open函数中O_CLOEXEC标志的开关

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Linux中，文件描述符有一个属性：CLOEXEC，即当调用exec（）函数成功后，文件描述符会自动关闭。在以往的内核版本（2.6.23以前）中，需要调用 fcntl(fd, F_SETFD, FD_CLOEXEC) 来设置这个属性。而新版本（2.6.23开始）中，可以在调用open函数的时候，通过 flags 参数设置 CLOEXEC 功能，如 open(filename, O_CLOEXEC)。

虽然新版本支持在open时设置CLOEXEC，但是在编译的时候还是会提示错误 - error: ‘O_CLOEXEC’ undeclared (first use in this function)。原来这个新功能要求我们手动去打开，需要设置一个宏(_GNU_SOURCE)。可通过以下两种方法来设置这个宏以打开新功能：

1. 在源代码中加入 #define _GNU_SOURCE
2. 在编译参数中加入 -D_GNU_SOURCE

gcc 还有很多其他的开关，具体可以参考

--------------------------- /usr/include/features.h -----------------------------------------------

These are defined by the user (or the compiler) to specify the desired environment:

   __STRICT_ANSI__      ISO Standard C.
   _ISOC99_SOURCE       Extensions to ISO C89 from ISO C99.
   _POSIX_SOURCE        IEEE Std 1003.1.
   _POSIX_C_SOURCE      If ==1, like _POSIX_SOURCE; if >=2 add IEEE Std 1003.2;
                        if >=199309L, add IEEE Std 1003.1b-1993;
                        if >=199506L, add IEEE Std 1003.1c-1995;
                        if >=200112L, all of IEEE 1003.1-2004
   _XOPEN_SOURCE        Includes POSIX and XPG things. Set to 500 if
                        Single Unix conformance is wanted, to 600 for the
                        upcoming sixth revision.
   _XOPEN_SOURCE_EXTENDED XPG things and X/Open Unix extensions.
   _LARGEFILE_SOURCE    Some more functions for correct standard I/O.
   _LARGEFILE64_SOURCE Additional functionality from LFS for large files.
   _FILE_OFFSET_BITS=N Select default filesystem interface.
   _BSD_SOURCE          ISO C, POSIX, and 4.3BSD things.
   _SVID_SOURCE         ISO C, POSIX, and SVID things.
   _ATFILE_SOURCE       Additional *at interfaces.
   _GNU_SOURCE          All of the above, plus GNU extensions.
   _REENTRANT           Select additionally reentrant object.
   _THREAD_SAFE         Same as _REENTRANT, often used by other systems.
   _FORTIFY_SOURCE      If set to numeric value > 0 additional security
                        measures are defined, according to level.

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