate memory for objects explicitly mentioned in the source code (in the case of "strings," this includes character arrays and string literals). The programmer must arrange (explicitly) for sufficient space for the results of run-time operations such as string concatenation, typically by declaring arrays, or by calling malloc.  (See also question 17.20.)

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 with sufficient space:

               char s1[20] = "Hello, ";

Since strcat returns the value of its first argument (s1, in this case), the s3 variable is superfluous.

References: CT&P Sec. 3.2 p. 32.

3.3: But the man page for strcat says that it takes two char *'s as arguments.  How am I supposed to know to allocate things?

In general, when using pointers you always have to consider memory allocation, at least to make sure that the compiler is doing it for you.  If a library routine'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 can be misleading.  The code fragments presented there are closer to the function definition used by the call's implementor than the invocation used by the caller.  In particular, many routines which accept pointers (e.g. to structs or strings), are usually called with the address of some object (a struct, or an array -- see questions 2.3 and 2.4.)  Another common example isstat().

3.4: I have a function that is supposed to return a string, but when it returns to its caller, the returned string is garbage.

Make sure that the memory to which the function returns a pointer is correctly allocated. The returned pointer should be to a statically-allocated buffer, or to a buffer passed in by the caller, or to memory obtained withmalloc(), but notto a local (auto) array.  In other words, never do something like

        char *f()

        {

                char buf[10];

                /* ... */

                return buf;

        }

One fix (which is imperfect, especially if f() is called recursively, or if several of its return values are needed simultaneously) would to to declare the buffer as

                static char buf[10];

See also question 17.5.

3.5: Why does some code carefully cast the values returned by malloc to the pointer type being allocated?

Before ANSI/ISO Standard C introduced the void * generic pointer type, these casts were typically required to silence warnings about assignment between incompatible pointer types.  (Under ANSI/ISO Standard C, these casts are not required.)

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

No.  Some early documentation for malloc stated that the contents of freed memory was "left undisturbed;" this ill-advised guarantee was never universal and is not required by ANSI.

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((char *)listp);

               }

and notice what would happen if the more-obvious loop iteration expression listp = listp->next were used, without the temporary nextp pointer.

References: ANSI Rationale Sec. 4.10.3.2 p. 102; CT&P Sec. 7.10 p. 95.

3.7: How does free() know how many bytes to free?

The malloc/free package remembers the size of each block it allocates and returns, so it is not necessary to remind it of the size when freeing.

3.8: So can I query the malloc package to find out how big an allocated block is?

Not portably.

3.9: I'm allocating structures which contain pointers to other dynamically-allocated objects.  When I free a structure, do I have to free each subsidiary pointer first?

Yes.  In general, you must arrange that each pointer returned from malloc be individually passed tofree, exactly once (if it is freed at all).

3.10: I have a program which mallocs but then frees a lot of memory, but memory usage (as reported by ps) doesn't seem to go back down.

Most implementations of malloc/free do not return freed memory to the operating system (if there is one), but merely make it available for futuremalloc calls within the same process.

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

You shouldn't have to.  A real operating system definitively reclaims all memory 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 it is a "quality of implementation issue."

References: ANSI Sec. 4.10.3.2 .

3.12: Is it legal to pass a null pointer as the first argument to realloc()?  Why would you want to?

ANSI C sanctions this usage (and the related realloc(..., 0), which frees), but several earlier implementations do not support it, so it is not widely portable.  Passing an initially-null pointer torealloccan make it easier to write a self-starting incremental allocation algorithm.

References: ANSI Sec. 4.10.3.4 .

3.13: What is the difference between calloc and malloc?  Is it safe to use calloc's zero-fill guarantee for pointer and floating-point values?  Does free work on memory allocated withcalloc,  or do you need a cfree?

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 zero values for pointers (see section 1 of this list) or floating-point values.  free can (and should) be used tofree the memory allocated bycalloc.

References: ANSI Secs. 4.10.3 to 4.10.3.2 .

3.14: What is alloca and why is its use discouraged?

alloca allocates memory which is automatically freed when the function which called alloca returns. That is, memory allocated withalloca 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 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 cannot be used in programs which must be widely portable, no matter how useful it might be.

References: ANSI Rationale Sec. 4.10.3 p. 102.

---

Section 4. Expressions

---

4.1: Why doesn't this code:

               a[i] = i++;

work?

The subexpression i++ causes a side effect -- it modifies i's value -- which leads to undefined behavior if iis also referenced elsewhere in the same expression.  (Note that although the language in K&R suggests that the behavior of this expression is unspecified, the ANSI/ISO C Standard makes the stronger statement that it is undefined -- see question 5.23.)

References: ANSI Sec. 3.3 p. 39.

4.2: 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?

Although the postincrement and postdecrement operators ++ and -- perform the operations after yielding the former value, the implication of "after" is often misunderstood.  It is not guaranteed that the operation 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). In the example, the compiler chose to multiply the previous value by itself and to perform both increments afterwards.

The behavior of code which contains multiple, ambiguous side effects has always been undefined (see question 5.23).  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 howthey are done on various machines, that innocence may help to protect you."

References: K&R I Sec. 2.12 p. 50; K&R II Sec. 2.12 p. 54; ANSI Sec. 3.3 p. 39; CT&P Sec. 3.7 p. 47; PCS Sec. 9.5 pp. 120-1. (Ignore H&S Sec. 7.12 pp. 190-1, which is obsolete.)

4.3: I've experimented with the code

              int i = 2;

              i = i++;

on several compilers.  Some gave i the value 2, some gave 3, but one gave 4.  I know the behavior is undefined, but how could it give 4?

Undefined behavior means anythingcan happen.  See question 5.23.

4.4: People keep saying the behavior is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected.

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 question 5.18.

4.5: Can I use explicit parentheses to force the order of evaluation I want?  Even if I don't, doesn't precedence dictate it?

Operator precedence and explicit parentheses impose only a partial ordering on the evaluation of an expression. Consider 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.

4.6: But what about the &&, ||, and comma operators? I see code like "if((c = getchar()) == EOF || c == '\n')" ...

There is a special exception for those operators, (as well as the ?:operator); each of them does imply a sequence point (i.e. left-to-right evaluation is guaranteed).  Any book on C should make this clear.

References: K&R I Sec. 2.6 p. 38, Secs. A7.11-12 pp. 190-1; K&R II Sec. 2.6 p. 41, Secs. A7.14-15 pp. 207-8; ANSI Secs. 3.3.13 p. 52, 3.3.14 p. 52, 3.3.15 p. 53, 3.3.17 p. 55, CT&P Sec. 3.7 pp. 46-7.

4.7: If I'm not using the value of the expression, should I use i++ or ++i to increment a variable?

Since the two forms differ only in the value yielded, they are entirely equivalent when only their side effect is needed.

4.8: Why doesn't the code

               int a = 1000, b = 1000;

               long int c = a * b;

work?

Under C's integral promotion rules, the multiplication is carried out using int arithmetic, and the result may overflow and/or be truncated before being assigned to the long int left- hand-side.  Use an explicit cast to force long arithmetic:

               long int c = (long int)a * b;

Note that the code (long int)(a * b) would not have the desired effect.

---

SECTION 5. ANSI C

---

5.1: What is the "ANSI C Standard?"

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 standardizes existing practice, with a few additions from C++ (most notably function prototypes) and support for multinational character sets (including the much-lambasted trigraph sequences).  The ANSI C standard also formalizes the C run-time library support routines.

The published Standard includes a "Rationale," which explains many of its decisions, and discusses a number of subtle points, including several of those covered here.  (The Rationale is "not part of ANSI Standard X3.159-1989, but is included for information only.")

The Standard has been adopted as an international standard, ISO/IEC 9899:1990, although the sections are numbered differently (briefly, ANSI sections 2 through 4 correspond roughly to ISO sections 5 through 7), and the Rationale is currently not included.

