An application that includes a header described by the POSIX.1 standard must specify the _POSIX_SOURCE feature test macro prior to any instance of that header being included in the source file. When _POSIX_SOURCE is specified in the source file, the following conditions are true:
All symbols required by the POSIX.1 standard are made visible to the application.
Symbols that are explicitly permitted, but not required, by the POSIX.1 standard are made visible to the application.
Additional symbols not required or explicitly permitted by the POSIX.1 standard are not made visible.
