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C standard library
The C standard library, sometimes referred to as libc, is the standard library for the C programming language, as specified in the ISO C standard. Starting from the original ANSI C standard, it was developed at the same time as the C POSIX library, which is a superset of it. Since ANSI C was adopted by the International Organization for Standardization, the C standard library is also called the ISO C library.
The C standard library provides macros, type definitions and functions for tasks such as string manipulation, mathematical computation, input/output processing, memory management, and input/output.
The application programming interface (API) of the C standard library is declared in a number of header files. Each header file contains one or more function declarations, data type definitions, and macros.
After a long period of stability, three new header files (<iso646.h>, <wchar.h>, and <wctype.h>) were added with Normative Addendum 1 (NA1), an addition to the C Standard ratified in 1995. Six more header files (<complex.h>, <fenv.h>, <inttypes.h>, <stdbool.h>, <stdint.h>, and <tgmath.h>) were added with C99, a revision to the C Standard published in 1999, five more files (<stdalign.h>, <stdatomic.h>, <stdnoreturn.h>, <threads.h>, and <uchar.h>) with C11 in 2011 and one more file (<stdbit.h>) with C23 in 2023. In total, there are now 30 header files:
Three of the header files (<complex.h>, <stdatomic.h>, and <threads.h>) 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 <fcntl.h> and <unistd.h>. A number of other groups are using other nonstandard headers – the GNU C Library has <alloca.h>, and OpenVMS has the va_count() function.
On Unix-like systems, the authoritative documentation of the API is provided in the form of man pages. On most systems, man pages on standard library functions are in section 3; section 7 may contain some more generic pages on underlying concepts (e.g. man 7 math_error in Linux).
Unix-like systems typically have a C library in shared library form, but the header files (and compiler toolchain) may be absent from an installation so C development may not be possible. The C library is considered part of the operating system on Unix-like systems; in addition to functions specified by the C standard, it includes other functions that are part of the operating system API, such as functions specified in the POSIX standard. The C library functions, including the ISO C standard ones, are widely used by programs, and are regarded as if they were not only an implementation of something in the C language, but also de facto part of the operating system interface. Unix-like operating systems generally cannot function if the C library is erased. This is true for applications which are dynamically as opposed to statically linked. Further, the kernel itself (at least in the case of Linux) operates independently of any libraries.
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C standard library
The C standard library, sometimes referred to as libc, is the standard library for the C programming language, as specified in the ISO C standard. Starting from the original ANSI C standard, it was developed at the same time as the C POSIX library, which is a superset of it. Since ANSI C was adopted by the International Organization for Standardization, the C standard library is also called the ISO C library.
The C standard library provides macros, type definitions and functions for tasks such as string manipulation, mathematical computation, input/output processing, memory management, and input/output.
The application programming interface (API) of the C standard library is declared in a number of header files. Each header file contains one or more function declarations, data type definitions, and macros.
After a long period of stability, three new header files (<iso646.h>, <wchar.h>, and <wctype.h>) were added with Normative Addendum 1 (NA1), an addition to the C Standard ratified in 1995. Six more header files (<complex.h>, <fenv.h>, <inttypes.h>, <stdbool.h>, <stdint.h>, and <tgmath.h>) were added with C99, a revision to the C Standard published in 1999, five more files (<stdalign.h>, <stdatomic.h>, <stdnoreturn.h>, <threads.h>, and <uchar.h>) with C11 in 2011 and one more file (<stdbit.h>) with C23 in 2023. In total, there are now 30 header files:
Three of the header files (<complex.h>, <stdatomic.h>, and <threads.h>) 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 <fcntl.h> and <unistd.h>. A number of other groups are using other nonstandard headers – the GNU C Library has <alloca.h>, and OpenVMS has the va_count() function.
On Unix-like systems, the authoritative documentation of the API is provided in the form of man pages. On most systems, man pages on standard library functions are in section 3; section 7 may contain some more generic pages on underlying concepts (e.g. man 7 math_error in Linux).
Unix-like systems typically have a C library in shared library form, but the header files (and compiler toolchain) may be absent from an installation so C development may not be possible. The C library is considered part of the operating system on Unix-like systems; in addition to functions specified by the C standard, it includes other functions that are part of the operating system API, such as functions specified in the POSIX standard. The C library functions, including the ISO C standard ones, are widely used by programs, and are regarded as if they were not only an implementation of something in the C language, but also de facto part of the operating system interface. Unix-like operating systems generally cannot function if the C library is erased. This is true for applications which are dynamically as opposed to statically linked. Further, the kernel itself (at least in the case of Linux) operates independently of any libraries.