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

ANSI X3.159 has been officially superseded by ISO 9899. 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

or

Global Engineering Documents
2805 McGaw Avenue
Irvine, CA  92714 USA
(+1) 714 261 1455
(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

The cost is $130.00 from ANSI or $162.50 from Global.  Copies of the original X3.159 (including the Rationale) are still available at $205.00 from ANSI or $200.50 from Global.  Note that ANSI derives revenues to support its operations from the sale of printed standards, so electronic copies are notavailable.

The mistitled Annotated ANSI C Standard, with annotations by Herbert Schildt, contains all but a few pages 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.)

The text of the Rationale (not the full Standard) is now available for anonymous ftp from ftp.uu.net (see question 17.12) in directory doc/standards/ansi/X3.159-1989 .  The Rationale has also been printed by Silicon Press, ISBN 0-929306-07-4.

5.3: Does anyone have a tool for converting old-style C programs to ANSI C, or vice versa, or for automatically generating prototypes?

Two programs, protoize and unprotoize, convert back and forth between prototyped and "old style" function definitions and declarations.  (These programs do nothandle full-blown translation between "Classic" C and ANSI C.)  These programs were once patches to the FSF GNU C compiler, gcc, but are now part of the main gcc distribution; look in pub/gnu at prep.ai.mit.edu (18.71.0.38), or at several other FSF archive sites.

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.

Several prototype generators exist, many as modifications to lint.  Version 3 of CPROTO was posted to comp.sources.misc in March, 1992.  There is another program called "cextract."  See also question 17.12.

Finally, are you sure you really need to convert lots of old code to ANSI C?  The old-style function syntax is still acceptable.

5.4: 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.

You must use something like the following two-step procedure to force the 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 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: ANSI Sec. 3.8.3.2, Sec. 3.8.3.5 example p. 93.

5.5: 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];

The const qualifier really means "read-only;" an object so qualified is a normal run-time object which cannot (normally) be assigned to.  The value of a const-qualified object is therefore nota 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 .

References: ANSI Sec. 3.4 .

5.6: What's the difference between "char const *p" and "char * const p"?

"char const *p" is a pointer to a constant character (you can't change the character); "char * const p" is a constant pointer to a (variable) character (i.e. you can't change the pointer). (Read these "inside out" to understand them.  See question 10.4.)

References: ANSI Sec. 3.5.4.1 .

5.7: Why can't I pass a char ** to a function which expects a const char **?

You can use a pointer-to-T (for any type T) where a pointer-to- const-T is expected, but the rule (an explicit exception) which permits slight mismatches in qualified pointer types is not applied recursively, but only at the top level.

You must use explicit casts (e.g. (const char **) in this case) when assigning (or passing) pointers which have qualifier mismatches at other than the first level of indirection.

References: ANSI Sec. 3.1.2.6 p. 26, Sec. 3.3.16.1 p. 54, Sec. 3.5.3 p. 65.

5.8: My ANSI compiler complains about a mismatch when it sees

               extern int func(float);

               int func(x)

               float x;

               {...

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 safe to mix the two styles (see question 5.9), but not in this case.  Old C (and ANSI C, in the absence of prototypes, and in variable-length argument lists) "widens" certain arguments when they are passed to functions.  floats are promoted to double, and characters and short integers are promoted to ints.  (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, as long as the address of that parameter is not taken.)

It may also be safer to avoid "narrow" (char, short int, and float) function arguments and return types.

References: ANSI Sec. 3.3.2.2 .

5.9: Can you mix old-style and new-style function syntax?

Doing so is perfectly legal, as long as you're careful (see especially question 5.8).  Note however that old-style syntax is marked as obsolescent, and support for it may be removed some day.

References: ANSI Secs. 3.7.1, 3.9.5 .

5.10: Why does the declaration

               extern f(struct x {int s;} *p);

give me an obscure warning message about "struct x introduced in prototype scope"?

In a quirk of C's normal block scoping rules, a struct declared only within a prototype cannot be compatible with other structs declared in the same source file, nor can the struct tag be used later as you'd expect (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 will reserve a place at file scope for struct x's definition, which will be completed by the struct declaration within the prototype.

References: ANSI Sec. 3.1.2.1 p. 21, Sec. 3.1.2.6 p. 26, Sec. 3.5.2.3 p. 63.

5.11: I'm getting strange syntax errors inside code which I've #ifdeffed out.

Under ANSI C, the text inside a "turned off" #if, #ifdef, or #ifndef must still consist of "valid preprocessing tokens." This means that there must be no unterminated comments or quotes (note particularly that an apostrophe within a contracted word could look like the beginning of a character constant), and no newlines inside quotes.  Therefore, natural-language comments and pseudocode should always be written between the "official" comment delimiters /* and */.  (But see also question 17.14, and 6.7.)

References: ANSI Sec. 2.1.1.2 p. 6, Sec. 3.1 p. 19 line 37.

5.12: Can I declare main as void, to shut off these annoying "main returns no value" messages?  (I'm calling exit(), so main doesn't return.)

No.  main must be declared as returning an int, and as taking either zero or two arguments (of the appropriate type).  If you're calling exit() but still getting warnings, you'll have to insert a redundantreturn statement (or use some kind of "notreached" directive, if available).

Declaring a function as void does not merely silence 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.

References: ANSI Sec. 2.1.2.2.1 pp. 7-8.

5.13: Is exit(status)truly equivalent to returning status from main?

Formally, yes, although discrepancies arise under a few older, nonconforming systems, or if data local tomain() might be needed during cleanup (due perhaps to a setbuf or atexitcall), or if main() is called recursively.

References: ANSI Sec. 2.1.2.2.3 p. 8.

5.14: Why does the ANSI Standard not guarantee more than six monocase characters of external identifier significance?

The problem is older linkers which are neither under the control of the ANSI standard nor the C compiler developers on the systems which have them.  The limitation is only that identifiers be significant in the first six characters, not that they be restricted to six characters in length.  This limitation is annoying, but certainly not unbearable, and is marked in the Standard as "obsolescent," i.e. a future revision will likely relax it.

This concession to current, restrictive linkers really had to be made, no matter how vehemently some people oppose it.  (The Rationale notes that its retention was "most painful.")  If you disagree, or have thought of a trick by which a compiler burdened with a restrictive linker could present the C programmer with the appearance of more significance in external identifiers, read the excellently-worded section 3.1.2 in the X3.159 Rationale (see question 5.1), which discusses several such schemes and explains why they could not be mandated.

References: ANSI Sec. 3.1.2 p. 21, Sec. 3.9.1 p. 96, Rationale Sec. 3.1.2 pp. 19-21.

5.15: What is the difference between memcpy and memmove?

memmoveoffers 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 usememmove.

References: ANSI Secs. 4.11.2.1, 4.11.2.2, Rationale Sec. 4.11.2.

5.16: My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors.

Perhaps it is a pre-ANSI compiler, unable to accept function prototypes and the like.  See also questions 5.17 and 17.2.

5.17: Why are some ANSI/ISO Standard library routines showing up as undefined, even though I've got an ANSI compiler?

It's not unusual to have a compiler available which accepts ANSI syntax, but not to have ANSI-compatible header files or run-time libraries installed.  See also questions 5.16 and 17.2.

5.18: 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.

Most 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 4.4.

5.19: Why can't I perform arithmetic on a void * pointer?

The compiler doesn't know the size of the pointed-to objects. Before performing arithmetic, cast the pointer either to char * or to the type you're trying to manipulate (but see question 2.18).

5.20: Is char a[3] = "abc"; legal?  What does it mean?

It is legal in ANSI C (and perhaps in a few pre-ANSI systems), though questionably useful. 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.

References: ANSI Sec. 3.5.7 pp. 72-3.

5.21: What are #pragmas and what are they good for?

The #pragma directive provides a single, well-defined "escape hatch" which can be used for all sorts of implementation- specific controls and extensions: source listing control, structure packing, warning suppression (like the old lint /* NOTREACHED */ comments), etc.

References: ANSI Sec. 3.8.6 .

5.22: What does "#pragma once" mean?  I found it in some header files.

It is an extension implemented by some preprocessors to help make header files idempotent; it is essentially equivalent to the #ifndef trick mentioned in question 6.4.

