The POSIX standard defines all of the functionality a POSIX-compliant operating system must implement. With respect to system programming, the POSIX standard defines the system call interface (that is, the APIs, not the ABIs) that the operating system must support.
Under the hood, most of the system-level APIs that C and C++ provide actually execute POSIX functions, or are POSIX functions themselves (as is this case with a lot of C library APIs). In fact, libc is generally considered to be a subset of the greater POSIX standard, while C++ leverages libc and POSIX to implement its higher-level APIs such as threading, memory management, error handling, file operations, and input/output. For more information, refer to https://ieeexplore.ieee.org/document/8277153/.
In this section, we will discuss some components of the POSIX standard that are relevant...