Stdlib.h

Functions to convert strings into numerical types.

Function	Description	Example
int atoi(const char *str)	Convert string to integer	atoi("1024") //1024
double atof(const char *str)	Convert string to double	atof("3.14") //3.14
long strtol(const char str, char *end, int base)	Convert string to long integer with base selection	strtol("1A", NULL, 16) // 26

Functions for dynamic memory allocation and deallocation.

Function	Description	Example
void* malloc(size_t size)	Allocate a block of uninitialized memory	int p = malloc(sizeof(int)5);
void* calloc(size_t nitems, size_t size)	Allocate memory and initialize bytes to zero	int *p = calloc(5, sizeof(int));
void realloc(void ptr, size_t size)	Resize a previously allocated memory block	p = realloc(p, sizeof(int)*10);
void free(void *ptr)	Deallocate/Free memory	free(p);

Basic integer math and random number generation.

Function	Description	Example
int abs(int x)	Absolute value of an integer	abs(-10) //10
void srand(unsigned int seed)	Seed the random number generator	srand(time(0));
int rand(void)	Generate a pseudo-random integer	rand() % 100 // 42

Process control and algorithm utilities.

Function	Description	Example
void exit(int status)	Terminate the program normally	exit(0);
int system(const char *command)	Execute a shell command	system("ls -l");
void qsort(void base, size_t n, size_t size, int (cmp)(...))	Sort an array (Quicksort)	qsort(arr, 5, sizeof(int), cmp);