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POSIX Basics
POSIX (Portable Operating System Interface) is a family of standards for maintaining compatibility between operating systems. It describes utilities, APIs, and services that a compliant OS should provide to software, thus making it easier to port programs from one system to another.
A practical example: in a Unix-like operating system, there are three standard streams, stdin, stdout and stderr - they are I/O connections that you will probably come across when using a terminal, as they manage the flow from the standard input (stdin), standard output (stdout) and standard error (stderr).
So, in this case, when we want to interact with any of these streams (through a process, for example), the POSIX operating system API makes it easier - for example, in the <unistd.h> C header where the stdin, stderr, and stdout are defined as STDIN_FILENO, STDERR_FILENO and STDOUT_FILENO.
POSIX also adds a standard for exit codes, filesystem semantics, and several other command line utility API conventions.
Free Content Summary of some POSIX implementations A guide to POSIX POSIX standard by IEEE