Editing C standard library (section)

== Application programming interface (API) ==

=== {{Anchor|headers}}Header files ===

The [[application programming interface]] (API) of the C standard library is declared in a number of [[header file]]s. Each header file contains one or more function declarations, data type definitions, and macros.

After a long period of stability, three new header files (<code>iso646.h</code>, <code>wchar.h</code>, and <code>wctype.h</code>) were added with ''Normative Addendum 1'' (NA1), an addition to the C Standard ratified in 1995. Six more header files (<code>complex.h</code>, <code>fenv.h</code>, <code>inttypes.h</code>, <code>stdbool.h</code>, <code>stdint.h</code>, and <code>tgmath.h</code>) were added with [[C99]], a revision to the C Standard published in 1999, five more files (<code>stdalign.h</code>, <code>stdatomic.h</code>, <code>stdnoreturn.h</code>, <code>threads.h</code>, and <code>uchar.h</code>) with [[C11 (C standard revision)|C11]] in 2011 and one more file (<code>stdbit.h</code>) with [[C23 (C standard revision)|C23]] in 2023. In total, there are now 30 header files:

{| class="wikitable sortable"
|-
! Name !! From !! Description
|-
| <code>[[assert.h|<assert.h>]]</code> || || Declares the [[assertion (computing)|assert]] macro, used to assist with detecting logical errors and other types of bugs while [[debugging]] a program.
|-
| <code>[[complex.h|<complex.h>]]</code> || C99 || Defines a [[C mathematical functions#complex.h|set of functions]] for manipulating [[complex number]]s.
|-
| <code>[[ctype.h|<ctype.h>]]</code> || || Defines [[C character classification|set of functions]] used to classify characters by their types or to convert between upper and lower case in a way that is independent of the used [[character set]] (typically [[ASCII]] or one of its extensions, although implementations utilizing [[EBCDIC]] are also known).
|-
| <code>[[errno.h|<errno.h>]]</code> || || For testing error codes reported by library functions.
|-
| <code>[[fenv.h|<fenv.h>]]</code> || C99 || Defines a [[C mathematical functions#fenv.h|set of functions]] for controlling [[floating-point]] environment.
|-
| <code>[[float.h|<float.h>]]</code> || || Defines [[C data types#float.h|macro constants]] specifying the implementation-specific properties of the [[floating-point]] library.
|-
| <code>[[inttypes.h|<inttypes.h>]]</code> || C99 || Defines [[C data types#inttypes.h|exact-width integer types]].
|-
| <code>[[iso646.h|<iso646.h>]]</code> || NA1 || Defines [[C alternative tokens|several macros]] that implement alternative ways to express several standard tokens. For programming in [[ISO 646]] variant character sets.
|-
| <code>[[limits.h|<limits.h>]]</code> || || Defines [[C data types#limits.h|macro constants]] specifying the implementation-specific properties of the integer types.
|-
| <code><locale.h></code> || || Defines [[C localization functions|localization functions]].
|-
| <code>[[math.h|<math.h>]]</code> || || Defines [[C mathematical functions|common mathematical functions]].
|-
| <code>[[setjmp.h|<setjmp.h>]]</code> || || Declares the macros <code>setjmp</code> and <code>longjmp</code>, which are used for non-local exits.
|-
| <code>[[signal.h|<signal.h>]]</code> || || Defines [[C signal handling|signal-handling functions]].
|-
| <code><stdalign.h></code> || C11 || For querying and specifying the [[Data structure alignment|alignment]] of objects.
|-
| <code>[[stdarg.h|<stdarg.h>]]</code> || || For accessing a varying number of arguments passed to functions.
|-
| <code><stdatomic.h></code> || C11 || For [[Linearizability|atomic operations]] on data shared between threads.
|-
| <code><stdbit.h></code> || C23 || For byte ordening and bit representation.
|-
| <code>[[stdbool.h|<stdbool.h>]]</code> || C99 || Defines [[C data types#stdbool.h|a Boolean data type]].
|-
| <code>[[stddef.h|<stddef.h>]]</code> || || Defines [[C data types#stddef.h|several useful types and macros]].
|-
| <code>[[stdint.h|<stdint.h>]]</code> || C99 || Defines [[C data types#stdint.h|exact-width integer types]].
|-
| <code>[[stdio.h|<stdio.h>]]</code> || || Defines [[C file input/output|core input and output functions]]
|-
| <code><stdlib.h></code> || || Defines [[C string handling#stdlib.h|numeric conversion functions]], [[C mathematical functions#stdlib.h|pseudo-random numbers generation functions]], [[C dynamic memory allocation|memory allocation]], [[C process control|process control functions]]
|-
| <code><stdnoreturn.h></code> || C11 || For specifying non-returning functions
|-
| <code>[[string.h|<string.h>]]</code> || || Defines [[C string handling|string-handling functions]]
|-
| <code>[[tgmath.h|<tgmath.h>]]</code> || C99 || Defines [[C mathematical functions#tgmath.h|type-generic mathematical functions]].
|-
| <code><threads.h></code> || C11 || Defines functions for managing multiple [[Thread (computer science)|threads]], [[Mutual exclusion|mutexes]] and [[Monitor (synchronization)|condition variables]]
|-
| <code>[[time.h|&lt;time.h&gt;]]</code> || || Defines [[C date and time functions|date- and time-handling functions]]
|-
| <code><uchar.h></code> || C11 || Types and functions for manipulating [[Unicode]] characters
|-
| <code>[[wchar.h|<wchar.h>]]</code> || NA1 || Defines [[C string handling|wide-string-handling functions]]
|-
| <code>[[wctype.h|<wctype.h>]]</code> || NA1 || Defines [[C character classification|set of functions]] used to classify wide characters by their types or to convert between upper and lower case
|}

Three of the header files (<code>complex.h</code>, <code>stdatomic.h</code>, and <code>threads.h</code>) are conditional features that implementations are not required to support.

The [[POSIX]] standard added several nonstandard C headers for Unix-specific functionality. Many have found their way to other architectures. Examples include <code>[[fcntl.h]]</code> and <code>[[unistd.h]]</code>. A number of other groups are using other nonstandard headers – the [[GNU C Library]] has <code>alloca.h</code>, and [[OpenVMS]] has the <code>va_count()</code> function.

=== Documentation ===
On Unix-like systems, the authoritative documentation of the API is provided in the form of [[man page]]s. On most systems, man pages on standard library functions are in section&nbsp;3; section&nbsp;7 may contain some more generic pages on underlying concepts (e.g. <code>man 7 math_error</code> in [[Linux]]).