.\" Copyright 1997 Nicolás Lichtmaier <nick@debian.org>
.\" Created Thu Aug  7 00:44:00 ART 1997
.\"
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.\" Added section stuff, aeb, 2002-04-22.
.\" Corrected include file, drepper, 2003-06-15.
.\"
.TH LOCKF 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
lockf \- apply, test or remove a POSIX lock on an open file
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int lockf(int " fd ", int " cmd ", off_t " len );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR lockf ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* Glibc since 2.19: */ _DEFAULT_SOURCE
        || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
Apply, test, or remove a POSIX lock on a section of an open file.
The file is specified by
.IR fd ,
a file descriptor open for writing, the action by
.IR cmd ,
and the section consists of byte positions
.IR pos .. pos + len \-1
if
.I len
is positive, and
.IR pos \- len .. pos \-1
if
.I len
is negative, where
.I pos
is the current file position, and if
.I 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.
.PP
On Linux,
.BR lockf ()
is just an interface on top of
.BR fcntl (2)
locking.
Many other systems implement
.BR lockf ()
in this way, but note that POSIX.1 leaves the relationship between
.BR lockf ()
and
.BR fcntl (2)
locks unspecified.
A portable application should probably avoid mixing calls
to these interfaces.
.PP
Valid operations are given below:
.TP
.B 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.
.TP
.B F_TLOCK
Same as
.B F_LOCK
but the call never blocks and returns an error instead if the file is
already locked.
.TP
.B F_ULOCK
Unlock the indicated section of the file.
This may cause a locked section to be split into two locked sections.
.TP
.B F_TEST
Test the lock: return 0 if the specified section
is unlocked or locked by this process; return \-1, set
.I errno
to
.B EAGAIN
.RB ( EACCES
on some other systems),
if another process holds a lock.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.BR EACCES " or " EAGAIN
The file is locked and
.B F_TLOCK
or
.B F_TEST
was specified, or the operation is prohibited because the file has
been memory-mapped by another process.
.TP
.B EBADF
.I fd
is not an open file descriptor; or
.I cmd
is
.B F_LOCK
or
.BR F_TLOCK
and
.I fd
is not a writable file descriptor.
.TP
.B EDEADLK
The command was
.B F_LOCK
and this lock operation would cause a deadlock.
.TP
.B EINTR
While waiting to acquire a lock, the call was interrupted by
delivery of a signal caught by a handler; see
.BR signal (7).
.TP
.B EINVAL
An invalid operation was specified in
.IR cmd .
.TP
.B ENOLCK
Too many segment locks open, lock table is full.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR lockf ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
.SH SEE ALSO
.BR fcntl (2),
.BR flock (2)
.PP
.I locks.txt
and
.I mandatory\-locking.txt
in the Linux kernel source directory
.IR Documentation/filesystems
(on older kernels, these files are directly under the
.I Documentation
directory, and
.I mandatory\-locking.txt
is called
.IR mandatory.txt )