NAME

       lockf - apply, test or remove a POSIX lock on an open file

SYNOPSIS

       #include <sys/file.h>

       int lockf(int fd, int cmd, off_t len);

DESCRIPTION

       Apply,  test  or  remove a POSIX lock on an open file.  The file is specified by fd.  This
       call is just an interface for fcntl(2).  Valid operations are given below:

              F_LOCK    Set an exclusive lock to the file. Only one process may hold an exclusive
                        lock  for  a given file at a given time. If the file is already locked it
                        blocks until the previous lock is released.

              F_TLOCK   Same as F_LOCK but never blocks and return error instead if the  file  is
                        already locked.

              F_ULOCK   Unlock the file.

              F_TEST    Test  the  lock:  return  0  if fd is unlocked or locked by this process;
                        return -1, set errno to EACCES, if another process holds the lock.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EAGAIN The file is locked and the LOCK_NB flag was selected, or  operation  is  prohibited
              because the file has been memory-mapped by another process.

       EBADF  fd is not an open file descriptor.

       EDEADLK
              Specified lock operation would cause a deadlock.

       EINVAL An invalid operation was specified in fd.

       ENOLCK Too many segment locks open, lock table is full.

CONFORMING TO

       SYSV

SEE ALSO

       fcntl(2),    flock(2).     There    are    also    locks.txt    and    mandatory.txt    in
       /usr/src/linux/Documentation.