编译文件时出现以下错误，但我已经加入了fcntl.h文件，
$ gcc -o open open.c
open.c: In function ‘main’:
open.c:25: error: ‘O_DIRECT’ undeclared (first use in this function)
open.c:25: error: (Each undeclared identifier is reported only once
open.c:25: error: for each function it appears in.)
open.c:25: error: ‘O_LARGEFILE’ undeclared (first use in this function)
后来发现是fcntl.h中
#ifdef __USE_GNU
# define O_DIRECT 040000 /* Direct disk access. */
# define O_DIRECTORY 0200000 /* Must be a directory. */
# define O_NOFOLLOW 0400000 /* Do not follow links. */
# define O_NOATIME 01000000 /* Do not set atime. */
#endif
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原来是需要定义
__USE_GNU，不过编译是加入-D__USE_GNU也没用，结果一样，最后发现是在编译是加入-D_GNU_SOURCE就行了，比如：gcc -o open open.c -D_GNU_SOURCE
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