[ ]
#define _XOPEN_SOURCE
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Glibc ：
1.ISO C: C .
2.POSIX: ISO/IEC 9945 (aka IEEE 1003) .
3.Berkeley Unix: BSD SunOS.
4.SVID: V .
5.XPG: The X/Open Portability Guide.
， 。 POSIX， _POSIX_SOURCE。 _BSD_SOURCE、_SVID_SOURCE、_XOPEN_SOURCE。
：
#define _XOPEN_SOURCE
5. The X/Open Portability Guide 。
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：
1） cc ， ：cc -D _POSIX_SOURCE file.c
2） ：＃define _POSIX_SOURCE 1
[_XOPEN_SOURCE ？]
， ：
Macro: _XOPEN_SOURCE
If you define this macro, functionality described in the X/Open Portability Guide is included. This is a superset of the POSIX.1 and POSIX.2 functionality and in fact _POSIX_SOURCE and _POSIX_C_SOURCE are automatically defined.
As the unification of all Unices, functionality only available in BSD and SVID is also included.
If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more functionality is available. The exa functions will make all functions available which are necessary for the X/Open Unix brand.