S
Steve Summit
Archive-name: C-faq/toc
Comp-lang-c-archive-name: C-FAQ-list.toc
URL: http://www.eskimo.com/~scs/C-faq/top.html
[Last modified July 3, 2004 by scs.]
This article is a table of contents for the comp.lang.c frequently-asked
questions (FAQ) list, listing the questions which the abridged and full
versions of the FAQ list answer. (Both lists answer all questions; the
wordings of the questions in this article are taken from the abridged
list.)
If you have only just come across this article, you will naturally be
wondering where the lists which it indexes can be found. The unabridged
version is normally posted to comp.lang.c on the first of each month,
and the abridged version twice per month, both with Expires: lines which
should keep them around all month. They can also be found in the
newsgroups comp.answers and news.answers . They are available for
anonymous ftp from ftp.eskimo.com in directoru u/s/scs/C-faq/.
Several sites archive news.answers postings and other FAQ lists,
including comp.lang.c's: two sites are rtfm.mit.edu (directories
pub/usenet/news.answers/C-faq/ and pub/usenet/comp.lang.c/ ) and
ftp.uu.net (directory usenet/news.answers/C-faq/ ).
A web version is at http://www.eskimo.com/~scs/C-faq/top.html .
See the meta-FAQ list in news.answers for more information.
Section 1. Declarations and Initializations
Section 2. Structures, Unions, and Enumerations
Section 3. Expressions
Section 4. Pointers
Section 5. Null Pointers
Section 6. Arrays and Pointers
Section 7. Memory Allocation
Section 8. Characters and Strings
Section 9. Boolean Expressions and Variables
Section 10. C Preprocessor
Section 11. ANSI/ISO Standard C
Section 12. Stdio
Section 13. Library Functions
Section 14. Floating Point
Section 15. Variable-Length Argument Lists
Section 16. Strange Problems
Section 17. Style
Section 18. Tools and Resources
Section 19. System Dependencies
Section 20. Miscellaneous
Bibliography
Acknowledgements
Section 1. Declarations and Initializations
1.1: How should I decide which integer type to use?
1.4: What should the 64-bit type be on a machine that can support it?
1.7: What's the best way to declare and define global variables?
1.11: What does extern mean in a function declaration?
1.12: What's the auto keyword good for?
1.14: I can't seem to define a linked list node which contains a
pointer to itself.
1.21: How do I declare an array of N pointers to functions returning
pointers to functions returning pointers to char?
1.25: My compiler is complaining about an invalid redeclaration of a
function, but I only define it once.
1.25b: What's the right declaration for main()?
1.30: What am I allowed to assume about the initial values of
variables which are not explicitly initialized?
1.31: Why can't I initialize a local array with a string?
1.31b: What's wrong with "char *p = malloc(10);" ?
1.32: What is the difference between char a[] = "string"; and
char *p = "string"; ?
1.34: How do I initialize a pointer to a function?
Section 2. Structures, Unions, and Enumerations
2.1: What's the difference between struct x1 { ... }; and
typedef struct { ... } x2; ?
2.2: Why doesn't "struct x { ... }; x thestruct;" work?
2.3: Can a structure contain a pointer to itself?
2.4: How can I implement opaque (abstract) data types in C?
2.4b: Is there a good way of simulating OOP-style inheritance in C?
2.6: 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 it act like the array had several elements.
Is this legal or portable?
2.8: Is there a way to compare structures automatically?
2.10: Can I pass constant values to functions which accept structure
arguments?
2.11: How can I read/write structures from/to data files?
2.12: How can I turn off structure padding?
2.13: Why does sizeof report a larger size than I expect for a
structure type?
2.14: How can I determine the byte offset of a field within a
structure?
2.15: How can I access structure fields by name at run time?
2.18: I have a program which works correctly, but dumps core after it
finishes. Why?
2.20: Can I initialize unions?
2.22: What's the difference between an enumeration and a set of
preprocessor #defines?
2.24: Is there an easy way to print enumeration values symbolically?
Section 3. Expressions
3.1: Why doesn't the code "a = i++;" work?
3.2: Under my compiler, the code "int i = 7;
printf("%d\n", i++ * i++);" prints 49. Regardless of the order
of evaluation, shouldn't it print 56?
3.3: What should the code "int i = 3; i = i++;" do?
3.3b: Here's a slick expression: "a ^= b ^= a ^= b". It swaps a and b
without using a temporary.
3.4: Don't precedence and parentheses dictate order of evaluation?
3.5: But what about the && and || operators?
3.8: What's a "sequence point"?
3.9: So given a = i++; we don't know which cell of a[] gets
written to, but i does get incremented by one, right?
3.12a: What's the difference between ++i and i++?
3.12b: If I'm not using the value of the expression, should I use ++i
or i++ to increment a variable?
3.14: Why doesn't the code "int a = 1000, b = 1000;
long int c = a * b;" work?
3.16: Can I use ?: on the left-hand side of an assignment expression?
Section 4. Pointers
4.2: What's wrong with "char *p; *p = malloc(10);"?
4.3: Does *p++ increment p, or what it points to?
4.5: I want to use a char * pointer to step over some ints. Why
doesn't "((int *)p)++;" work?
4.8: I have a function which accepts, and is supposed to initialize,
a pointer, but the pointer in the caller remains unchanged.
4.9: Can I use a void ** pointer as a parameter so that a function
can accept a generic pointer by reference?
4.10: I have a function which accepts a pointer to an int. How can I
pass a constant like 5 to it?
4.11: Does C even have "pass by reference"?
4.12: I've seen different syntax used for calling functions via
pointers.
4.15: How do I convert an int to a char *?
Section 5. Null Pointers
5.1: What is this infamous null pointer, anyway?
5.2: How do I get a null pointer in my programs?
5.3: Is the abbreviated pointer comparison "if(p)" to test for non-
null pointers valid?
5.4: What is NULL and how is it defined?
5.5: How should NULL be defined on a machine which uses a nonzero bit
pattern as the internal representation of a null pointer?
5.6: If NULL were defined as "((char *)0)," wouldn't that make
function calls which pass an uncast NULL work?
5.9: If NULL and 0 are equivalent as null pointer constants, which
should I use?
5.10: But wouldn't it be better to use NULL, in case the value of NULL
changes?
5.12: I use the preprocessor macro "#define Nullptr(type) (type *)0"
to help me build null pointers of the correct type.
5.13: This is strange. NULL is guaranteed to be 0, but the null
pointer is not?
5.14: Why is there so much confusion surrounding null pointers?
5.15: I'm confused. I just can't understand all this null pointer
stuff.
5.16: Given all the confusion surrounding null pointers, wouldn't it
be easier simply to require them to be represented internally by
zeroes?
5.17: Seriously, have any actual machines really used nonzero null
pointers?
5.20: What does a run-time "null pointer assignment" error mean?
