# C Programming/stdlib.h

stdlib.h is the header of the general purpose standard library of C programming language which includes functions involving memory allocation, process control, conversions and others. It is compatible with C++ and is known as cstdlib in C++. The name "stdlib" stands for "standard library".

## Member functions

Members of the stdlib.h can be classified into the following categories: conversion, memory, process control, sort and search, mathematics.
| Name | Description |
| Type Conversion ||
| atof | string to double (NOT float) |
| atoi | string to integer |
| atol | string to long |
| strtod | string to double |
| strtol | string to long int |
| strtoul | string to unsigned long int |
| strtoll | string to long long int |
| strtoull | string to unsigned long long int |
| Pseudo-random sequence generation ||
| int rand(void) | generates a pseudo-random number |
| int random(void) | generates a pseudo-random number (not standard C; provided by POSIX) |
| void srand(unsigned int seed) | set the rand pseudo-random generator seed [common convention uses time to seed] |
| void srandom(unsigned int seed) | set the random pseudo-random generator seed [common convention uses time to seed] (not standard C; provided by POSIX) |
| Memory allocation and deallocation ||
| malloc calloc realloc | allocate memory from the heap |
| free | release memory back to the heap |
| Process control ||
| /abort/ | terminate execution abnormally |
| atexit | register a callback function for program exit |
| exit | terminate program execution |
| getenv | retrieve an environment variable |
| system | execute an external command |
| Sorting, searching and comparison ||
| bsearch | binary search an array |
| qsort | sort an array |
| Mathematics ||
| int abs(int) | absolute value of an integer. |
| long int labs(long int) | absolute value of a long integer. |
| div | integer division (returns quotient and remainder) |
| ldiv | long integer division (returns quotient and remainder) |
| Multibyte / Wide Characters ||
| mblen | size of multibyte char |
| mbtowc, wctomb, mbstowcs, wcstombs | multibyte & wide character conversion |

## Member constants
### NULL

The stdlib.h and stddef.h header files define the macro NULL, which yields a null pointer constant, and represents a pointer value that is guaranteed not to point to a valid address in memory.

#### Variants

NULL may be defined as a constant expression equal to int zero, long int zero, or zero cast to a void * pointer:
Although the null pointer constant is always represented in C by the symbolic constant 0 or by 0 cast to a void pointer, the actual bit representation of such a pointer is system-specific and may contain one-bits.

## Member data types
### size_t

The size_t definition shall be provided to a referencing piece of code by including this header file. In fact most implementations don't have it defined literally in this file but instead include the file stddef.h as, for example, the standard library of the GNU C compiler does. The direct inclusion of stddef.h for application code is totally valid and thus can replace stdlib.h in cases where no other members from this file are needed or desired. This whole header file design conforms with e.g. the C99 ISO/ANSI standard definition.

### div_t, ldiv_t

Two less widely used datatypes, div_t and ldiv_t, are also defined. They are the return types of the div and ldiv functions. The standard defines them as:

## Nonstandard functions
### itoa

/itoa/ is a common function that is included in many implementations of stdlib.h, but the standard does not define the function. Although the same result can be achieved with sprintf, which is defined in the standard, calling itoa directly involves considerably less overhead than calling it (or an equivalent) via sprintf.

## See also

* stdio.h
* C standard library
* wchar_t (wide characters)
* div (C) (division function)

## References

* ↑ C++ Resources Network - stdlib.h accessed 2009 12 15
* ↑ C++ Resources Network - stdlib.h accessed 2009 12 15
* ↑ http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1124.pdf chapter 7.20
* stdlib.h summary on cplusplus.com, accessed in February 2006.
* Why size_t matters, accessed in May 2016.