This is not the function you are interested in. Look at readdir(3) for the POSIX conforming C library interface. This page documents the bare kernel system call interface, which can change, and which is superseded by getdents(2).
readdir reads one dirent structure from the directory pointed at by fd into the memory area pointed to by dirp. The parameter count is ignored; at most one dirent structure is read.
The dirent structure is declared as follows:
long d_ino; /* inode number */
off_t d_off; /* offset to this dirent */
unsigned short d_reclen; /* length of this d_name */
char d_name [NAME_MAX+1]; /* file name (null-terminated) */
d_ino is an inode number. d_off is the distance from the start of the directory to this dirent. d_reclen is the size of d_name, not counting the null terminator. d_name is a null-terminated file name.
On success, 1 is returned. On end of directory, 0 is returned. On error, -1 is returned, and errno is set appropriately.