Section 6. Arrays and Pointers
6.1: I had the definition char a[6] in one source file, and in
another I declared extern char *a. Why didn't it work?
6.2: But I heard that char a[] was identical to char *a.
6.3: So what is meant by the "equivalence of pointers and arrays" in
C?
6.4: Why are array and pointer declarations interchangeable as
function formal parameters?
6.7: How can an array be an lvalue, if you can't assign to it?
6.8: What is the real difference between arrays and pointers?
6.9: Someone explained to me that arrays were really just constant
pointers.
6.11: I came across some "joke" code containing the "expression"
5["abcdef"] . How can this be legal C?
6.12: What's the difference between array and &array?
6.13: How do I declare a pointer to an array?
6.14: How can I set an array's size at run time?
6.15: How can I declare local arrays of a size matching a passed-in
array?
6.16: How can I dynamically allocate a multidimensional array?
6.17: Can I simulate a non-0-based array with a pointer?
6.18: My compiler complained when I passed a two-dimensional array to
a function expecting a pointer to a pointer.
6.19: How do I write functions which accept two-dimensional arrays
when the width is not known at compile time?
6.20: How can I use statically- and dynamically-allocated
multidimensional arrays interchangeably when passing them to
functions?
6.21: Why doesn't sizeof properly report the size of an array which is
a parameter to a function?
Section 7. Memory Allocation
7.1: Why doesn't the code "char *answer; gets(answer);" work?
7.2: I can't get strcat() to work. I tried "char *s3 =
strcat(s1, s2);" but I got strange results.
7.3: But the man page for strcat() says that it takes two char *'s as
arguments. How am I supposed to know to allocate things?
7.3b: I just tried the code "char *p; strcpy(p, "abc");" and it
worked. Why didn't it crash?
7.3c: How much memory does a pointer variable allocate?
7.5a: I have a function that is supposed to return a string, but when
it returns to its caller, the returned string is garbage.
7.5b: So what's the right way to return a string?
7.6: Why am I getting "warning: assignment of pointer from integer
lacks a cast" for calls to malloc()?
7.7: Why does some code carefully cast the values returned by malloc
to the pointer type being allocated?
7.7c: In a call to malloc(), what does an error like "Cannot convert
`void *' to `int *'" mean?
7.8: Why does so much code leave out the multiplication by
sizeof(char) when allocating strings?
7.11: How can I dynamically allocate arrays?
7.14: I've heard that some operating systems don't actually allocate
malloc'ed memory until the program tries to use it. Is this
legal?
7.16: I'm allocating a large array for some numeric work, but malloc()
is acting strangely.
7.17: I've got 8 meg of memory in my PC. Why can I only seem to
malloc 640K or so?
7.19: My program is crashing, apparently somewhere down inside malloc.
7.20: You can't use dynamically-allocated memory after you free it,
can you?
7.21: Why isn't a pointer null after calling free()?
7.22: When I call malloc() to allocate memory for a local pointer, do
I have to explicitly free() it?
7.23: When I free a dynamically-allocated structure containing
pointers, do I also have to free each subsidiary pointer?
7.24: Must I free allocated memory before the program exits?
7.25: Why doesn't my program's memory usage go down when I free
memory?
7.26: How does free() know how many bytes to free?
7.27: So can I query the malloc package to find out how big an
allocated block is?
7.30: Is it legal to pass a null pointer as the first argument to
realloc()?
7.31: What's the difference between calloc() and malloc()?
7.32: What is alloca() and why is its use discouraged?
Section 8. Characters and Strings
8.1: Why doesn't "strcat(string, '!');" work?
8.2: Why won't the test if(string == "value") correctly compare
string against the value?
8.3: Why can't I assign strings to character arrays?
8.6: How can I get the numeric (character set) value corresponding to
a character?
8.9: Why is sizeof('a') not 1?
Section 9. Boolean Expressions and Variables
9.1: What is the right type to use for Boolean values in C?
9.2: What if a built-in logical or relational operator "returns"
something other than 1?
9.3: Is if(p), where p is a pointer, valid?
Section 10. C Preprocessor
10.2: I've got some cute preprocessor macros that let me write C code
that looks more like Pascal. What do y'all think?
10.3: How can I write a generic macro to swap two values?
10.4: What's the best way to write a multi-statement macro?
10.6: What are .h files and what should I put in them?
10.7: Is it acceptable for one header file to #include another?
10.8a: What's the difference between #include <> and #include "" ?
10.8b: What are the complete rules for header file searching?
10.9: I'm getting strange syntax errors on the very first declaration
in a file, but it looks fine.
10.10b: I'm #including the header file for a function, but the linker
keeps saying it's undefined.
10.11: Where can I get a copy of a missing header file?
10.12: How can I construct preprocessor #if expressions which compare
strings?
10.13: Does the sizeof operator work in preprocessor #if directives?
10.14: Can I use an #ifdef in a #define line, to define something two
different ways?
10.15: Is there anything like an #ifdef for typedefs?
10.16: How can I use a preprocessor #if expression to detect
endianness?
10.18: How can I preprocess some code to remove selected conditional
compilations, without preprocessing everything?
10.19: How can I list all of the predefined identifiers?
10.20: I have some old code that tries to construct identifiers with a
macro like "#define Paste(a, b) a/**/b", but it doesn't work any
more.
10.22: What does the message "warning: macro replacement within a
string literal" mean?
10.23-4: I'm having trouble using macro arguments inside string
literals, using the `#' operator.
10.25: I've got this tricky preprocessing I want to do and I can't
figure out a way to do it.
10.26: How can I write a macro which takes a variable number of
arguments?
Section 11. ANSI/ISO Standard C
11.1: What is the "ANSI C Standard?"
11.2: How can I get a copy of the Standard?
11.2b: Where can I get information about updates to the Standard?
11.3: My ANSI compiler is complaining about prototype mismatches for
parameters declared float.
11.4: Can you mix old-style and new-style function syntax?
11.5: Why does the declaration "extern int f(struct x *p);" give me a
warning message?
11.8: Why can't I use const values in initializers and array
dimensions?
11.8b: If you can't modify string literals, why aren't they defined as
being arrays of const characters?
11.9: What's the difference between "const char *p" and
"char * const p"?
11.10: Why can't I pass a char ** to a function which expects a
const char **?
11.12a: What's the correct declaration of main()?
11.12b: Can I declare main() as void, to shut off these annoying
"main returns no value" messages?
11.13: But what about main's third argument, envp?
11.14a: I believe that declaring void main() can't fail, since I'm
calling exit() instead of returning.
11.14b: Are there systems where void main() doesn't work?
11.15: The book I've been using always uses void main().
11.16: Is exit(status) truly equivalent to returning the same status
from main()?
11.17: How do I get the ANSI "stringizing" preprocessing operator `#'
to stringize the macro's value instead of its name?
11.18: What does the message "warning: macro replacement within a
string literal" mean?