5.23: People seem to make a point of distinguishing between implementation-defined, unspecified, and undefined behavior. What's the difference?

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.

If you're interested in writing portable code, you can ignore the distinctions, as you'll want to avoid code that depends on any of the three behaviors.

References: ANSI Sec. 1.6, especially the Rationale.

---

SECTION 6. C PREPROCESSOR

---

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

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 (and the "obvious" supercompressed implementation for integral types a^=b^=a^=b is in fact illegal due to multiple side-effects; see questions 4.1 and 4.2).  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 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.

6.2: 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.

That comments disappeared entirely and could therefore be used for token pasting was an undocumented feature of some early preprocessor implementations, notably Reiser's.  ANSI affirms (as did K&R) 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 5.4.)

References: ANSI Sec. 3.8.3.3 p. 91, Rationale pp. 66-7.

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

The usual goal is to write a macro that can be invoked as if it were a single function-call statement.  This means that the "caller" will be supplying the final semicolon, so the macro body should not.  The macro body cannot be a simple brace- delineated 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 is to use

               #define Func() 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.  (See the example under question 6.10 below. This technique also allows a value to be "returned.")

References: CT&P Sec. 6.3 pp. 82-3.

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

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 14.3) disparages them; they can make it harder to find relevant definitions; they can lead to multiple-declaration errors if a file is #included twice; 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 #includes what it needs itself, rather than requiring each #includer to do so, a requirement that can lead to intractable headaches); a tool like grep (or a tags file) makes it easy to find definitions no matter where they are; a popular trick:

               #ifndef HEADERUSED

               #define HEADERUSED

               ...header file contents...

               #endif

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) handle dependency generation in the face of nested #include files easily.  See also section 14.

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

No.  Preprocessing happens during an earlier pass of compilation, before type names have been parsed. Consider using the predefined constants in ANSI's <limits.h>, if applicable, or a "configure" script, instead. (Better yet, try to write code which is inherently insensitive to type sizes.)

References: ANSI Sec. 2.1.1.2 pp. 6-7, Sec. 3.8.1 p. 87 footnote 83.

6.6: How can I use a preprocessor #if expression to tell if a machine is big-endian or little-endian?

You probably can't.  (Preprocessor arithmetic uses only long ints, 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.

6.7: I've got this tricky processing I want to do at compile time and I can't figure out a way to get cpp to do it.

cpp is not intended as a general-purpose preprocessor.  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 (such as m4).

6.8: 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 cpp and expanding all of the #include's and #define's as well?

There are programs floating around called unifdef, rmifdef, and scpp which do exactly this. (See question 17.12.)

6.9: How can I list all of the pre#defined identifiers?

There's no standard way, although it is a frequent need.  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(1) utility.  Beware that many traditional system-selective pre#defined identifiers (e.g. "unix") are non-Standard (because they clash with the user's namespace) and are being removed or renamed.

6.10: How can I write a cpp macro which takes a variable number of arguments?

One popular trick is to define the macro with a single argument, and call it with a double set of parentheses, which appear to the preprocessor to indicate a single argument:

               #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.  Other solutions are to use different macros (DEBUG1, DEBUG2, etc.) depending on the number of arguments, or to play games with commas:

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

        #define _ ,

        DEBUG("i = %d" _ i)

It is often better to use a bona-fide function, which can take a variable number of arguments in a well-defined way.  See questions 7.1 and 7.2.

---

SECTION 7. VARIABLE-LENGTH ARGUMENT LISTS

---

7.1: How can I write a function that takes a variable number of arguments?

Use the <stdarg.h> header (or, if you must, the older <varargs.h>).

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(char *first, ...)

               {

                               size_t len = 0;

                               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);

                               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.  (Also note that the caller must free the returned, malloc'ed storage.)

Under a pre-ANSI compiler, rewrite the function definition without a prototype ("char *vstrcat(first) char *first;{"), include <stdio.h> rather than <stdlib.h>, add "extern char *malloc();", and use intinstead of size_t.  You may also have to delete the (void) casts, and use the older varargs package instead of stdarg.  See the next question for hints.

Remember that in variable-length argument lists, function prototypes do not supply parameter type information; therefore, default argument promotions apply (see question 5.8), and null pointer arguments must be typed explicitly (see question 1.2).

References: K&R II Sec. 7.3 p. 155, Sec. B7 p. 254; H&S Sec. 13.4 pp. 286-9; ANSI Secs. 4.8 through 4.8.1.3 .

7.2: 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?

Use vprintf, vfprintf, or vsprintf.

Here is an "error" routine which prints an error message, preceded by the string "error: " and terminated with a newline:

               #include <stdio.h>

               #include <stdarg.h>

               void

               error(char *fmt, ...)

               {

                               va_list argp;

                               fprintf(stderr, "error: ");

                               va_start(argp, fmt);

                               vfprintf(stderr, fmt, argp);

                               va_end(argp);

                               fprintf(stderr, "\n");

               }

To use the older <varargs.h> package, instead of <stdarg.h>, change the function header to:

               void error(va_alist)

               va_dcl

               {

                               char *fmt;

change the va_start line to

               va_start(argp);

and add the line

               fmt = va_arg(argp, char *);

between the calls to va_start and vfprintf.  (Note that there is no semicolon after va_dcl.)

References: K&R II Sec. 8.3 p. 174, Sec. B1.2 p. 245; H&S Sec. 17.12 p. 337; ANSI Secs. 4.9.6.7, 4.9.6.8, 4.9.6.9 .

7.3: How can I discover how many arguments a function was actually called with?

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 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 (useful 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 theexecl and vstrcat examples under questions 1.2 and 7.1 above).

7.4: I can't get the va_arg macro to pull in an argument of type pointer-to-function.

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.

References: ANSI Sec. 4.8.1.2 p. 124.

7.5: 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)?

In general, you cannot.  You must provide a version of that other function which accepts a va_list pointer, as does vfprintf in the example above.  If the arguments must be passed directly as actual arguments (not indirectly through a va_listpointer) to another function which is itself variadic (for which you do not have the option of creating an alternate, va_list-accepting version) no portable solution is possible.  (The problem can be solved by resorting to machine-specific assembly language.)

7.6: How can I call a function with an argument list built up at run time?

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...  (See also question 16.11.)

---

SECTION 8. BOOLEAN EXPRESSIONS AND VARIABLES

---

8.1: What is the right type to use for boolean values in C?  Why isn't it a standard type?  Should #defines or enums be used for the true and false values?

C does not provide a standard boolean type, because picking one involves a space/time tradeoff which is best decided by the programmer.  (Using an int for a boolean may be faster, while using char may save data space.)

The choice between #defines and enums is arbitrary and not terribly interesting (see also question 9.1). 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 enum may be preferable if your debugger expands enum values 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 8.2 below; see also question 1.6).

8.2: Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered "true" in C?  What if a built-in boolean or relational operator "returns" something other than 1?

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)

will work as expected (as long as TRUE is 1), but it is obviously silly.  In general, explicit tests against TRUEand FALSE are undesirable, because some library functions (notably isupper, isalpha, etc.) return, on success, a nonzero value which is notnecessarily 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 are used for code readability, not because the underlying values might ever change.  (See also questions 1.7 and 1.9.)

References: K&R I Sec. 2.7 p. 41; K&R II Sec. 2.6 p. 42, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206; ANSI Secs. 3.3.3.3, 3.3.8, 3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, 3.6.5; Achilles and the Tortoise.

---

SECTION 9. STRUCTS, ENUMS, AND UNIONS

---

9.1: What is the difference between an enum and a series of preprocessor #defines?

At the present time, there is little difference.  Although many people might have wished otherwise, the ANSI standard says that enumerations may be freely intermixed with integral types, without errors.  (If such intermixing were disallowed without explicit casts, judicious use of enums could catch certain programming errors.)

Some advantages of enums are that the numeric values are automatically assigned, that a debugger may be able to display the symbolic values when enum variables are examined, and that they obey block scope.  (A compiler may also generate nonfatal warnings when enums and ints are indiscriminately mixed, since doing so can still be considered bad style even though it is not strictly illegal).  A disadvantage is that the programmer has little control over the size (or over those nonfatal warnings).

