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.