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 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

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 section 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  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; or cmd is F_LOCK or F_TLOCK and  fd  is  not  a  writable  file
              descriptor.

       EDEADLK
              The command was F_LOCK and this lock operation would cause a deadlock.

       EINVAL An invalid operation was specified in cmd.

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

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │Interface │ Attribute     │ Value   │
       ├──────────┼───────────────┼─────────┤
       │lockf()   │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4.

SEE ALSO

       fcntl(2), flock(2)

       locks.txt  and  mandatory-locking.txt  in the Linux 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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       http://www.kernel.org/doc/man-pages/.