References: K&R II Sec. 2.3 p. 39, Sec. A4.2 p. 196; H&S Sec. 5.5 p. 100; ANSI Secs. 3.1.2.5, 3.5.2, 3.5.2.2 .

9.2: I heard that structures could be assigned to variables and passed to and from functions, but K&R I says not.

What K&R I said was that the restrictions on struct operations would be lifted in a forthcoming version of the compiler, and in fact struct assignment and passing were fully functional in Ritchie's compiler even as K&R I was being published. Although a few early C compilers lacked struct assignment, all modern compilers support it, and it is part of the ANSI C standard, so there should be no reluctance to use it.

References: K&R I Sec. 6.2 p. 121; K&R II Sec. 6.2 p. 129; H&S Sec. 5.6.2 p. 103; ANSI Secs. 3.1.2.5, 3.2.2.1, 3.3.16 .

9.3: How does struct passing and returning work?

When structures are passed as arguments to functions, the entire struct is typically pushed on the stack, using as many words as are required.  (Programmers often choose to use pointers to structures instead, precisely to avoid this overhead.)

Structures are often returned from functions in a location pointed to by an extra, compiler-supplied "hidden" argument to the function.  Some older compilers used a special, static location for structure returns, although this made struct-valued functions nonreentrant, which ANSI C disallows.

References: ANSI Sec. 2.2.3 p. 13.

9.4: The following 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 missing semicolon causes the compiler to believe that main returns a structure.  (The connection is hard to see because of the intervening comment.)  Since struct-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 question 17.21.

References: CT&P Sec. 2.3 pp. 21-2.

9.5: Why can't you compare structs?

There is no reasonable way for a compiler to implement struct comparison which is consistent with C's low-level flavor.  A byte-by-byte comparison could be invalidated by random bits present in unused "holes" in the structure (such padding is used to keep the alignment of later fields correct; see questions 9.10 and 9.11).  A field-by- field comparison would require unacceptable amounts of repetitive, in-line code for large structures.

If you want to compare two structures, you must write your own function to do so.  C++ would let you arrange for the == operator to map to your function.

References: K&R II Sec. 6.2 p. 129; H&S Sec. 5.6.2 p. 103; ANSI Rationale Sec. 3.3.9 p. 47.

9.6: How can I read/write structs from/to data files?

It is relatively straightforward to write a struct out using fwrite:

                fwrite((char *)&somestruct, sizeof(somestruct), 1, fp);

and a corresponding fread invocation can read it back in. However, data files so written will not be very portable (see questions 9.11 and 17.3). Note also that on many systems you must use the "b" flag when fopening the files.

9.7: I came across some code that declared a structure like this:

               struct name

                               {

                               int namelen;

                               char name[1];

                               };

and then did some tricky allocation to make the name array act like it had several elements.  Is this legal and/or portable?

This technique is popular, although Dennis Ritchie has called it "unwarranted chumminess with the C implementation."  An ANSI Interpretation Ruling has deemed it (more precisely, access beyond the declared size of the name field) to be not strictly conforming, although a thorough treatment of the arguments surrounding the legality of the technique is beyond the scope of this list.  It seems, however, to be portable to all known implementations.  (Compilers which check array bounds carefully might issue warnings.)

To be on the safe side, it may be preferable to declare the variable-size element very large, rather than very small; in the case of the above example:

                ...

                char name[MAXSIZE];

                ...

where MAXSIZE is larger than any name which will be stored. (The trick so modified is said to be in conformance with the Standard.)

References: ANSI Rationale Sec. 3.5.4.2 pp. 54-5.

9.8: How can I determine the byte offset of a field within a structure?

ANSI C defines the offsetof macro, which should be used if available; see <stddef.h>.  If you don't have it, a suggested implementation is

               #define offsetof(type, mem) ((size_t) \

                               ((char *)&((type *) 0)->mem - (char *)((type *) 0)))

This implementation is not 100% portable; some compilers may legitimately refuse to accept it.

See the next question for a usage hint.

References: ANSI Sec. 4.1.5, Rationale Sec. 3.5.4.2 p. 55.

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

Build a table of names and offsets, using the offsetof()macro. The offset of field b in struct a is

               offsetb = offsetof(struct a, b)

If structp is a pointer to an instance of this structure, and b is an int field with offset as computed above, b's value can be set indirectly with

               *(int *)((char *)structp + offsetb) = value;

9.10: Why does sizeofreport a larger size than I expect for a structure type, as if there was padding at the end?

Structures may have this padding (as well as internal padding; see also question 9.5), so that alignment properties will be preserved when an array of contiguous structures is allocated.

9.11: My compiler is leaving holes in structures, which is wasting space and preventing "binary" I/O to external data files.  Can I turn off the padding, or otherwise control the alignment of structs?

Your compiler may provide an extension to give you this control (perhaps a #pragma), but there is no standard method.  See also question 17.3.

9.12: Can I initialize unions?

ANSI Standard C allows an initializer for the first member of a union.  There is no standard way of initializing the other members (nor, under a pre-ANSI compiler, is there generally any way of initializing any of them).

9.13: How can I pass constant values to routines which accept struct arguments?

C has no way of generating anonymous struct values.  You will have to use a temporary struct variable.

---

SECTION 10. DECLARATIONS

---

10.1: How do you decide which integer type to use?

If you might need large values (above 32767 or below -32767), use long.  Otherwise, if space is very important (there are large arrays or many structures), use short.  Otherwise, use int.  If well-defined overflow characteristics are important and/or negative values are not, use the corresponding unsignedtypes.  (But beware of mixing signed and unsigned in expressions.)  Similar arguments apply when deciding between float and double.

Although char or unsigned char can be used as a "tiny" int type, doing so is often more trouble than it's worth, due to unpredictable sign extension and increased code size.

These rules obviously don't apply if the address of a variable is taken and 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 17.3), be sure to encapsulate the choice behind an appropriate typedef.

10.2: What should the 64-bit type on new, 64-bit machines be?

Some vendors of C products for 64-bit machines support 64-bit long ints.  Others fear that too much existing code depends on sizeof(int)== sizeof(long) == 32 bits, and introduce a new 64-bit long long int (or__longlong)  type instead.

Programmers interested in writing portable code should therefore insulate their 64-bit type needs behind appropriate typedefs. Vendors who feel compelled to introduce a new, longer integral type should advertise it as being "at least 64 bits" (which is truly new; a type traditional C doesn't have), and not "exactly 64 bits."

10.3: 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 struct in C contain a pointer to itself?

Structs 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 this example is that the NODEPTR typedef is not complete at the point where the "next" field is declared.  To fix it, first give the structure a tag ("struct node").  Then, declare the "next" field as "struct node *next;", and/or move the typedef declaration wholly before or wholly after the struct declaration.  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.

References: K&R I Sec. 6.5 p. 101; K&R II Sec. 6.5 p. 139; H&S Sec. 5.6.1 p. 102; ANSI Sec. 3.5.2.3 .

10.4: How do I declare an array of N pointers to functions returning pointers to functions returning pointers to characters?

This question can be answered in at least three ways:

1. char *(*(*a[N])())();
2. Build the declaration up in stages, 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 arguments go in (for complicated function definitions, like the above). Versions of cdecl are in volume 14 of comp.sources.unix (see question 17.12) and K&R II.

Any good book on C should explain how to read these complicated C declarations "inside out" to understand them ("declaration mimics use").

References: K&R II Sec. 5.12 p. 122; H&S Sec. 5.10.1 p. 116.

10.5: I'm building a state machine with a bunch of functions, one for each state.  I want to implement state transitions by having each function return a pointer to the next state function. I find a limitation in C's declaration mechanism: there's no way to declare these functions as returning a pointer to a function returning a pointer to a function returning a pointer to a function...

You can't do it directly.  Either have the function return a generic function pointer type, and apply a cast before calling through it; or have it return a structure containing only a pointer to a function returning that structure.

10.6: My compiler is complaining about an invalid redeclaration of a function, but I only define it once and call it once.

