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_SOURCE2. 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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