11.19: I'm getting strange syntax errors inside lines I've #ifdeffed
out.
11.20: What are #pragmas ?
11.21: What does "#pragma once" mean?
11.22: Is char a[3] = "abc"; legal?
11.24: Why can't I perform arithmetic on a void * pointer?
11.25: What's the difference between memcpy() and memmove()?
11.26: What should malloc(0) do?
11.27: Why does the ANSI Standard place limits on the length and case-
significance of external identifiers?
11.29: My compiler is rejecting the simplest possible test programs,
with all kinds of syntax errors.
11.30: Why are some ANSI/ISO Standard library functions showing up as
undefined, even though I've got an ANSI compiler?
11.31: Does anyone have a tool for converting old-style C programs to
ANSI C, or for automatically generating prototypes?
11.32: Why won't frobozz-cc, which claims to be ANSI compliant, accept
this code?
11.33: What's the difference between implementation-defined,
unspecified, and undefined behavior?
11.33b: What does it really mean for a program to be "conforming"?
11.34: I'm appalled that the ANSI Standard leaves so many issues
undefined.
11.35: I just tried some allegedly-undefined code on an ANSI-conforming
compiler, and got the results I expected.
Section 12. Stdio
12.1: What's wrong with the code "char c; while((c = getchar()) !=
EOF) ..."?
12.1b: How do I enter EOF from the keyboard?
12.2: Why won't the code "while(!feof(infp)) {
fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?
12.4: My program's prompts and intermediate output don't always show
up on the screen.
12.5: How can I read one character at a time, without waiting for the
RETURN key?
12.6: How can I print a '%' character with printf?
12.9: How can printf() use %f for type double, if scanf() requires
%lf?
12.9b: What printf format should I use for a typedef when I don't know
the underlying type?
12.10: How can I implement a variable field width with printf?
12.11: How can I print numbers with commas separating the thousands?
12.12: Why doesn't the call scanf("%d", i) work?
12.12b: Why *does* the call "char s[30]; scanf("%s", s);" work?
12.13: Why doesn't the code "double d; scanf("%f", &d);" work?
12.15: How can I specify a variable width in a scanf() format string?
12.17: When I read numbers from the keyboard with scanf "%d\n", it
seems to hang until I type one extra line of input.
12.18a: I'm reading a number with scanf %d and then a string with
gets(), but the compiler seems to be skipping the call to
gets()!
12.19: I'm re-prompting the user if scanf() fails, but sometimes it
seems to go into an infinite loop.
12.20: Why does everyone say not to use scanf()? What should I use
instead?
12.21: How can I tell how much destination buffer space I'll need for
an arbitrary sprintf call? How can I avoid overflowing the
destination buffer with sprintf()?
12.23: Why does everyone say not to use gets()?
12.24: Why does errno contain ENOTTY after a call to printf()?
12.25: What's the difference between fgetpos/fsetpos and ftell/fseek?
12.26a: Will fflush(stdin) flush unread characters from the standard
input stream?
12.26b: If fflush() won't work, what can I use to flush input?
12.27: fopen() is failing for certain pathnames.
12.30: I'm trying to update a file in place, by using fopen mode "r+",
but it's not working.
12.33: How can I redirect stdin or stdout from within a program?
12.34: Once I've used freopen(), how can I get the original stream
back?
12.36b: How can I arrange to have output go two places at once?
12.38: How can I read a binary data file properly?
Section 13. Library Functions
13.1: How can I convert numbers to strings?
13.2: Why does strncpy() not always write a '\0'?
13.5: Why do some versions of toupper() act strangely if given an
upper-case letter?
13.6: How can I split up a string into whitespace-separated fields?
13.7: I need some code to do regular expression and wildcard matching.
13.8: I'm trying to sort an array of strings with qsort(), using
strcmp() as the comparison function, but it's not working.
13.9: Now I'm trying to sort an array of structures, but the compiler
is complaining that the function is of the wrong type for
qsort().
13.10: How can I sort a linked list?
13.11: How can I sort more data than will fit in memory?
13.12: How can I get the time of day in a C program?
13.13: How can I convert a struct tm or a string into a time_t?
13.14: How can I perform calendar manipulations?
13.15: I need a random number generator.
13.16: How can I get random integers in a certain range?
13.17: Each time I run my program, I get the same sequence of numbers
back from rand().
13.18: I need a random true/false value, so I'm just taking rand() % 2,
but it's alternating 0, 1, 0, 1, 0...
13.20: How can I generate random numbers with a normal or Gaussian
distribution?
13.25: I get errors due to library functions being undefined even
though I #include the right header files.
13.26: I'm still getting errors due to library functions being
undefined, even though I'm requesting the right libraries.
13.28: What does it mean when the linker says that _end is undefined?
13.29: My compiler is complaining that printf is undefined!
Section 14. Floating Point
14.1: When I set a float variable to 3.1, why is printf printing it as
3.0999999?
14.2: Why is sqrt(144.) giving me crazy numbers?
14.3: I keep getting "undefined: sin" compilation errors.
14.4a: My floating-point calculations are acting strangely and giving
me different answers on different machines.
14.5: What's a good way to check for "close enough" floating-point
equality?
14.6: How do I round numbers?
14.7: Where is C's exponentiation operator?
14.8: The predefined constant M_PI seems to be missing from <math.h>.
14.9: How do I test for IEEE NaN and other special values?
14.11: What's a good way to implement complex numbers in C?
14.12: I'm looking for some mathematical library code.
14.13: I'm having trouble with a Turbo C program which crashes and says
something like "floating point formats not linked."
Section 15. Variable-Length Argument Lists
15.1: I heard that you have to #include <stdio.h> before calling
printf(). Why?
15.2: How can %f be used for both float and double arguments in
printf()?
15.3: Why don't function prototypes guard against mismatches in
printf's arguments?
15.4: How can I write a function that takes a variable number of
arguments?
15.5: How can I write a function that takes a format string and a
variable number of arguments, like printf(), and passes them to
printf() to do most of the work?
15.6: How can I write a function analogous to scanf(), that calls
scanf() to do most of the work?
15.8: How can I discover how many arguments a function was actually
called with?
15.9: My compiler isn't letting me declare a function that accepts
*only* variable arguments.
15.10: Why isn't "va_arg(argp, float)" working?
15.11: I can't get va_arg() to pull in an argument of type pointer-to-
function.
15.12: How can I write a function which takes a variable number of
arguments and passes them to some other function ?
15.13: How can I call a function with an argument list built up at run
time?
Section 16. Strange Problems
16.1b: I'm getting baffling syntax errors which make no sense at all,
and it seems like large chunks of my program aren't being
compiled.
16.1c: Why isn't my procedure call working?
16.3: This program crashes before it even runs!
16.4: I have a program that seems to run correctly, but then crashes
as it's exiting.
16.5: This program runs perfectly on one machine, but I get weird
results on another.