Functions which are called without a declaration in scope (or before they are declared) are assumed to be declared as returning int, leading to discrepancies if the function is later declared otherwise.  Non-int functions must be declared before they are called.

References: K&R I Sec. 4.2 pp. 70; K&R II Sec. 4.2 p. 72; ANSI Sec. 3.3.2.2 .

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

First, though there can be many declarations(and in many translation units) of a single "global" (strictly speaking, "external") variable (or function), there must be exactly one definition.  (The definition is the declaration that actually allocates space, and provides an initialization value, if any.) It is best to place the definition in some central (to the program, or to the module) .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 header file containing the external declaration, so that the compiler can check that the declarations match.

This rule promotes a high degree of portability, and is consistent with the requirements of the ANSI C Standard.  Note that Unix compilers and linkers typically use a "common model" which allows multiple (uninitialized) definitions.  A few very odd systems may require an explicit initializer to distinguish a definition from an external declaration.

It is possible to use preprocessor tricks to arrange that the declaration need only be typed once, in the header file, and "turned into" a definition, during exactly one #inclusion, via a special #define.

References: K&R I Sec. 4.5 pp. 76-7; K&R II Sec. 4.4 pp. 80-1; ANSI Sec. 3.1.2.2 (esp. Rationale), Secs. 3.7, 3.7.2, Sec. F.5.11; H&S Sec. 4.8 pp. 79-80; CT&P Sec. 4.2 pp. 54-56.

10.8: What does extern mean in a function declaration?

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: ANSI Sec. 3.1.2.2 .

10.9: I finally figured out the syntax for declaring pointers to functions, but now how do I initialize one?

Use something like

               extern int func();

               int (*fp)() = func;

When the name of a function appears in an expression but is not being called (i.e. is not followed by a "("), it "decays" into a pointer (i.e. it has its address implicitly taken), much as an array name does.

An explicit extern declaration for the function is normally needed, since implicit external function declaration does not happen in this case (again, because the function name is not followed by a "(").

10.10: I've seen different methods used for calling through pointers to functions. What's the story?

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 through pointers, but that "real" functions decay implicitly into pointers (in expressions, as they do in initializations) and so cause no trouble.  This reasoning, made widespread through pcc and adopted in the ANSI standard, means that

               r = fp();

is legal and works correctly, whether fp is 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 through it.)  An explicit * is harmless, and still allowed (and recommended, if portability to older compilers is important).

References: ANSI Sec. 3.3.2.2 p. 41, Rationale p. 41.

10.11: What's the auto keyword good for?

Nothing; it's obsolete.

---

SECTION 11. STDIO

---

11.1: What's wrong with this code:

               char c;

               while((c = getchar()) != EOF)...

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 passing getchar's return value through 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: CT&P Sec. 5.1 p. 70.

11.2: How can I print a'%' character in a printf format string?  I tried \%, but it didn't work.

Simply double the percent sign: %% .

References: K&R I Sec. 7.3 p. 147; K&R II Sec. 7.2 p. 154; ANSI Sec. 4.9.6.1 .

11.3: Why doesn't the code scanf("%d", i); work? scanf needs pointers to the variables it is to fill in; you must call scanf("%d", &i);

11.4: Why doesn't this code:

               double d;

               scanf("%f", &d);

work?

scanf uses %lf for values of type double,  and %f for float. (Note the discrepancy with printf , which uses %f for both double and float,  due to C's default argument promotion rules.)

11.5: Why won't the code

               while(!feof(infp)) {

                               fgets(buf, MAXLINE, infp);

                               fputs(buf, outfp);

               }

work?

C's I/O is not like Pascal's.  EOF is only indicated after an input routine has tried to read, and has reached end-of-file. Usually, you should just check the return value of the input routine (fgets in this case); often, you don't need to use feof() at all.

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

It cannot be told the size of the buffer it's to read into, so it cannot be prevented from overflowing that buffer.  See question 3.1 for a code fragment illustrating the replacement of gets() with fgets().

11.7: Why does errno contain ENOTTY after a call to printf?

Many implementations of the stdio package adjust their behavior slightly if stdout is a terminal.  To make the determination, these implementations perform an operation which fails (with ENOTTY) if stdout is not a terminal.  Although the output operation goes on to complete successfully, errno still contains ENOTTY.

References: CT&P Sec. 5.4 p. 73.

11.8: My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program.

It is best to use an explicit fflush(stdout) whenever output should definitely be visible.  Several mechanisms attempt to perform the fflush for you, at the "right time," but they tend to apply only whenstdout is a terminal.  (See question 11.7.)

11.9: When I read from the keyboard with scanf, it seems to hang until I type one extra line of input.

scanf was designed for free-format input, which is seldom what you want when reading from the keyboard.  In particular, "\n" in a format string does notmean to expect a newline, but rather to read and discard characters as long as each is a whitespace character.

A related problem is that unexpected non-numeric input can cause scanf to "jam."  Because of these problems, it is usually better to use fgets to read a whole line, and then use sscanf or other string functions to pick apart the line buffer. If you do use sscanf, don't forget to check the return value to make sure that the expected number of items were found.

11.10: I'm trying to update a file in place, by using fopen mode "r+", then reading a certain string, and finally writing back a modified string, but it's not working.

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; see also question 17.4.

References: ANSI Sec. 4.9.5.3 p. 131.

11.11: How can I read one character at a time, without waiting for the RETURN key?

See question 16.1.

11.12: How can I flush pending input so that a user's typeahead isn't read at the next prompt?  Will fflush(stdin) work?

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 forfflush on input streams.  There is no standard way to discard unread characters from a stdio input buffer, nor would such a way be sufficient; unread characters can also accumulate in other, OS-level input buffers.

11.13: How can I redirect stdin or stdout to a file from within a program?

Use freopen.

11.14: Once I've used freopen, how can I get the original stdout (or stdin) back?

If you need to switch back and forth, the best all-around solution is not to use 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.

11.15: How can I recover the file name given an open file descriptor?

This problem is, in general, insoluble.  Under Unix, for instance, a scan of the entire disk, (perhaps requiring special permissions) would theoretically be required, and would fail if the file descriptor was 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 when you open them (perhaps with a wrapper function aroundfopen).

---

Section 12. Library Subroutines

---

12.1: Why does strncpy not always place a '\0' termination in the destination string?

strncpy was first designed to handle a now-obsolete data structure, the fixed-length, not-necessarily-\0-terminated "string."  strncpy is admittedly a bit cumbersome to use in other contexts, since you must often append a '\0' to the destination string by hand.

12.2: I'm trying to sort an array of strings with qsort, using strcmp as the comparison function, but it's not working.

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, of course, accepts simple pointers to char.)

The comparison routine's arguments are expressed as "generic pointers," const void * or char *.  They must be converted back to what they "really are" (char **) and dereferenced, yielding char *'s which can be usefully compared.  Write a comparison function like this:

               int pstrcmp(p1, p2)   /* compare strings through pointers */

               char *p1, *p2;                          /* const void * for ANSI C */

               {

                               return strcmp(*(char **)p1, *(char **)p2);

               }

Beware of the discussion in K&R II Sec. 5.11 pp. 119-20, which is not discussing Standard library qsort.

12.3: Now I'm trying to sort an array of structures with qsort.  My comparison routine 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?

The conversions must be in the comparison function, which must be declared as accepting "generic pointers" (const void * or char *) as discussed in question 12.2 above. The code might look like

        int mystructcmp(p1, p2)

        char *p1, *p2;          /* const void * for ANSI C */

        {

                struct mystruct *sp1 = (struct mystruct *)p1;

                struct mystruct *sp2 = (struct mystruct *)p2;

                /* now compare sp1->whatever and sp2-> ... */

        }

(If, on the other hand, you're sorting pointers to structures, you'll need indirection, as in question 12.2:

        sp1 = *(struct mystruct **)p1 .)

12.4: How can I convert numbers to strings (the opposite of atoi)?  Is there anitoafunction?

