.\" Copyright 1997 Nicolás Lichtmaier .\" Created Thu Aug 7 00:44:00 ART 1997 .\" .\" %%%LICENSE_START(GPLv2+_DOC_FULL) .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .\" Added section stuff, aeb, 2002-04-22. .\" Corrected include file, drepper, 2003-06-15. .\" .TH LOCKF 3 2017-09-15 "GNU" "Linux Programmer's Manual" .SH NAME lockf \- apply, test or remove a POSIX lock on an open file .SH SYNOPSIS .B #include .PP .BI "int lockf(int " fd ", int " cmd ", off_t " len ); .PP .in -4n Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .in .PP .BR lockf (): .ad l .RS 4 _XOPEN_SOURCE\ >=\ 500 .\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED || /* Glibc since 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE .RE .ad .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 appropriately. .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 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). .TS allbox; lb lb lb l l l. Interface Attribute Value T{ .BR lockf () T} Thread safety MT-Safe .TE .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 ) .SH COLOPHON This page is part of release 4.16 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.