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