Just use sprintf.  (You'll have to allocate space for the result somewhere anyway; see questions 3.1 and 3.2. Don't worry that sprintf may be overkill, potentially wasting run time or code space; it works well in practice.)

References: K&R I Sec. 3.6 p. 60; K&R II Sec. 3.6 p. 64.

12.5: How can I get the current date or time of day in a C program?

Just use the time, ctime, and/or localtime functions. (These routines have been around for years, and are in the ANSI standard.)  Here is a simple example:

               #include <stdio.h>

               #include <time.h>

               main()

               {

                               time_t now = time((time_t *)NULL);

                               printf("It's %.24s.\n", ctime(&now));

                               return 0;

               }

References: ANSI Sec. 4.12 .

12.6: I know that the library routine localtime will convert a time_t into a broken-downstruct 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?

ANSI C specifies a library routine, mktime, which converts a struct tm to a time_t.  Several public-domain versions of this routine are available in case your compiler does not support it yet.

Converting a string to a time_t is harder, because of the wide variety of date and time formats which should be parsed.  Some systems provide a strptime function; another popular routine is partime (widely distributed with the RCS package), but these are less likely to become standardized.

References: K&R II Sec. B10 p. 256; H&S Sec. 20.4 p. 361; ANSI Sec. 4.12.2.3 .

12.7: How can I add n days to a date?  How can I find the difference between two dates?

The ANSI/ISO Standard C mktime and difftimefunctions provide some 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 convert to atime_tvalue).  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.  (Note, however, that these solutions only work for dates in the range which can be represented as time_t's, and that not all days are 86400 seconds long.)  See also questions 12.6 and 17.28.

References: K&R II Sec. B10 p. 256; H&S Secs. 20.4, 20.5 pp. 361-362; ANSI Secs. 4.12.2.2, 4.12.2.3 .

12.8: I need a random number generator.

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.

References: ANSI Sec. 4.10.2.1 p. 154; Knuth Vol. 2 Chap. 3 pp. 1-177.

12.9: How can I get random integers in a certain range?

The obvious way,

        rand() % N

(where N is of course the range) is poor, because the low-order bits of many random number generators are distressingly non-random.  (See question 12.11.)  A better method is something like

        (int)((double)rand() / ((double)RAND_MAX + 1) * N)

If you're worried about using floating point, you could try

        rand() / (RAND_MAX / N + 1)

Both methods obviously require knowingRAND_MAX (which ANSI defines in <stdlib.h>), and assume that Nis much less than RAND_MAX.

12.10: Each time I run my program, I get the same sequence of numbers back fromrand().

You can call srand() to seed the pseudo-random number generator with a more random initial value. Popular seed values are the time of day, or the elapsed time before the user presses a key (although keypress times are hard to determine portably; see question 16.10).

References: ANSI Sec. 4.10.2.2 p. 154.

12.11: I need a random true/false value, so I'm taking rand() % 2,  but it's just alternating 0, 1, 0, 1, 0...

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 12.9.

12.12 I'm trying to port this old program.  Why do I get "undefined external" errors for ..:

Those routines are variously obsolete; you should instead...:

index?

use strchr.

rindex?

use strrchr.

bcopy?

use memmove, after interchanging the first and second arguments (see also question 5.15).

bcmp?

use memcmp.

bzero?

use memset, with a second argument of 0.

12.13: I keep getting errors due to library routines being undefined, but I'm #including all the right header files.

In some cases (especially if the routines are nonstandard) you may have to explicitly ask for the correct libraries to be searched when you link the program.  See also question 15.2.

12.14: I'm still getting errors due to library routines being undefined, even though I'm using -l to request the libraries while linking.

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 switches towards the end of the command line.)

12.15: I need some code to do regular expression matching.

Look for the regexp library (supplied with many Unix systems), or get Henry Spencer's regexp package from ftp.cs.toronto.edu in pub/regexp.shar.Z (see also question 17.12).

12.16: How can I split up a command line into whitespace-separated arguments, likemain's argc and argv?

Most systems have a routine called strtok, although it can be tricky to use and it may not do everything you want it to (e.g., quoting).

References: ANSI Sec. 4.11.5.8; K&R II Sec. B3 p. 250; H&S Sec. 15.7; PCS p. 178.

---

SECTION 13. LINT

---

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

Try running lint first (perhaps with the -a, -c, -h, -p and/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.

13.2: How can I shut off the "warning: possible pointer alignment problem" message lint gives me for each call to malloc?

The problem is that traditional versions of lint do not know, and cannot be told, that malloc "returns a pointer to space suitably aligned for storage of any type of object."  It is possible to provide a pseudoimplementation of malloc, using a #define inside of #ifdef lint, which effectively shuts this warning off, but a simpleminded #definition will also suppress meaningful messages about truly incorrect invocations.  It may be easier simply to ignore the message, perhaps in an automated way with grep -v.

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

A product called FlexeLint is available (in "shrouded source form," for compilation on 'most any system) from

Gimpel Software
3207 Hogarth Lane
Collegeville, PA  19426 USA
(+1) 610 584 4261
gimpel@netaxs.com

The 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 ANSI­compatible lint (which can also perform higher­level formal verification) is LCLint, available via anonymous ftp from larch.lcs.mit.edu://pub/Larch/lclint/.

In the absence of lint, many modern compilers attempt to diagnose almost as many problems as a good lint does.

---

SECTION 14. STYLE

---

14.1: Here's a neat trick:

               if(!strcmp(s1, s2))

Is this good style?

It is not particularly good style, although it is a popular idiom.  The test succeeds if the two strings are equal, but its form suggests that it tests for inequality.

Another solution is to use a macro:

               #define Streq(s1, s2) (strcmp((s1), (s2)) == 0)

Opinions on code style, like those on religion, can be debated endlessly.  Though good style is a worthy goal, and can usually be recognized, it cannot be codified.

14.2: What's the best style for code layout in C?

K&R, while providing the example most often copied, also supply a good excuse for avoiding 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.

References: K&R Sec. 1.2 p. 10.

14.3: Where can I get the "Indian Hill Style Guide" and other coding standards?

Various documents are available for anonymous ftp from:

• cs.washington.edu (128.95.1.4), as pub/cstyle.tar.Z
  (the updated Indian Hill guide)
• ftp.cs.toronto.edu, in directory doc/programming
• ftp.cs.umd.edu, in directory pub/style-guide

---

SECTION 15. FLOATING POINT

---

15.1: My floating-point calculations are acting strangely and giving me different answers on different machines.

First, make sure that you have #included <math.h>, and correctly declared other functions returning double.

If the problem isn't that simple, recall that most digital computers 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.)

These problems are no worse for C than they are for any other computer language.  Floating-point semantics are usually defined as "however the processor does them;" 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 programming text should cover the basics.

References: EoPS Sec. 6 pp. 115-8.

15.2: I'm trying to do some simple trig, and I am #including <math.h>;, but I keep getting "undefined: _sin" compilation errors.

Make sure you're linking with the correct math library.  For instance, under Unix, you usually need to use the -lm option, and at the endof the command line, when compiling/linking. See also question 12.14.

15.3: Why doesn't C have an exponentiation operator?

Because few processors have an exponentiation instruction. Instead, you can #include <math.h> and use thepow()function, although explicit multiplication is often better for small positive integral exponents.

References: ANSI Sec. 4.5.5.1 .

15.4: How do I round numbers?

The simplest and most straightforward way is with code like

        (int)(x + 0.5)

This won't work properly for negative numbers, though.

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

Many systems with high-quality IEEE floating-point implementations provide facilities (e.g. an isnan() macro) to deal with these values cleanly, and the Numerical C Extensions Group (NCEG) is working 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.

15.6: I'm having trouble with a Turbo C program which crashes and says something like "floating point formats not linked."

Some compilers for small machines, including Turbo C (and Ritchie's original PDP-11 compiler), leave out 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 Turbo C's heuristics for determining whether the program uses floating point are insufficient, and the programmer must sometimes insert an extra, explicit call to a floating-point library routine to force loading of floating-point support.

---

SECTION 16. SYSTEM DEPENDENCIES

---

16.1: How can I read a single character from the keyboard without waiting for a newline?

