16 Library introduction [library]

16.5 Library-wide requirements [requirements]

16.5.4 Constraints on programs [constraints]

16.5.4.2 Namespace use [namespace.constraints]

16.5.4.2.2 Namespace posix [namespace.posix]

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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.