今天在写程序的时候遇到了如下的错误，核对了半天，感觉头文件已经包含，后来发现是缺少了#define _GNU_SOURCE
gcc example.c -o example example.c: 在函数‘call’中: example.c:58:17: 错误：数组元素的类型不完全 struct mmsghdr msgs[VLEN]; example.c: In function ‘call’: example.c:59:17: error: array type has incomplete element type struct mmsghdr msgs[VLEN];
查看/usr/include下面的定义发现原因。 由于mmsghdr是recvmmsg所需要的，而recvmmsg只在linux中存在，所以在sys/socket.h中有如下定义：
#ifdef __USE_GNU /* For `recvmmsg' and `sendmmsg'. */ struct mmsghdr { struct msghdr msg_hdr; /* Actual message header. */ unsigned int msg_len; /* Number of received or sent bytes for the entry. */ }; #endif
在features.h文件中可以看到
#ifdef _GNU_SOURCE # define __USE_GNU 1 #endif
这样就可以理解为什么需要添加#define _GNU_SOURCE,之前也有注意到The recvmmsg() system call was added in Linux 2.6.33. Support in glibc was added in version 2.12.但是写程序的时候居然忘记了。 以下截取部分features.h里面的说明内容：
/* 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. _ISOC11_SOURCE Extensions to ISO C99 from ISO C11. _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 if >=200809L, all of IEEE 1003.1-2008 _XOPEN_SOURCE Includes POSIX and XPG things. Set to 500 if Single Unix conformance is wanted, to 600 for the sixth revision, to 700 for the seventh 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. The `-ansi' switch to the GNU C compiler defines __STRICT_ANSI__. If none of these are defined, the default is to have _SVID_SOURCE, _BSD_SOURCE, and _POSIX_SOURCE set to one and _POSIX_C_SOURCE set to 200112L. If more than one of these are defined, they accumulate. For example __STRICT_ANSI__, _POSIX_SOURCE and _POSIX_C_SOURCE together give you ISO C, 1003.1, and 1003.2, but nothing else. These are defined by this file and are used by the header files to decide what to declare or define: __USE_ISOC11 Define ISO C11 things. __USE_ISOC99 Define ISO C99 things. __USE_ISOC95 Define ISO C90 AMD1 (C95) things. __USE_POSIX Define IEEE Std 1003.1 things. __USE_POSIX2 Define IEEE Std 1003.2 things. __USE_POSIX199309 Define IEEE Std 1003.1, and .1b things. __USE_POSIX199506 Define IEEE Std 1003.1, .1b, .1c and .1i things. __USE_XOPEN Define XPG things. __USE_XOPEN_EXTENDED Define X/Open Unix things. __USE_UNIX98 Define Single Unix V2 things. __USE_XOPEN2K Define XPG6 things. __USE_XOPEN2KXSI Define XPG6 XSI things. __USE_XOPEN2K8 Define XPG7 things. __USE_XOPEN2K8XSI Define XPG7 XSI things. __USE_LARGEFILE Define correct standard I/O things. __USE_LARGEFILE64 Define LFS things with separate names. __USE_FILE_OFFSET64 Define 64bit interface as default. __USE_BSD Define 4.3BSD things. __USE_SVID Define SVID things. __USE_MISC Define things common to BSD and System V Unix. __USE_ATFILE Define *at interfaces and AT_* constants for them. __USE_GNU Define GNU extensions. __USE_REENTRANT Define reentrant/thread-safe *_r functions. __USE_FORTIFY_LEVEL Additional security measures used, according to level. __FAVOR_BSD Favor 4.3BSD things in cases of conflict. The macros `__GNU_LIBRARY__', `__GLIBC__', and `__GLIBC_MINOR__' are defined by this file unconditionally. `__GNU_LIBRARY__' is provided only for compatibility. All new code should use the other symbols to test for features. All macros listed above as possibly being defined by this file are explicitly undefined if they are not explicitly defined. Feature-test macros that are not defined by the user or compiler but are implied by the other feature-test macros defined (or by the lack of any definitions) are defined by the file. */ /* If _GNU_SOURCE was defined by the user, turn on all the other features. */ #ifdef _GNU_SOURCE # undef _ISOC95_SOURCE # define _ISOC95_SOURCE 1 # undef _ISOC99_SOURCE # define _ISOC99_SOURCE 1 # undef _ISOC11_SOURCE # define _ISOC11_SOURCE 1 # undef _POSIX_SOURCE # define _POSIX_SOURCE 1 # undef _POSIX_C_SOURCE # define _POSIX_C_SOURCE 200809L # undef _XOPEN_SOURCE # define _XOPEN_SOURCE 700 # undef _XOPEN_SOURCE_EXTENDED # define _XOPEN_SOURCE_EXTENDED 1 # undef _LARGEFILE64_SOURCE # define _LARGEFILE64_SOURCE 1 # undef _BSD_SOURCE # define _BSD_SOURCE 1 # undef _SVID_SOURCE # define _SVID_SOURCE 1 # undef _ATFILE_SOURCE # define _ATFILE_SOURCE 1 #endif