Contrary to popular belief and many people's wishes, this is not a C-related question.  (Nor are closely-related questions concerning the echo of keyboard input.)  The delivery of characters from a "keyboard" to a C program is a function of the operating system in use, and has not been standardized by the C language. Some versions of curses have a cbreak() function which does what you want. If you're specifically trying to read a short password without echo, you might try getpass().  Under Unix, 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). Under MS-DOS, use getch(). Under VMS, try the Screen Management (SMG$) routines, or curses, or issue low-level $QIO's with the IO$_READVBLK (and perhapsIO$M_NOECHO) function codes to ask for one character at a time. Under other operating systems, you're on your own. Beware that some operating systems make this sort of thing impossible, because character collection into input lines is done by peripheral processors not under direct control of the CPU running your program.

Operating system specific questions are not appropriate for comp.lang.c .  Many common questions are answered in frequently-asked questions postings in such groups as comp.unix.questions and comp.os.msdos.programmer .  Note that the answers are often not unique even across different variants of a system; bear in mind when answering system-specific questions that the answer that applies to your system may not apply to everyone else's.

References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1.

16.2: 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?

These, too, are entirely operating-system-specific.  Some versions of curses have anodelay() function. Depending on your system, you may also be able to use "nonblocking I/O", or a system call named "select", or the FIONREAD ioctl, or kbhit(), or rdchk(), or the O_NDELAY option toopen() or fcntl().

16.3: How can I clear the screen?  How can I print things in inverse video?

Such things depend on the terminal type (or display) you're using.  You will have to use a library such astermcap or curses, or some system-specific routines, to perform these functions.

16.4: How do I read the mouse?

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, Macintosh, and probably every other system.

16.5: How can my program discover the complete pathname to the executable file from which it was invoked?

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 or portable solution.

16.6: How can a process change an environment variable in its caller?

In general, it cannot.  Different operating systems implement 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() and/orputenv()functions to do this), and the modified environment is usually passed on to any child processes, but it is notpropagated back to the parent process.

16.7: How can I check whether a file exists?  I want to query the user before overwriting existing files.

On Unix-like systems, you can try the access() routine, although it's got a few problems.  (It isn't atomic with respect to the following action, and can have anomalies if used in setuid programs.)  Another option (perhaps preferable) is to call stat() on the file.  Otherwise, the only guaranteed and portable way to test for file existence is to try opening the file (which doesn't help if you're trying to avoid overwriting an existing file, unless you've got something like the BSD Unix O_EXCL open option available).

16.8: How can I find out the size of a file, prior to reading it in?

If the "size of a file" is the number of characters you'll be able to read from it in C, it is in general impossible to determine this number in advance.  Under Unix, thestat call will give you an exact answer, and several other systems supply a Unix-like stat which will give an approximate answer. You can fseek to the end and then use ftell, but this usage is nonportable (it gives you an accurate answer only under Unix, and otherwise a quasi-accurate answer only for ANSI C "binary" files).  Some systems provide routines calledfilesize or filelength.

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.

16.9: How can a file be shortened in-place without completely clearing or rewriting it?

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 usewrite(fd, "", 0). However, there is no truly portable solution.

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

Unfortunately, there is no portable way.  V7 Unix, and derived systems, provided a fairly useful ftime()routine with resolution up to a millisecond, but it has disappeared from System V and Posix.  Other routines you might look for on your system include nap(), setitimer(), msleep(), usleep(), clock(), andgettimeofday().  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.

16.11: How can I read in an object file and jump to routines in it?

You want a dynamic linker and/or loader.  It is 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 usesystem() and ld -A to do the linking for you.  Many (most?) versions of SunOS and System V have the -ldllibrary which allows object files to be dynamically loaded. There is also a GNU package called "dld". See also question 7.6.

16.12: How can I invoke an operating system command from within a program?

Use system().

References: K&R II Sec. B6 p. 253; ANSI Sec. 4.10.4.5; H&S Sec. 21.2; PCS Sec. 11 p. 179;

16.13: How can I invoke an operating system command and trap its output?

Unix and some other systems provide a popen() routine, 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). Alternately, invoke the command simply (see question 16.12) in such a way that it writes its output to a file, then open and read that file.

References: PCS Sec. 11 p. 169 .

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

See if you can use the opendir() and readdir() routines, which are available on most Unix systems. Implementations also exist for MS-DOS, VMS, and other systems.  (MS-DOS also has FINDFIRST and FINDNEXTroutines which do essentially the same thing.)

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

It's system-dependent.  Under Unix, you typically open, read, and write a device in /dev, and use the facilities of the terminal driver to adjust its characteristics.  Under MS-DOS, you can either use some primitive BIOS interrupts, or (if you require decent performance) one of any number of interrupt-driven serial I/O packages.

---

SECTION 17. MISCELLANEOUS

---

17.1: What can I safely assume about the initial values of variables which are not explicitly initialized?  If global variables start out as "zero," is that good enough for null pointers and floating-point zeroes?

Variables with "static" duration (that is, those declared outside of functions, and those declared with the storage class static), are guaranteed initialized to zero, as if the programmer had typed "= 0".  Therefore, such variables are initialized to the null pointer (of the correct type; see also Section 1) 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.

Dynamically-allocated memory obtained with malloc and realloc is also likely to contain garbage, and must be initialized by the calling program, as appropriate.  Memory obtained with calloc contains all-bits-0, but this is not necessarily useful for pointer or floating-point values (see question 3.13, and section 1).

17.2: This code, straight out of a book, isn't compiling:

        f()

        {

        char a[] = "Hello, world!";

        }

Perhaps you have a pre-ANSI compiler, which doesn't allow initialization of "automatic aggregates" (i.e. non-static local arrays and structures).  As a workaround, you can make the array global or static, and initialize it with strcpy when f is called.  (You can always initialize local char * variables with string literals, but see question 17.20).  See also questions 5.16 and 5.17.

17.3: How can I write data files which can be read on other machines with different word size, byte order, or floating point formats?

The best 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 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, CCITT's X.409, or ISO 8825 "Basic Encoding Rules." See also question 9.11.

17.4: How can I insert or delete a line (or record) from the middle of a file?

Short of rewriting the file, you probably can't.  See also question 16.9.

17.5: How can I return several values from a function?

Either pass pointers to locations which the function can fill in, or have the function return a structure containing the desired values, or (in a pinch) consider global variables.  See also questions 2.17, 3.4, and 9.2.

17.6: If I have a char * variable pointing to the name of a function as a string, how can I call that function?

The most straightforward thing to do is maintain a correspondence table of names and function pointers:

               int function1(), function2();

               struct {char *name; int (*funcptr)(); } symtab[] =

                               {

                               "function1",              function1,

                               "function2",              function2,

                               };

Then, just search the table for the name, and call through the associated function pointer.  See also questions 9.9 and 16.11.

17.7: I seem to be missing the system header file <sgtty.h>.  Can someone send me a copy?

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.  Ask your compiler vendor why the file was not provided (or to send a replacement copy).

17.8: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions from C?  (And vice versa?)

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.  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.

cfortran.h, a C header file, simplifies C/FORTRAN interfacing on many popular machines. It is available via anonymous ftp from zebra.desy.de (131.169.2.244).

In C++, a "C" modifier in an external function declaration indicates that the function is to be called using C calling conventions.

17.9: Does anyone know of a program for converting Pascal or FORTRAN (or LISP, Ada, awk, "Old" C, ...) to C?

Several public-domain 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 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 other commercial translation products, and some for more obscure languages.

See also question 5.3.

17.10: Is C++ a superset of C? Can I use a C++ compiler to compile C code?

C++ was derived from C, and is largely based on it, but there are some legal C constructs which are not legal C++.  (Many C programs will nevertheless compile correctly in a C++ environment.)

17.11: I need...

Look for programs (see also question 17.12) named:

...a C cross-reference generator

cflow, calls, cscope

...a C beautifier/pretty-printer

cb, indent

17.12: Where can I get copies of all these public-domain programs?

