Several standard, low-level libraries are defined for C. Their contents are declared in standard header files, which should be included in source files that use them. When a binary executable is built, the library binaries have to be explicitly linked to the executable by the compiler.

In Unix, get full info for a given function by typing

man funcname

stdlib	memory allocation integer functions (abs, div,) some system functions (exit, abort, getenv, etc.)
stdio	formatting text output; file manipulation and I/O
string	string manipulation, comparison; memory compare/copy/move
math	floating-point math functions
time	date and time
ctype	types of characters (upper/lower case, space, digit, etc.)
assert	the assert macro
errno	error description for many library functions
stdarg	allows variable numbers of arguments for functions
limits	limits on integer types
float	limits on floating-point types
signal	Unix signals
setjmp	means of return from deeply nested function call
locale	character sets, multi-byte characters, alphabetization

other libraries

unistd	environ, exec, exit, fork, getcwd, gethostename, getlogin, nice, pipe, sleep, usleep, etc.
sys/*	fcntl, socket, wait,
X11/*	XWindows
etc., etc., etc.