Man page 9 : sys_flock

sys_flock - flock system call.

long sys_flock(unsigned int  fd , unsigned int  cmd );

fd
    the file descriptor to lock.

cmd
    the type of lock to apply.

Apply a FL_FLOCK style lock to an open file descriptor. The cmd can be one of

LOCK_SH -- a shared lock.

LOCK_EX -- an exclusive lock.

LOCK_UN -- remove an existing lock.

LOCK_MAND -- a `mandatoryAq flock. This exists to emulate Windows Share Modes.

LOCK_MAND can be combined with LOCK_READ or LOCK_WRITE to allow other processes read and write access respectively.