# C Cheat Sheet

This is a collection of important C features.

## Memory Allocation
Allocate `size` bytes of memory on the heap:
**`void *malloc(size_t size)`**
MALLOC MUST BE NULL CHECKED!!!

Free previously allocated memory on the heap:
**`void free(void *ptr)`**

Both are defined in the `stdlib.h` header file.

## String operations

Find the length of a string:
**`size_t strlen(const char *str)`**
returns the length of str.

Compare two strings:
**`int strcmp(const char *Str1, const char *str2)`**
returns 0 if equal

Copy a string:
**`char *strcpy(char *dest, const char *src)`**
returns pointer to destination string dest.

All are defined in the `string.h` header file.

## Moving and manipulating memory

Move n bytes from src to dest:
**`void *memcpy(void *dest, const void *src, size_t n)`**
returns pointer to dest.
does not work on overlapping src and dest

**`void *memmove(void *dest, const void *src, size_t n)`**
returns pointer to dest.
Does work for overlapping src and dest

Fill a block of memory (dest) with a particular value ( c) for n bytes
**`void *memset(void *dest, int c, size_t n) //string.h`**
returns a pointer to dest

## fork and exec

**`pid_t fork() //unistd.h`**
returns -1 on error, 0 to the child, and the child pid to the parent.

**`void exit(int status) //stdlib.h`**

Run another program. The v is for argv and the p is for looking in PATH.
**`int execvp(const char *file, char *const argv[]) //unistd.h`**
only returns on error with -1.

Wait for a child process to end:
**`pid_t waitpid(pid_t pid, int *status, int options) //sys/wait.h`**
returns the PID of ended process on success.
Methods on the status variable:
**`WIFEXITED(status)`** returns true if child terminated normally
**`WEXITSTATUS(status)`** returns the exit status of the child

Wait for any child process to end:
**`pid_t wait(int *status) //sys/wait.h`**

## thread management

### Main Methods

#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg)

returns 0 on success, error number on error

#include <pthread.h>
int pthread_join(pthread_t thread, void **retval)

returns 0 on success, error number on error

#include <pthread.h>
void pthread_exit(void *retval)

### Example usage

#include <pthread.h>
...
void *myfunc(void *arg) {
    ...
    pthread_exit(NULL);
}
...
pthread_t tid;
int myarg = 1;
pthread_create(&tid, NULL, myfunc, &myarg);
...
pthread_join(tid, NULL);
...

## Synchronization

### pthread mutex
Example usage:

#include <pthread.h>
...
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
...
pthread_mutex_lock(&mutex);
// critical section
pthread_mutex_unlock(&mutex);
...

## File I/O
fopen
fclose
fgets

## POSIX message queues