This section introduces the system calls and other system services you can
use to develop application programs.
Each application performs a different function, but
goes through the same basic steps:
input, processing, and output.
For the input and output steps, most applications
interact with an end user at a terminal.
During the processing step, sometimes
an application needs access to special services provided
by the operating system (for example, to interact with
the file system, control processes, manage memory, and more).
Some of these services are provided through system calls and some through
libraries of functions.
Libraries and header files