C Cheat Sheet

A Programming › C cheat sheet in English by pmg

Development Programming C Bsd

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read file char-by-char

#include <stdio.h>

FILE *h;

int ch;

h = fopen("filename", "rb");

/* error checking missing */

while ((ch = fgetc(h)) != EOF) {

/* deal with ch */

}

/* if needed test why last read failed */

if (feof(h) || ferror(h)) /* whatever */;

fclose(h);

You can replace fgetc(h) with getchar() to read from standard input.

read file line-by-line

#include <stdio.h>

FILE *h;
char line[100];
h = fopen("filename", "rb");
/* error checking missing */
while (fgets(line, sizeof line, h)) {
/* deal with line */
}
/* if needed test why last read failed */
if (feof(h) || ferror(h)) /* whatever */;
fclose(h);

Flexible Array Member

How to declare a FAM?

By using empty brackets as the last member of a struct.

How to define the size for an object containg a FAM?

ptr = malloc(sizeof *ptr + sizeof (FAMTYPE[wantedsize]));

Do not use FAMs! They were known as struct hack before C99 and, now as then, feel like a dirty hack.

<stdio.h> functions with a FILE pointer at the end

char *fgets(char *, int, FILE *);

int fputc(int, FILE *);

int fputs(char *, FILE *);

size_t fread(void *, size_t, size_t, FILE *);

FILE *freopen(char *, char *, FILE *);

size_t fwrite(void *, size_t, size_t, FILE *);

int ungetc(int, FILE *);

dynamic memory

Remember to #include <stdlib.h>
Allocate
malloc	*ptr = malloc(n sizeof ptr);*
calloc	*ptr = calloc(n, sizeof ptr);**
Change size
realloc	*newsize = n sizeof ptr; tmp = realloc(ptr, newsize); if (tmp) ptr = tmp; else / ptr is still valid /;*
Release
free	free(ptr);

remove trailing newline

How do I remove the final newline in a string?

len = strlen(data);
if (len && data[len - 1] == '\n') data[--len] = 0;

or, if you don't need to keep and update data length

data[strcspn(data, "\n")] = 0;

If len is known in advance, do not call strlen(). You can pass the updated len to the caller.

Casting

^{Casts in C are almost always wrong. When are they right?}
<ctype.h>	isupper((unsigned char)ch)
%p printf specifiers	printf("%p", *(void)**ptr)

Specifically a cast to the return value of malloc() is a definite sign the code author either didn't know what he was doing or didn't choose a good language for the implementation of whatever he's doing.

(BSD) sockets

Headers needed
#include <arpa/inet.h>
#include <netdb.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

initialize with
getaddrinfo()

loop to find and connect a socket
socket()
connect()
if needed: close()
after loop: freeaddrinfo()

getpeername(), getsockname()
send() or recv() or sendto() or recvfrom()

close()

Predefined C macros

__FILE__

"filename.c" or something like that

__LINE__

42 or another integer

__STDC__

__STDC_VERSION__

undefined for C89; 199901L for C99; 201112L for C11

__DATE__

"Feb 17 2012" for example

__TIME__

"15:16:17" for example

__func__

"main" for example

__STDC_HOSTED__

0 or 1

Reserved identifiers

Reserved for all uses anywhere
_[A-Z]; __	E[A-Z]; E[0-9]
is[a-z]; to[a-z]	SIG[A-Z]; SIG_[A-Z]
LC_[A-Z]*	*_t
str[a-z]; mem[a-z]; wcs[a-z]*
all math functions possibly followed by f or l
When #include <limits.h> is present
*_MAX
When #include <signal.h> is present
SA_*	sa_*
POSIX adds a few other identifiers
<dirent.h>	d_*
<fcntl.h>	l_; F_; O_; S_
<grp.h>	gr_*
<pwd.h>	pw_*
<sys/stat.h>	st_; S_
<sys/times.h>	tms_*
<termios.h>	C_; V_; I_; O_; TC; B[0-9]

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