The header file <stdlib.h> defines a number of types, macros, and functions of general utility.

Types
div_t | Structure containing quotient and remainder after division of ints
ldiv_t | Structure containing quotient and remainder after division of longs
lldiv_t | Structure containing quotient and remainder after division of long longs
String to number conversions
atoi | Convert string to int
atol | Convert string to long
atoll | Convert string to long long
strtol | Convert string to long
strtoll | Convert string to long long
strtoul | Convert string to unsigned long
strtoull | Convert string to unsigned long long
Number to string conversions
itoa | Convert int to string
ltoa | Convert long to string
lltoa | Convert long long to string
utoa | Convert unsigned to string
ultoa | Convert unsigned long to string
ulltoa | Convert unsigned long long to string
Integer arithmetic functions
div | Divide two ints returning quotient and remainder
ldiv | Divide two longs returning quotient and remainder
lldiv | Divide two long longs returning quotient and remainder
Pseudo-random sequence generation functions
RAND_MAX | Maximum value returned by rand
rand | Return next random number in sequence
srand | Set seed of random number sequence
Memory allocation functions
calloc | Allocate space for an array of objects and initialize them to zero
free | Frees allocated memory for reuse
malloc | Allocate space for a single object
realloc | Resizes allocated memory space or allocates memory space
Search and sort functions
bsearch | Search a sorted array.
qsort | Sort an array.
Exit functions
atexit | Register functions to call on exit.
exit | Terminate the application.