Mac OS X Leopard - Technology - UNIX. In: Leopard Technology Overview. Apple Inc., abgerufen am 11. Juni 2007: „Leopard is now an Open Brand UNIX 03 Registered Product, conforming to the SUSv3 and POSIX 1003.1 specifications for the C API, Shell Utilities, and Threads.“
Open Group Base Specifications Issue 7, 2008 edition. The Open Group, abgerufen am 29. August 2018 (The _POSIX_C_SOURCE Feature Test Macro): „Note that the definition of _XOPEN_SOURCE with the value 700 makes the definition of _POSIX_C_SOURCE redundant and it can safely be omitted.“