Search notes:

POSIX

A standard for operating systems.
POSIX = ISO/IEC 9945 (aka IEEE 1003).
To some extent, the System V Interface Description is a superset of POSIX.

Syscalls

Syscalls are not part of POSIX. POSIX defines a programming interface - if the functionality is implemented with syscalls or not is an implementation detail.
Stackoverflow: Are Unix Linux system calls part of posix library functions.

See also

POSIXLY_CORRECT, Perl module POSIX
Originally, Windows came with a POSIX subsystem: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\SubSystems
The POSIX standard also covers the line editor ed and its visual «variant» vi.
Linux Standard Base

Index