该结构struct dirent是指目录条目。

http://www.gnu.org/software/libc/manual/html_node/Directory-
Entries.html

在linux中，它定义为：

struct dirent {
    ino_t          d_ino;       /* inode number */
    off_t          d_off;       /* offset to the next dirent */
    unsigned short d_reclen;    /* length of this record */
    unsigned char  d_type;      /* type of file; not supported
                                   by all file system types */
    char           d_name[256]; /* filename */
};

参考： man readdir

或者只是在包含目录中查找“ dirent.h”。