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The C standard library is the standard library for the C language, as specified in the ISO C (ANSI C) standard
The 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
On Unix-like systems, the documentation of the API is provided by man pages
POSIX is a family of standards that defines both the system and user-level API, along with shells and utility interfaces, for software compatibility with variants of UNIX and other operating systems
The Single UNIX Specification (SUS) is a standard for operating systems compliance, with which being required to qualify for using the UNIX trademark
The C POSIX library is a part of POSIX, a specification of a C standard library for POSIX compatible systems. It is a superset of the C standard library, which adds several nonstandard C headers for Unix-specific functionality. For example, unistd.h provides access to the POSIX OS API
There are two kinds of the C implementations: hosted, where the C standard library is available; and freestanding, where only a few headers are usable that contains only definitions and types
An OS kernel is an example of a program running in a freestanding environment; a program using the facilities of an OS is an example of a program running in a hosted environment
Unix typically has a C library (libc) in a shared library form, and is linked automatically into every executable
The C library is considered part of the operating system on Unix 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
There are couple of implementations of the libc: BSD libc, glibc, musl etc.
GCC provides built-in versions of many of the functions in the libc; that is, the implementations of the functions are written into the compiled object file, and the program calls the built-in versions instead of the functions in the C library shared object file
A C standard library for Linux includes wrappers around the system calls of the Linux kernel. Many library functions don't make any use of system calls (e.g., the string-manipulation functions). On the other hand, some library functions are layered on top of system calls
The combination of the Linux kernel system call interface and a C standard library is what builds the Linux API
Linux API provides additional capabilities that are not part of POSIX. For example: