LOCKF(3)                                            Linux Programmer's Manual                                           LOCKF(3)



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

SYNOPSIS
       #include <unistd.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       lockf(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION
       Apply, test or remove a POSIX lock on a section of an open file.  The file is specified by fd, a file descriptor open for
       writing, the action by cmd, and  the  section  consists  of  byte  positions  pos..pos+len-1  if  len  is  positive,  and
       pos-len..pos-1  if  len is negative, where pos is the current file position, and if len is zero, the section extends from
       the current file position to infinity, encompassing the present and future end-of-file positions.  In all cases, the sec-
       tion may extend past current end-of-file.

       On Linux, lockf() is just an interface on top of fcntl(2) locking.  Many other systems implement lockf() in this way, but
       note that POSIX.1-2001 leaves the relationship between lockf() and fcntl(2) locks unspecified.   A  portable  application
       should probably avoid mixing calls to these interfaces.

       Valid operations are given below:

       F_LOCK Set an exclusive lock on the specified section of the file.  If (part of) this section is already locked, the call
              blocks until the previous lock is released.  If this section overlaps an earlier locked section, both are  merged.
              File  locks  are  released  as  soon as the process holding the locks closes some file descriptor for the file.  A
              child process does not inherit these locks.

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

       F_ULOCK
              Unlock the indicated section of the file.  This may cause a locked section to be split into two locked sections.

       F_TEST Test the lock: return 0 if the specified section is unlocked or locked by this process; return -1,  set  errno  to
              EAGAIN (EACCES on some other systems), if another process holds a lock.

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

ERRORS
       EACCES or EAGAIN
              The  file  is locked and F_TLOCK or F_TEST was specified, or the operation is prohibited because the file has been
              memory-mapped by another process.

       EBADF  fd is not an open file descriptor.

       EDEADLK
              The command was T_LOCK and this 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
       SVr4, POSIX.1-2001.

SEE ALSO
       fcntl(2), flock(2)
       There are also locks.txt and mandatory-locking.txt in the kernel source directory Documentation/filesystems.   (On  older
       kernels, these files are directly under the Documentation/ directory, and mandatory-locking.txt is called mandatory.txt.)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                                                        2009-07-25                                                   LOCKF(3)