16.6: Why does the code "char *p = "hello, world!"; p[0] = 'H';"
crash?
16.8: What does "Segmentation violation" mean?
Section 17. Style
17.1: What's the best style for code layout in C?
17.3: Is the code "if(!strcmp(s1, s2))" good style?
17.4: Why do some people write if(0 == x) instead of if(x == 0)?
17.4b: What are the extra parentheses and underscores in
"extern int func __((int, int));" for?
17.5: I came across some code that puts a (void) cast before each call
to printf(). Why?
17.8: What is "Hungarian Notation"?
17.9: Where can I get the "Indian Hill Style Guide" and other coding
standards?
17.10: Some people say that goto's are evil and that I should never use
them. Isn't that a bit extreme?
Section 18. Tools and Resources
18.1: I'm looking for C development tools (cross-reference generators,
code beautifiers, etc.).
18.2: How can I track down these pesky malloc problems?
18.3: What's a free or cheap C compiler I can use?
18.4: I just typed in this program, and it's acting strangely. Can
you see anything wrong with it?
18.7: Where can I get an ANSI-compatible lint?
18.8: Don't ANSI function prototypes render lint obsolete?
18.9: Are there any C tutorials or other resources on the net?
18.9b: Where can I find some good code examples to study and learn
from?
18.10: What's a good book for learning C?
18.13: Where can I find the sources of the standard C libraries?
18.13b: Is there an on-line C reference manual?
18.13c: Where can I get a copy of the ANSI/ISO C Standard?
18.14: I need code to parse and evaluate expressions.
18.15: Where can I get a BNF or YACC grammar for C?
18.15b: Does anyone have a C compiler test suite I can use?
18.15c: Where are some collections of useful code fragments and
examples?
18.15d: I need code for performing multiple precision arithmetic.
18.16: Where and how can I get copies of all these freely distributable
programs?
Section 19. System Dependencies
19.1: How can I read a single character from the keyboard without
waiting for the RETURN key?
19.2: How can I find out how many characters are available for
reading, or do a non-blocking read?
19.3: How can I display a percentage-done indication that updates
itself in place, or show one of those "twirling baton" progress
indicators?
19.4: How can I clear the screen, or print text in color, or move the
cursor?
19.5: How do I read the arrow keys? What about function keys?
19.6: How do I read the mouse?
19.7: How can I do serial ("comm") port I/O?
19.8: How can I direct output to the printer?
19.9: How do I send escape sequences to control a terminal or other
device?
19.9b: How can I access an I/O board directly?
19.10: How can I do graphics?
19.10b: How can I display GIF and JPEG images?
19.11: How can I check whether a file exists?
19.12: How can I find out the size of a file, prior to reading it in?
19.12b: How can I find the modification date of a file?
19.13: How can a file be shortened in-place without completely clearing
or rewriting it?
19.14: How can I insert or delete a line in the middle of a file?
19.15: How can I recover the file name given an open file descriptor?
19.16: How can I delete a file?
19.16b: How do I copy files?
19.17: What's wrong with the call fopen("c:\newdir\file.dat", "r")?
19.17b: fopen() isn't letting me open files like "$HOME/.profile".
19.17c: How can I suppress the "Abort, Retry, Ignore?" message?
19.18: How can I increase the allowable number of simultaneously open
files?
19.20: How can I read a directory in a C program?
19.22: How can I find out how much memory is available?
19.23: How can I allocate arrays or structures bigger than 64K?
19.24: What does the error message "DGROUP exceeds 64K" mean?
19.25: How can I access memory located at a certain address?
19.27: How can I invoke another program from within a C program?
19.30: How can I invoke another program and trap its output?
19.31: How can my program discover the complete pathname to the
executable from which it was invoked?
19.32: How can I automatically locate a program's configuration files
in the same directory as the executable?
19.33: How can a process change an environment variable in its caller?
19.36: How can I read in an object file and jump to locations in it?
19.37: How can I implement a delay, or time a user's response,
with sub-second resolution?
19.38: How can I trap or ignore keyboard interrupts like control-C?
19.39: How can I handle floating-point exceptions gracefully?
19.40: How do I... Use sockets? Do networking? Write client/server
applications?
19.40b: How do I... Use BIOS calls? Write ISR's? Create TSR's?
19.40c: I'm trying to compile a program in which "union REGS" and
int86() are undefined.
19.40d: What are "near" and "far" pointers?
19.41: But I can't use all these nonstandard, system-dependent
functions, because my program has to be ANSI compatible!
Section 20. Miscellaneous
20.1: How can I return multiple values from a function?
20.3: How do I access command-line arguments?
20.5: How can I write data files which can be read on other machines
with different data formats?
20.6: How can I call a function, given its name as a string?
20.8: How can I implement sets or arrays of bits?
20.9: How can I determine whether a machine's byte order is big-endian
or little-endian?
20.9b: How do I swap bytes?
20.10: How can I convert integers to binary or hexadecimal?
20.11: Can I use base-2 constants (something like 0b101010)?
Is there a printf() format for binary?
20.12: What is the most efficient way to count the number of bits which
are set in an integer?
20.13: What's the best way of making my program efficient?
20.14: Are pointers really faster than arrays? How much do function
calls slow things down?
20.15b: People claim that optimizing compilers are good, but mine can't
even replace i/=2 with a shift.
20.15c: How can I swap two values without using a temporary?
20.17: Is there a way to switch on strings?
20.18: Is there a way to have non-constant case labels (i.e. ranges or
arbitrary expressions)?
20.19: Are the outer parentheses in return statements really optional?
20.20: Why don't C comments nest? Are they legal inside quoted
strings?
20.21b: What does a+++++b mean ?
20.24: Why doesn't C have nested functions?
20.24b: What is assert()?
20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions
from C?
20.26: Does anyone know of a program for converting Pascal or FORTRAN
to C?
20.27: Can I use a C++ compiler to compile C code?
20.28: I need to compare two strings for close, but not necessarily
exact, equality.
20.29: What is hashing?
20.31: How can I find the day of the week given the date?
20.32: Is (year % 4 == 0) an accurate test for leap years?
20.34: How do you write a program which produces its own source code as
output?
20.35: What is "Duff's Device"?
20.36: When will the next Obfuscated C Code Contest be held?
How can I get a copy of previous winning entries?
20.37: What was the entry keyword mentioned in K&R1?
20.38: Where does the name "C" come from, anyway?
20.39: How do you pronounce "char"?
20.39b: What do "lvalue" and "rvalue" mean?
20.40: Where can I get extra copies of this list?
Steve Summit
(e-mail address removed)
This article is Copyright 1990-2004 by Steve Summit.
Content from the book _C Programming FAQs: Frequently Asked Questions_
is made available here by permission of the author and the publisher as
a service to the community. It is intended to complement the use of the
published text and is protected by international copyright laws. The
content is made available here and may be accessed freely for personal
use but may not be republished without permission.
