20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.4 Constraints on programs [constraints] Namespace use [namespace.constraints] Namespace posix [namespace.posix]

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace posix or to a namespace within namespace posix unless otherwise specified.
The namespace posix is reserved for use by ISO/IEC 9945 and other POSIX standards.