struct flock {
        short   l_type;         /* F_RDLCK, F_WRLCK, or F_UNLCK */
        short   l_whence;       /* flag to choose starting offset */
        long    l_start;        /* relative offset, in bytes */
        long    l_len;          /* length, in bytes; 0 means lock to EOF */
        short   l_pid;          /* returned with F_GETLK */
        short   l_xxx;          /* reserved for future use */
};