Comp-lang-c-archive-name: C-FAQ-list.toc
URL: http://www.eskimo.com/~scs/C-faq/top.html
[Last modified July 3, 2004 by scs.]
This article is a table of contents for the comp.lang.c frequently-asked
questions (FAQ) list, listing the questions which the abridged and full
versions of the FAQ list answer. (Both lists answer all questions; the
wordings of the questions in this article are taken from the abridged
list.)
If you have only just come across this article, you will naturally be
wondering where the lists which it indexes can be found. The unabridged
version is normally posted to comp.lang.c on the first of each month,
and the abridged version twice per month, both with Expires: lines which
should keep them around all month. They can also be found in the
newsgroups comp.answers and news.answers . They are available for
anonymous ftp from ftp.eskimo.com in directoru u/s/scs/C-faq/.
Several sites archive news.answers postings and other FAQ lists,
including comp.lang.c's: two sites are rtfm.mit.edu (directories
pub/usenet/news.answers/C-faq/ and pub/usenet/comp.lang.c/ ) and
ftp.uu.net (directory usenet/news.answers/C-faq/ ).
A web version is at http://www.eskimo.com/~scs/C-faq/top.html .
See the meta-FAQ list in news.answers for more information.
Section 1. Declarations and Initializations
Section 2. Structures, Unions, and Enumerations
Section 3. Expressions
Section 4. Pointers
Section 5. Null Pointers
Section 6. Arrays and Pointers
Section 7. Memory Allocation
Section 8. Characters and Strings
Section 9. Boolean Expressions and Variables
Section 10. C Preprocessor
Section 11. ANSI/ISO Standard C
Section 12. Stdio
Section 13. Library Functions
Section 14. Floating Point
Section 15. Variable-Length Argument Lists
Section 16. Strange Problems
Section 17. Style
Section 18. Tools and Resources
Section 19. System Dependencies
Section 20. Miscellaneous
Bibliography
Acknowledgements
Section 1. Declarations and Initializations
1.1: How should I decide which integer type to use?
1.4: What should the 64-bit type be on a machine that can support it?
1.7: What's the best way to declare and define global variables?
1.11: What does extern mean in a function declaration?
1.12: What's the auto keyword good for?
1.14: I can't seem to define a linked list node which contains a
pointer to itself.
1.21: How do I declare an array of N pointers to functions returning
pointers to functions returning pointers to char?
1.25: My compiler is complaining about an invalid redeclaration of a
function, but I only define it once.
1.25b: What's the right declaration for main()?
1.30: What am I allowed to assume about the initial values of
variables which are not explicitly initialized?
1.31: Why can't I initialize a local array with a string?
1.31b: What's wrong with "char *p = malloc(10);" ?
1.32: What is the difference between char a[] = "string"; and
char *p = "string"; ?
1.34: How do I initialize a pointer to a function?
Section 2. Structures, Unions, and Enumerations
2.1: What's the difference between struct x1 { ... }; and
typedef struct { ... } x2; ?
2.2: Why doesn't "struct x { ... }; x thestruct;" work?
2.3: Can a structure contain a pointer to itself?
2.4: How can I implement opaque (abstract) data types in C?
2.4b: Is there a good way of simulating OOP-style inheritance in C?
2.6: 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 it act like the array had several elements.
Is this legal or portable?
2.8: Is there a way to compare structures automatically?
2.10: Can I pass constant values to functions which accept structure
arguments?
2.11: How can I read/write structures from/to data files?
2.12: How can I turn off structure padding?
2.13: Why does sizeof report a larger size than I expect for a
structure type?
2.14: How can I determine the byte offset of a field within a
structure?
2.15: How can I access structure fields by name at run time?
2.18: I have a program which works correctly, but dumps core after it
finishes. Why?
2.20: Can I initialize unions?
2.22: What's the difference between an enumeration and a set of
preprocessor #defines?
2.24: Is there an easy way to print enumeration values symbolically?
Section 3. Expressions
3.1: Why doesn't the code "a = i++;" work?
3.2: Under my compiler, the code "int i = 7;
printf("%d\n", i++ * i++);" prints 49. Regardless of the order
of evaluation, shouldn't it print 56?
3.3: What should the code "int i = 3; i = i++;" do?
3.3b: Here's a slick expression: "a ^= b ^= a ^= b". It swaps a and b
without using a temporary.
3.4: Don't precedence and parentheses dictate order of evaluation?
3.5: But what about the && and || operators?
3.8: What's a "sequence point"?
3.9: So given a = i++; we don't know which cell of a[] gets
written to, but i does get incremented by one, right?
3.12a: What's the difference between ++i and i++?
3.12b: If I'm not using the value of the expression, should I use ++i
or i++ to increment a variable?
3.14: Why doesn't the code "int a = 1000, b = 1000;
long int c = a * b;" work?
3.16: Can I use ?: on the left-hand side of an assignment expression?
Section 4. Pointers
4.2: What's wrong with "char *p; *p = malloc(10);"?
4.3: Does *p++ increment p, or what it points to?
4.5: I want to use a char * pointer to step over some ints. Why
doesn't "((int *)p)++;" work?
4.8: I have a function which accepts, and is supposed to initialize,
a pointer, but the pointer in the caller remains unchanged.
4.9: Can I use a void ** pointer as a parameter so that a function
can accept a generic pointer by reference?
4.10: I have a function which accepts a pointer to an int. How can I
pass a constant like 5 to it?
4.11: Does C even have "pass by reference"?
4.12: I've seen different syntax used for calling functions via
pointers.
4.15: How do I convert an int to a char *?
Section 5. Null Pointers
5.1: What is this infamous null pointer, anyway?
5.2: How do I get a null pointer in my programs?
5.3: Is the abbreviated pointer comparison "if(p)" to test for non-
null pointers valid?
5.4: What is NULL and how is it defined?
5.5: How should NULL be defined on a machine which uses a nonzero bit
pattern as the internal representation of a null pointer?
5.6: If NULL were defined as "((char *)0)," wouldn't that make
function calls which pass an uncast NULL work?
5.9: If NULL and 0 are equivalent as null pointer constants, which
should I use?
5.10: But wouldn't it be better to use NULL, in case the value of NULL
changes?
5.12: I use the preprocessor macro "#define Nullptr(type) (type *)0"
to help me build null pointers of the correct type.
5.13: This is strange. NULL is guaranteed to be 0, but the null
pointer is not?
5.14: Why is there so much confusion surrounding null pointers?
5.15: I'm confused. I just can't understand all this null pointer
stuff.
5.16: Given all the confusion surrounding null pointers, wouldn't it
be easier simply to require them to be represented internally by
zeroes?
5.17: Seriously, have any actual machines really used nonzero null
pointers?
5.20: What does a run-time "null pointer assignment" error mean?
