Components of a libc

A modern, standards compliant libc implementation consists of the following libraries, tools and sub-systems.

1. The C runtime, commonly referred to as the CRT - This component is responsible for the start-up and termination of an application. More information about the CRT can be found here.
2. The C standard library - This component provides the various C standard libraries like math.h, stdio.h, thread.h etc.
3. The POSIX extensions - This component provides various POSIX extension headers like pthread.h, unistd.h etc.
4. Library files - Most of libc is made available as libc.a or libc.so. However, for POSIX compliance, few parts of the libc implementation are grouped into their own .a or .so files. For example, POSIX requires that the math functions be packaged as libm.a for static linking and libm.so for dynamic linking. Hence, though the C standard does not require such a packaging scheme, all POSIX compliant libc implementations provide libm.a and libm.so separate from libc.a and libc.so. Same is true with pthread and a bunch of other library files as well. Though they are packaged into separate library files, they are all considered as part of, and provided by, a single libc implementation.
5. Non-standard extensions - These are extension utilities provided by a typical modern libc. For example, syscall.h.
6. The dynamic loader - This component is responsible for dynamic loading of shared libraries.