Overview header files
test functions
macros and functions for classifying character
error numbers and constant numbers
symbolic constant for floating point operations
boundaries for compiling enviroment
mathematical functions and constant numbers
not local jumps
signal communication between processes
variable number of parameters for function calls
common used data types and constant values and functions. For example rand()
common I/O functions
some sorting and converting algorithms
functions for string operations
common functions to handle the system time