Section 6. Arrays and Pointers
6.1: I had the definition char a[6] in one source file, and in
another I declared extern char *a. Why didn't it work?
6.2: But I heard that char a[] was identical to char *a.
6.3: So what is meant by the "equivalence of pointers and arrays" in
C?
6.4: Why are array and pointer declarations interchangeable as
function formal parameters?
6.7: How can an array be an lvalue, if you can't assign to it?
6.8: What is the real difference between arrays and pointers?
6.9: Someone explained to me that arrays were really just constant
pointers.
6.11: I came across some "joke" code containing the "expression"
5["abcdef"] . How can this be legal C?
6.12: What's the difference between array and &array?
6.13: How do I declare a pointer to an array?
6.14: How can I set an array's size at run time?
6.15: How can I declare local arrays of a size matching a passed-in
array?
6.16: How can I dynamically allocate a multidimensional array?
6.17: Can I simulate a non-0-based array with a pointer?
6.18: My compiler complained when I passed a two-dimensional array to
a function expecting a pointer to a pointer.
6.19: How do I write functions which accept two-dimensional arrays
when the width is not known at compile time?
6.20: How can I use statically- and dynamically-allocated
multidimensional arrays interchangeably when passing them to
functions?
6.21: Why doesn't sizeof properly report the size of an array which is
a parameter to a function?
Section 7. Memory Allocation
7.1: Why doesn't the code "char *answer; gets(answer);" work?
7.2: I can't get strcat() to work. I tried "char *s3 =
strcat(s1, s2);" but I got strange results.
7.3: But the man page for strcat() says that it takes two char *'s as
arguments. How am I supposed to know to allocate things?
7.3b: I just tried the code "char *p; strcpy(p, "abc");" and it
worked. Why didn't it crash?
7.3c: How much memory does a pointer variable allocate?
7.5a: I have a function that is supposed to return a string, but when
it returns to its caller, the returned string is garbage.
7.5b: So what's the right way to return a string?
7.6: Why am I getting "warning: assignment of pointer from integer
lacks a cast" for calls to malloc()?
7.7: Why does some code carefully cast the values returned by malloc
to the pointer type being allocated?
7.7c: In a call to malloc(), what does an error like "Cannot convert
`void *' to `int *'" mean?
7.8: Why does so much code leave out the multiplication by
sizeof(char) when allocating strings?
7.11: How can I dynamically allocate arrays?
7.14: I've heard that some operating systems don't actually allocate
malloc'ed memory until the program tries to use it. Is this
legal?
7.16: I'm allocating a large array for some numeric work, but malloc()
is acting strangely.
7.17: I've got 8 meg of memory in my PC. Why can I only seem to
malloc 640K or so?
7.19: My program is crashing, apparently somewhere down inside malloc.
7.20: You can't use dynamically-allocated memory after you free it,
can you?
7.21: Why isn't a pointer null after calling free()?
7.22: When I call malloc() to allocate memory for a local pointer, do
I have to explicitly free() it?
7.23: When I free a dynamically-allocated structure containing
pointers, do I also have to free each subsidiary pointer?
7.24: Must I free allocated memory before the program exits?
7.25: Why doesn't my program's memory usage go down when I free
memory?
7.26: How does free() know how many bytes to free?
7.27: So can I query the malloc package to find out how big an
allocated block is?
7.30: Is it legal to pass a null pointer as the first argument to
realloc()?
7.31: What's the difference between calloc() and malloc()?
7.32: What is alloca() and why is its use discouraged?
Section 8. Characters and Strings
8.1: Why doesn't "strcat(string, '!');" work?
8.2: Why won't the test if(string == "value") correctly compare
string against the value?
8.3: Why can't I assign strings to character arrays?
8.6: How can I get the numeric (character set) value corresponding to
a character?
8.9: Why is sizeof('a') not 1?
Section 9. Boolean Expressions and Variables
9.1: What is the right type to use for Boolean values in C?
9.2: What if a built-in logical or relational operator "returns"
something other than 1?
9.3: Is if(p), where p is a pointer, valid?
Section 10. C Preprocessor
10.2: I've got some cute preprocessor macros that let me write C code
that looks more like Pascal. What do y'all think?
10.3: How can I write a generic macro to swap two values?
10.4: What's the best way to write a multi-statement macro?
10.6: What are .h files and what should I put in them?
10.7: Is it acceptable for one header file to #include another?
10.8a: What's the difference between #include <> and #include "" ?
10.8b: What are the complete rules for header file searching?
10.9: I'm getting strange syntax errors on the very first declaration
in a file, but it looks fine.
10.10b: I'm #including the header file for a function, but the linker
keeps saying it's undefined.
10.11: Where can I get a copy of a missing header file?
10.12: How can I construct preprocessor #if expressions which compare
strings?
10.13: Does the sizeof operator work in preprocessor #if directives?
10.14: Can I use an #ifdef in a #define line, to define something two
different ways?
10.15: Is there anything like an #ifdef for typedefs?
10.16: How can I use a preprocessor #if expression to detect
endianness?
10.18: How can I preprocess some code to remove selected conditional
compilations, without preprocessing everything?
10.19: How can I list all of the predefined identifiers?
10.20: I have some old code that tries to construct identifiers with a
macro like "#define Paste(a, b) a/**/b", but it doesn't work any
more.
10.22: What does the message "warning: macro replacement within a
string literal" mean?
10.23-4: I'm having trouble using macro arguments inside string
literals, using the `#' operator.
10.25: I've got this tricky preprocessing I want to do and I can't
figure out a way to do it.
10.26: How can I write a macro which takes a variable number of
arguments?
Section 11. ANSI/ISO Standard C
11.1: What is the "ANSI C Standard?"
11.2: How can I get a copy of the Standard?
11.2b: Where can I get information about updates to the Standard?
11.3: My ANSI compiler is complaining about prototype mismatches for
parameters declared float.
11.4: Can you mix old-style and new-style function syntax?
11.5: Why does the declaration "extern int f(struct x *p);" give me a
warning message?
11.8: Why can't I use const values in initializers and array
dimensions?
11.8b: If you can't modify string literals, why aren't they defined as
being arrays of const characters?
11.9: What's the difference between "const char *p" and
"char * const p"?
11.10: Why can't I pass a char ** to a function which expects a
const char **?
11.12a: What's the correct declaration of main()?
11.12b: Can I declare main() as void, to shut off these annoying
"main returns no value" messages?
11.13: But what about main's third argument, envp?
11.14a: I believe that declaring void main() can't fail, since I'm
calling exit() instead of returning.
11.14b: Are there systems where void main() doesn't work?
11.15: The book I've been using always uses void main().
11.16: Is exit(status) truly equivalent to returning the same status
from main()?
11.17: How do I get the ANSI "stringizing" preprocessing operator `#'
to stringize the macro's value instead of its name?
11.18: What does the message "warning: macro replacement within a
string literal" mean?
