The C standard library is a standardized collection of headers and library functions for the C programming language. The library provides a set of commonly used macros, type definitions, and functions to perform common tasks such as string handling, input/output processing, memory allocation, math computations, thread management, and various other common services.

Standard Headers

Related functions, necessary types to use those functions and related macros are grouped into headers. Collectively, they are known as the standard headers.

Standard C provides 29 standard headers. The headers may be included in any order. The headers may also be included more than once - the effect of that is as if it was only included once. All standard C functions have external linkage.

<assert.h>	<complex.h>	<ctype.h>	<errno.h>	<fenv.h>
<float.h>	<inttypes.h>	<iso646.h>	<limits.h>	<locale.h>
<math.h>	<setjmp.h>	<signal.h>	<stdalign.h>	<stdarg.h>
<stdatomic.h>	<stdbool.h>	<stddef.h>	<stdint.h>	<stdio.h>
<stdlib.h>	<stdnoreturn.h>	<string.h>	<tgmath.h>	<threads.h>
<time.h>	<uchar.h>	<wchar.h>	<wctype.h>

Optional headers

3 of the 29 standard headers are optional:

• <complex.h>
• <stdatomic.h>
• <threads.h>

While implementations are encouraged to support those headers, they are not required to.

Detailed description

Header Name	Since	Description
<assert.h>	C89	Provides a set of assertion macros used in assisting with debugging and detecting logic errors.
<complex.h>	C99	Provides a set of functions used to manipulate complex numbers.
<ctype.h>	C89	Provides a set of functions used for classifying characters.
<errno.h>	C89	Provides a set of predefined values for errno.
<fenv.h>	C99	Provides a set of functions for controlling the floating-point environment.
<float.h>	C89	Provides a set of macros defining the implementation of the floating-point environment.
<inttypes.h>	C99	Provides a set of exact width integer types.
<iso646.h>	NA1	Provides a set of macros that implement an alternative way of expressing various C operators
<limits.h>	C89	Provides a set of macros specifying the implementation-specific limits of the integer types.
<locale.h>	C89	Provides a set of functions localization functions
<math.h>	C89	Provides a set of common mathematical functions.
<setjmp.h>	C89	Provides macros to perform non-local jump operators.
<signal.h>	C89	Provides a set of signal handling functions.
<stdalign.h>	C11	Provides a set of macros for working with object alignments.
<stdarg.h>	C89	Provides a set of macros for working with variadic functions
<stdatomic.h>	C11	Provides a set of functions for performing atomic operations on data.
<stdbool.h>	C99	Provides boolean macros.
<stddef.h>	C89	Provides a set of commonly used type definitions within the standard library.
<stdint.h>	C99	Provides a set of exact width integer types.
<stdio.h>	C89	Provides a set of standard input/output processing functions.
<stdlib.h>	C89	Provides core library functions such as memory management and random number generation.
<stdnoreturn.h>	C11	Provides a macro for specifying non-returning functions.
<string.h>	C89	Provides a set of string handling functions.
<tgmath.h>	C99	Provides a set of type-generic math functions.
<threads.h>	C11	Provides a set of thread management functions.
<time.h>	C89	Provides a set of date and time handling functions.
<uchar.h>	C11	Provides a set of Unicode character manipulation functions.
<wchar.h>	NA1	Provides a set of wide string handling functions.
<wctype.h>	NA1	Provides a set of functions used for classifying wide characters.

Embedded C

Main article: Embedded C

Embedded C introduced two optional headers <stdfix.h> and <iohw.h>. The headers not formally part of the standard library.

See also

• List of standard c functions