_XOPEN_SOURCE

[误解]
#define _XOPEN_SOURCE
决不是简单的宏定义
它是使程序符合系统环境的不可缺少的部分

[概念]
Glibc 所实现全部或部分规范下的功能有&＃xff1a;
1.ISO C: C语言国际标准.
2.POSIX: 操作系统的 ISO/IEC 9945 (aka IEEE 1003) 标准.
3.Berkeley Unix: BSD 和 SunOS.
4.SVID: V 系统接口描述.
5.XPG: The X/Open Portability Guide.

程序中&＃xff0c;为了实现上述功能需要定义对应的宏。如第二个功能POSIX&＃xff0c;应定义 _POSIX_SOURCE。类似的有 _BSD_SOURCE、_SVID_SOURCE、_XOPEN_SOURCE。
也就是说&＃xff1a;
#define _XOPEN_SOURCE
是为了可以使用 5. The X/Open Portability Guide 的功能。

[实用方法]
使用上述五种功能的方法有两种&＃xff1a;
    1&＃xff09;在cc命令中指定&＃xff0c;如&＃xff1a;cc -D _POSIX_SOURCE file.c
    2&＃xff09;将源程序的第一行设置为&＃xff1a;&＃xff03;define _POSIX_SOURCE 1

[_XOPEN_SOURCE是什么&＃xff1f;]
翻译成汉语表达很蹩脚&＃xff0c;英文原版解释如下&＃xff1a;
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.

[其它]