11.19: I'm getting strange syntax errors inside lines I've #ifdeffed
out.
11.20: What are #pragmas ?
11.21: What does "#pragma once" mean?
11.22: Is char a[3] = "abc"; legal?
11.24: Why can't I perform arithmetic on a void * pointer?
11.25: What's the difference between memcpy() and memmove()?
11.26: What should malloc(0) do?
11.27: Why does the ANSI Standard place limits on the length and case-
significance of external identifiers?
11.29: My compiler is rejecting the simplest possible test programs,
with all kinds of syntax errors.
11.30: Why are some ANSI/ISO Standard library functions showing up as
undefined, even though I've got an ANSI compiler?
11.31: Does anyone have a tool for converting old-style C programs to
ANSI C, or for automatically generating prototypes?
11.32: Why won't frobozz-cc, which claims to be ANSI compliant, accept
this code?
11.33: What's the difference between implementation-defined,
unspecified, and undefined behavior?
11.33b: What does it really mean for a program to be "conforming"?
11.34: I'm appalled that the ANSI Standard leaves so many issues
undefined.
11.35: I just tried some allegedly-undefined code on an ANSI-conforming
compiler, and got the results I expected.
Section 12. Stdio
12.1: What's wrong with the code "char c; while((c = getchar()) !=
EOF) ..."?
12.1b: How do I enter EOF from the keyboard?
12.2: Why won't the code "while(!feof(infp)) {
fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?
12.4: My program's prompts and intermediate output don't always show
up on the screen.
12.5: How can I read one character at a time, without waiting for the
RETURN key?
12.6: How can I print a '%' character with printf?
12.9: How can printf() use %f for type double, if scanf() requires
%lf?
12.9b: What printf format should I use for a typedef when I don't know
the underlying type?
12.10: How can I implement a variable field width with printf?
12.11: How can I print numbers with commas separating the thousands?
12.12: Why doesn't the call scanf("%d", i) work?
12.12b: Why *does* the call "char s[30]; scanf("%s", s);" work?
12.13: Why doesn't the code "double d; scanf("%f", &d);" work?
12.15: How can I specify a variable width in a scanf() format string?
12.17: When I read numbers from the keyboard with scanf "%d\n", it
seems to hang until I type one extra line of input.
12.18a: I'm reading a number with scanf %d and then a string with
gets(), but the compiler seems to be skipping the call to
gets()!
12.19: I'm re-prompting the user if scanf() fails, but sometimes it
seems to go into an infinite loop.
12.20: Why does everyone say not to use scanf()? What should I use
instead?
12.21: How can I tell how much destination buffer space I'll need for
an arbitrary sprintf call? How can I avoid overflowing the
destination buffer with sprintf()?
12.23: Why does everyone say not to use gets()?
12.24: Why does errno contain ENOTTY after a call to printf()?
12.25: What's the difference between fgetpos/fsetpos and ftell/fseek?
12.26a: Will fflush(stdin) flush unread characters from the standard
input stream?
12.26b: If fflush() won't work, what can I use to flush input?
12.27: fopen() is failing for certain pathnames.
12.30: I'm trying to update a file in place, by using fopen mode "r+",
but it's not working.
12.33: How can I redirect stdin or stdout from within a program?
12.34: Once I've used freopen(), how can I get the original stream
back?
12.36b: How can I arrange to have output go two places at once?
12.38: How can I read a binary data file properly?
Section 13. Library Functions
13.1: How can I convert numbers to strings?
13.2: Why does strncpy() not always write a '\0'?
13.5: Why do some versions of toupper() act strangely if given an
upper-case letter?
13.6: How can I split up a string into whitespace-separated fields?
13.7: I need some code to do regular expression and wildcard matching.
13.8: I'm trying to sort an array of strings with qsort(), using
strcmp() as the comparison function, but it's not working.
13.9: Now I'm trying to sort an array of structures, but the compiler
is complaining that the function is of the wrong type for
qsort().
13.10: How can I sort a linked list?
13.11: How can I sort more data than will fit in memory?
13.12: How can I get the time of day in a C program?
13.13: How can I convert a struct tm or a string into a time_t?
13.14: How can I perform calendar manipulations?
13.15: I need a random number generator.
13.16: How can I get random integers in a certain range?
13.17: Each time I run my program, I get the same sequence of numbers
back from rand().
13.18: I need a random true/false value, so I'm just taking rand() % 2,
but it's alternating 0, 1, 0, 1, 0...
13.20: How can I generate random numbers with a normal or Gaussian
distribution?
13.25: I get errors due to library functions being undefined even
though I #include the right header files.
13.26: I'm still getting errors due to library functions being
undefined, even though I'm requesting the right libraries.
13.28: What does it mean when the linker says that _end is undefined?
13.29: My compiler is complaining that printf is undefined!
Section 14. Floating Point
14.1: When I set a float variable to 3.1, why is printf printing it as
3.0999999?
14.2: Why is sqrt(144.) giving me crazy numbers?
14.3: I keep getting "undefined: sin" compilation errors.
14.4a: My floating-point calculations are acting strangely and giving
me different answers on different machines.
14.5: What's a good way to check for "close enough" floating-point
equality?
14.6: How do I round numbers?
14.7: Where is C's exponentiation operator?
14.8: The predefined constant M_PI seems to be missing from <math.h>.
14.9: How do I test for IEEE NaN and other special values?
14.11: What's a good way to implement complex numbers in C?
14.12: I'm looking for some mathematical library code.
14.13: I'm having trouble with a Turbo C program which crashes and says
something like "floating point formats not linked."
Section 15. Variable-Length Argument Lists
15.1: I heard that you have to #include <stdio.h> before calling
printf(). Why?
15.2: How can %f be used for both float and double arguments in
printf()?
15.3: Why don't function prototypes guard against mismatches in
printf's arguments?
15.4: How can I write a function that takes a variable number of
arguments?
15.5: How can I write a function that takes a format string and a
variable number of arguments, like printf(), and passes them to
printf() to do most of the work?
15.6: How can I write a function analogous to scanf(), that calls
scanf() to do most of the work?
15.8: How can I discover how many arguments a function was actually
called with?
15.9: My compiler isn't letting me declare a function that accepts
*only* variable arguments.
15.10: Why isn't "va_arg(argp, float)" working?
15.11: I can't get va_arg() to pull in an argument of type pointer-to-
function.
15.12: How can I write a function which takes a variable number of
arguments and passes them to some other function ?
15.13: How can I call a function with an argument list built up at run
time?
Section 16. Strange Problems
16.1b: I'm getting baffling syntax errors which make no sense at all,
and it seems like large chunks of my program aren't being
compiled.
16.1c: Why isn't my procedure call working?
16.3: This program crashes before it even runs!
16.4: I have a program that seems to run correctly, but then crashes
as it's exiting.
16.5: This program runs perfectly on one machine, but I get weird
results on another.
