??? Linux中，文件描述符有一個(gè)屬性：CLOEXEC，即 當(dāng)調(diào)用exec（）函數(shù)成功后，文件描述符會(huì)自動(dòng)關(guān)閉。在以往的內(nèi)核版本（2.6.23以前）中，需要調(diào)用 fcntl(fd, F_SETFD, FD_CLOEXEC) 來設(shè)置這個(gè)屬性。而 新版本（2.6.23開始）中，可以在調(diào)用open函數(shù)的時(shí)候，通過 flags 參數(shù)設(shè)置 CLOEXEC 功能，如 open(filename, O_CLOEXEC)。

????? 雖然新版本支持在open時(shí)設(shè)置CLOEXEC，但是在編譯的時(shí)候還是會(huì)提示錯(cuò)誤 - error: ‘O_CLOEXEC’ undeclared (first use in this function)。原來這個(gè)新功能要求我們手動(dòng)去打開，需要設(shè)置一個(gè)宏(_GNU_SOURCE)?？赏ㄟ^以下兩種方法來設(shè)置這個(gè)宏以打開新功能：

1. 在源代碼中加入 #define _GNU_SOURCE
2. 在編譯參數(shù)中加入 -D_GNU_SOURCE

?

gcc 還有很多其他的開關(guān)，具體可以參考

--------------------------- /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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