If you have access to Usenet, see the regular postings in the comp.sources.unix and comp.sources.misc newsgroups, which describe, in some detail, the archiving policies and how to retrieve copies.  The usual approach is to use anonymous ftp and/or uucp from a central, public-spirited site, such as uunet (ftp.uu.net, 192.48.96.9).  However, this article cannot track or list all of the available archive sites and how to access them.

Ajay Shah maintains an index of free numerical software; it is posted periodically, and available where this FAQ list is archived (see question 17.33).  The comp.archives newsgroup contains numerous announcements of anonymous ftp availability of various items.  The "archie" mailserver can tell you which anonymous ftp sites have which packages; send the mail message "help" to archie@quiche.cs.mcgill.ca for information. 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.

17.13: 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?

The contest schedule is tied to the dates of the USENIX conferences at which the winners are announced. At the time of this writing, it is expected that the yearly contest will open in October. To obtain a current copy of the rules and guidelines, send e-mail with the Subject: line "send rules" to:

{apple,pyramid,sun,uunet}!hoptoad!judges  or
judges@toad.com

(Note that these are not the addresses for submitting entries.)

Contest winners should be announced at the winter USENIX conference in January, and are posted to the net sometime thereafter. Winning entries from previous years (to 1984) are archived at uunet (see question 17.12) under the directory ~/pub/ioccc.

As a last resort, previous winners may be obtained by sending e-mail to the above address, using the Subject: "send YEAR winners", where YEAR is a single four-digit year, a year range, or "all".

17.14: Why don't C comments nest?  How am I supposed to comment out code containing comments?  Are comments legal inside quoted strings?

Nested comments would cause more harm than good, mostly because of the possibility of accidentally leaving comments unclosed by including the characters "/*" within them.  For this reason, it is usually better to "comment out" large sections of code, which might contain comments, with#ifdef or #if 0 (but see question 5.11).

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.

References: ANSI Appendix E p. 198, Rationale Sec. 3.1.9 p. 33.

17.15: How can I get the ASCII value corresponding to a character, or vice versa?

In C, characters are represented by small integers corresponding to their values (in the machine's character set) so you don't need a conversion routine: if you have the character, you have its value.

17.16: How can I implement sets and/or arrays of bits?

Use arrays of char or int, with a few macros to access the right bit at the right index (try using 8 forCHAR_BIT if you don't have <limits.h>):

               #include <limits.h>                  /* for CHAR_BIT */

               #define BITMASK(bit) (1 << ((bit) % CHAR_BIT))

               #define BITSLOT(bit) ((bit) / CHAR_BIT)

               #define BITSET(ary, bit) ((ary)[BITSLOT(bit)] |= BITMASK(bit))

               #define BITTEST(ary, bit) ((ary)[BITSLOT(bit)] & BITMASK(bit))

17.17: What is the most efficient way to count the number of bits which are set in a value?

This and many other similar bit-twiddling problems can often be sped up and streamlined using lookup tables (but see the next question).

17.18: How can I make this code more efficient?

Efficiency, though a favorite comp.lang.c topic, is not important nearly as often 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 far 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 even "inefficient" code can 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.

For the small fraction of code that is time-critical, it is vital to pick a good algorithm; it is less important to "microoptimize" 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 degraded.

For more discussion of efficiency tradeoffs, as well as good advice on how to increase 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.

17.19: Are pointers really faster than arrays?  How much do function calls slow things down?  Is ++i faster than i = i + 1 ?

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.)

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 C compiler, not a keystroke-programmable calculator.  Any decent compiler will generate identical code for ++i, i += 1, andi = i + 1.  The reasons for using ++i or i += 1 over i = i + 1 have to do with style, not efficiency.  (See also question 4.7.)

17.20: Why does this code:

        char *p = "Hello, world!";

        p[0] = tolower(p[0]);

crash?

String literals are not necessarily modifiable, except (in effect) when they are used as array initializers. Try

        char a[] = "Hello, world!";

(For compiling old code, some compilers have a switch controlling whether strings are writable or not.) See also questions 2.1, 2.2, 2.8, and 17.2.

References: ANSI Sec. 3.1.4 .

17.21: This program crashes before it even runs! (When single-stepping with a debugger, it dies before the first statement in main.)

You probably have one or more very large (kilobyte or more) local arrays.  Many systems have fixed-size stacks, and 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).

(See also question 9.4.)

17.22: What do "Segmentation violation" and "Bus error" mean?

These generally mean that your program tried to access memory it shouldn't have, invariably as a result of improper pointer use, often involving uninitialized or improperly allocated pointers (see questions 3.1 and 3.2), or malloc (see question 17.23) or perhapsscanf (see question 11.3).

17.23: My program is crashing, apparently somewhere down inside malloc, but I can't see anything wrong with it.

It is unfortunately very easy to corrupt malloc's internal data structures, and the resulting problems can be hard to track down.  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 involve freeing pointers not obtained from malloc, or trying to realloc a null pointer (see question 3.12).

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 September of 1992.  Others are "leak," available in volume 27 of the comp.sources.unix archives; JMalloc.c and JMalloc.h in Fidonet's C_ECHO Snippets (or ask archie; see question 17.12); and MEMDEBUG from ftp.crpht.lu in pub/sources/memdebug .  See also question 17.12.

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

Plum Hall (formerly in Cardiff, NJ; now in Hawaii) sells one.  The FSF's GNU C (gcc) distribution includes a c-torture-test.tar.Z 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.

17.25: Where can I get a YACC grammar for C?

The definitive grammar is of course the one in the ANSI standard.  A fleshed-out, working instance of the ANSI grammar (due to Jeff Lee) is on uunet (see question 17.12) 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&R II.

References: ANSI Sec. A.2 .

17.26: I need code to parse and evaluate expressions.

Two available packages are "defunc," posted to comp.source.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.

17.27: I need a sort of an "approximate"strcmp routine, for comparing two strings for close, but not necessarily exact, equality.

The traditional routine for doing this sort of thing involves the "soundex" algorithm, which maps similar-sounding words to the same numeric codes.  Soundex is described in the Searching and Sorting volume of Donald Knuth's classic The Art of Computer Programming.

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

Use mktime (see questions 12.6 and 12.7), or Zeller's congruence, or see the sci.math FAQ list, or try this code posted by Tomohiko Sakamoto:

    dayofweek(y, m, d)      /* 0 = Sunday */

    int y, m, d;            /* 1 <= m <= 12,  y > 1752 or so */

    {

            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;

    }

17.29: Will 2000 be a leap year?  Is (year % 4 == 0) an accurate test for leap years?

Yes and no, respectively.  The full expression for the Gregorian calendar is

        year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)

See a good astronomical almanac or other reference for details.

17.30: How do you pronounce "char"?

You can pronounce the C keyword "char" in at least three ways: like the English words "char," "care," or "car;" the choice is arbitrary.

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

Mitch Wright maintains an annotated bibliography of C and Unix books; it is available for anonymous ftp from ftp.rahul.net in directory pub/mitch/YABL.

This FAQ list's editor maintains a collection of previous answers to this question, which is available upon request.

17.32: Are there any C tutorials on the net?

There are at least two of them:

"Notes for C programmers," by Christopher Sawtell, available from:

• svr-ftp.eng.cam.ac.uk:misc/sawtell_C.shar
• garbo.uwasa.fi:/pc/c-lang/c-lesson.zip
• paris7.jussieu.fr:/contributions/docs

Tim Love's "C for Programmers," available from svr-ftp.eng.cam.ac.uk in the misc directory.

17.33: Where can I get extra copies of this list? What about back issues?

For now, 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. An abridged version is also available (and posted), as is a list of changes accompanying each significantly updated version.  These lists can also be found in 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 (directory pub/usenet), and ftp.uu.net (directory usenet).  The archie server should help you find others; query it for "prog C-faq". See the meta-FAQ list in news.answers for more information; see also question 17.12.

This list is an evolving document of questions which have been Frequent since before the Great Renaming, not just a collection of this month's interesting questions. Older copies are obsolete and don't contain much, except the occasional typo, that the current list doesn't.

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