16.6: Why does the code "char *p = "hello, world!"; p[0] = 'H';"
crash?
16.8: What does "Segmentation violation" mean?
Section 17. Style
17.1: What's the best style for code layout in C?
17.3: Is the code "if(!strcmp(s1, s2))" good style?
17.4: Why do some people write if(0 == x) instead of if(x == 0)?
17.4b: What are the extra parentheses and underscores in
"extern int func __((int, int));" for?
17.5: I came across some code that puts a (void) cast before each call
to printf(). Why?
17.8: What is "Hungarian Notation"?
17.9: Where can I get the "Indian Hill Style Guide" and other coding
standards?
17.10: Some people say that goto's are evil and that I should never use
them. Isn't that a bit extreme?
Section 18. Tools and Resources
18.1: I'm looking for C development tools (cross-reference generators,
code beautifiers, etc.).
18.2: How can I track down these pesky malloc problems?
18.3: What's a free or cheap C compiler I can use?
18.4: I just typed in this program, and it's acting strangely. Can
you see anything wrong with it?
18.7: Where can I get an ANSI-compatible lint?
18.8: Don't ANSI function prototypes render lint obsolete?
18.9: Are there any C tutorials or other resources on the net?
18.9b: Where can I find some good code examples to study and learn
from?
18.10: What's a good book for learning C?
18.13: Where can I find the sources of the standard C libraries?
18.13b: Is there an on-line C reference manual?
18.13c: Where can I get a copy of the ANSI/ISO C Standard?
18.14: I need code to parse and evaluate expressions.
18.15: Where can I get a BNF or YACC grammar for C?
18.15b: Does anyone have a C compiler test suite I can use?
18.15c: Where are some collections of useful code fragments and
examples?
18.15d: I need code for performing multiple precision arithmetic.
18.16: Where and how can I get copies of all these freely distributable
programs?
Section 19. System Dependencies
19.1: How can I read a single character from the keyboard without
waiting for the RETURN key?
19.2: How can I find out how many characters are available for
reading, or do a non-blocking read?
19.3: How can I display a percentage-done indication that updates
itself in place, or show one of those "twirling baton" progress
indicators?
19.4: How can I clear the screen, or print text in color, or move the
cursor?
19.5: How do I read the arrow keys? What about function keys?
19.6: How do I read the mouse?
19.7: How can I do serial ("comm") port I/O?
19.8: How can I direct output to the printer?
19.9: How do I send escape sequences to control a terminal or other
device?
19.9b: How can I access an I/O board directly?
19.10: How can I do graphics?
19.10b: How can I display GIF and JPEG images?
19.11: How can I check whether a file exists?
19.12: How can I find out the size of a file, prior to reading it in?
19.12b: How can I find the modification date of a file?
19.13: How can a file be shortened in-place without completely clearing
or rewriting it?
19.14: How can I insert or delete a line in the middle of a file?
19.15: How can I recover the file name given an open file descriptor?
19.16: How can I delete a file?
19.16b: How do I copy files?
19.17: What's wrong with the call fopen("c:\newdir\file.dat", "r")?
19.17b: fopen() isn't letting me open files like "$HOME/.profile".
19.17c: How can I suppress the "Abort, Retry, Ignore?" message?
19.18: How can I increase the allowable number of simultaneously open
files?
19.20: How can I read a directory in a C program?
19.22: How can I find out how much memory is available?
19.23: How can I allocate arrays or structures bigger than 64K?
19.24: What does the error message "DGROUP exceeds 64K" mean?
19.25: How can I access memory located at a certain address?
19.27: How can I invoke another program from within a C program?
19.30: How can I invoke another program and trap its output?
19.31: How can my program discover the complete pathname to the
executable from which it was invoked?
19.32: How can I automatically locate a program's configuration files
in the same directory as the executable?
19.33: How can a process change an environment variable in its caller?
19.36: How can I read in an object file and jump to locations in it?
19.37: How can I implement a delay, or time a user's response,
with sub-second resolution?
19.38: How can I trap or ignore keyboard interrupts like control-C?
19.39: How can I handle floating-point exceptions gracefully?
19.40: How do I... Use sockets? Do networking? Write client/server
applications?
19.40b: How do I... Use BIOS calls? Write ISR's? Create TSR's?
19.40c: I'm trying to compile a program in which "union REGS" and
int86() are undefined.
19.40d: What are "near" and "far" pointers?
19.41: But I can't use all these nonstandard, system-dependent
functions, because my program has to be ANSI compatible!
Section 20. Miscellaneous
20.1: How can I return multiple values from a function?
20.3: How do I access command-line arguments?
20.5: How can I write data files which can be read on other machines
with different data formats?
20.6: How can I call a function, given its name as a string?
20.8: How can I implement sets or arrays of bits?
20.9: How can I determine whether a machine's byte order is big-endian
or little-endian?
20.9b: How do I swap bytes?
20.10: How can I convert integers to binary or hexadecimal?
20.11: Can I use base-2 constants (something like 0b101010)?
Is there a printf() format for binary?
20.12: What is the most efficient way to count the number of bits which
are set in an integer?
20.13: What's the best way of making my program efficient?
20.14: Are pointers really faster than arrays? How much do function
calls slow things down?
20.15b: People claim that optimizing compilers are good, but mine can't
even replace i/=2 with a shift.
20.15c: How can I swap two values without using a temporary?
20.17: Is there a way to switch on strings?
20.18: Is there a way to have non-constant case labels (i.e. ranges or
arbitrary expressions)?
20.19: Are the outer parentheses in return statements really optional?
20.20: Why don't C comments nest? Are they legal inside quoted
strings?
20.21b: What does a+++++b mean ?
20.24: Why doesn't C have nested functions?
20.24b: What is assert()?
20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions
from C?
20.26: Does anyone know of a program for converting Pascal or FORTRAN
to C?
20.27: Can I use a C++ compiler to compile C code?
20.28: I need to compare two strings for close, but not necessarily
exact, equality.
20.29: What is hashing?
20.31: How can I find the day of the week given the date?
20.32: Is (year % 4 == 0) an accurate test for leap years?
20.34: How do you write a program which produces its own source code as
output?
20.35: What is "Duff's Device"?
20.36: When will the next Obfuscated C Code Contest be held?
How can I get a copy of previous winning entries?
20.37: What was the entry keyword mentioned in K&R1?
20.38: Where does the name "C" come from, anyway?
20.39: How do you pronounce "char"?
20.39b: What do "lvalue" and "rvalue" mean?
20.40: Where can I get extra copies of this list?
Steve Summit
(e-mail address removed)
This article is Copyright 1990-2004 by Steve Summit.
Content from the book _C Programming FAQs: Frequently Asked Questions_
is made available here by permission of the author and the publisher as
a service to the community. It is intended to complement the use of the
published text and is protected by international copyright laws. The
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