Style:

1TBS of course ;)

Vi:

set: :set noai tabstop=8 shiftwidth=8 softtabstop=8 noexpandtab
modeline: /* vim: set noai tabstop=8 shiftwidth=8 softtabstop=8 noexpandtab: */
 

Strings:

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};
char str[12] = "Hello World";
char str[0] = '\0';
char buf[];

copy: strcpy(str2, str1)

concat: strcat(str1, str2)

length: strlen(str)

duplicate: ptr = strdup(str)

substring: strstr(str1, str2)

extract: strsep(*strptr, delimstr)
strstr():

locate subscring
or: “check if ‘sent’ contains ‘word'”
if(strstr(sent, word) != NULL) //..or skip NULL check


strsep():

extract token
or: “split substring using delim”
like deprecated strtok()
while((found = strsep(&string,",")) != NULL)


Things to remember:

set it before using it (initialize)
strings are char arrays terminated by \0
strings in double quotes are auto terminated by \0
strcpy : watch out for buffer overruns with long strings
use : checks, char buf[1024], \0 term, malloc(strlen(str) + xx)
buf[] has no size and no space allocated
convert int to string: itoa, sprintf, snprintf(buf, 10, "%d", num);

tutorialspoint.com/cprogramming/c_strings.htm
 
Print:
printf outputs to stdout stream

fprintf goes to a file handle FILE*

sprintf goes to a buffer you allocated (char*)
printf("This is a str: %s\n", str);
fprintf(fh, "log: %s", str);
sprintf(buf, "%s %s", str1, str2);

int: %d

long int: %ld

long long int: %lld

unsigned long long int: %llu
“%s” format specifier for printf always expects a char* argument
 
Pointers:
// ptr is going to store address of integer value
int i = 20;
int *ptr;

                Operator        Operator Name           Purpose
*ptr            *               Value at Operator       gives Value stored at particular address
type* ptr;                      Same                    Same
&ptr            &               Address Operator        gives Address of Variable
**ptr                           Double Pointer          declares a Pointer to a Pointer


using *ptr is actually the first element (not its address)
while int *var is the same as int* var, *var is better/more clear to use
initialize (NULL or valid address)

 
Extern:

tell compiler variable is declared elsewhere
use only in one place

example.h:
extern int global_variable;

example.c:
#include "example.h"
int global_variable = 1337;

example2.c:
#include "example.h"
void use_it(void)
{
    printf("Global variable: %d\n", global_variable++);
}

 
Compiler messages:
warning: implicit declaration of function 'function' [-Wimplicit-function-declaration]
You are using a function for which the compiler has not seen a declaration (“prototype”) yet.
int main()
{
    fun(2, "21"); /* The compiler has not seen the declaration. */
    return 0;
}

int fun(int x, char *p)
{
    /* ... */
}

You need to declare your function before main, like this, either directly or in a header:
int fun(int x, char *p);

From stackoverflow.com/a/8440833
 
Macro:
ifdef : “if defined” (quelle surprise;)
example.c:
#ifdef DEBUG
 printf("DEBUG: %s\n", dbgstuff);
#endif

gcc -DDEBUG -c
gcc.gnu.org/onlinedocs/cpp/Ifdef.html
 
Include guard:
ifndef : if NOT defined
#ifndef checks whether the given token has been #defined earlier in the file or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif statement. #ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was not set at the top of that file.
example.h:
#ifndef _INCL_GUARD
#define _INCL_GUARD
#endif

cprogramming.com/reference/preprocessor/ifndef.html
Malloc
char *buf = (char *) malloc(bufferSize);

where bufferSize is the runtime result of some computation
Examples:
char *string = (char*)malloc(n+1 * sizeof(char));
size_t length = strlen(str1) + strlen(str2) + 1;
char *concat = malloc(sizeof(char) * length);
char* buffer = (char*)malloc(256);
rhost = (char *)malloc(sizeof(char)*128);


if allocatable buffer space is variable, use malloc instead of buf[1024]
when done use free: free(buf);
strdupe already does malloc, no malloc needed

Execute command:
system("ls -la")
get output: popen()
FILE *fp = popen("ls -la", "r");
if (fp)
	fscanf(fp, "%100s", var);

-or-
	fgets(var, 100, fp);

Return:
return 0 or 1:
int function {
	if (success) {	return 0; }
	return 1;
}

return char* value:
char* function {
	char *buf;
	...
	return buf;
}

(use malloc or buf[1024] and free)
Examples:
str functions (example):
printf ("\nDEBUG: strncmp %s %i\n", flist_getfilename(ftmp), strncmp(flist_getfilename(ftmp), ".mp3", 4));
printf ("\nDEBUG: strcmp %s %i\n", flist_getfilename(ftmp), strcmp(flist_getfilename(ftmp), ".mp3"));
printf ("\nDEBUG: strcasecmp %s %d\n", flist_getfilename(ftmp), strcasecmp(flist_getfilename(ftmp), ".mp3"));

more strcmp:
if (!strncmp(flist_getfilename(ftmp), ".mp3", 4))
if (strncmp(flist_getfilename(ftmp), ".mp3", 4) != 4 )