1 .\" Copyright 1997 Nicolás Lichtmaier <nick@debian.org>
2 .\" Created Thu Aug 7 00:44:00 ART 1997
4 .\" This is free documentation; you can redistribute it and/or
5 .\" modify it under the terms of the GNU General Public License as
6 .\" published by the Free Software Foundation; either version 2 of
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19 .\" Added section stuff, aeb, 2002-04-22.
20 .\" Corrected include file, drepper, 2003-06-15.
22 .TH LOCKF 3 2011-09-11 "GNU" "Linux Programmer's Manual"
24 lockf \- apply, test or remove a POSIX lock on an open file
26 .B #include <unistd.h>
28 .BI "int lockf(int " fd ", int " cmd ", off_t " len );
31 Feature Test Macro Requirements for glibc (see
32 .BR feature_test_macros (7)):
38 _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
39 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
43 Apply, test or remove a POSIX lock on a section of an open file.
44 The file is specified by
46 a file descriptor open for writing, the action by
48 and the section consists of byte positions
49 .IR pos .. pos + len \-1
53 .IR pos \- len .. pos \-1
58 is the current file position, and if
60 is zero, the section extends from the current file position to
61 infinity, encompassing the present and future end-of-file positions.
62 In all cases, the section may extend past current end-of-file.
66 is just an interface on top of
69 Many other systems implement
71 in this way, but note that POSIX.1-2001 leaves the relationship between
76 A portable application should probably avoid mixing calls
79 Valid operations are given below:
82 Set an exclusive lock on the specified section of the file.
83 If (part of) this section is already locked, the call
84 blocks until the previous lock is released.
85 If this section overlaps an earlier locked section,
87 File locks are released as soon as the process holding the locks
88 closes some file descriptor for the file.
89 A child process does not inherit these locks.
94 but the call never blocks and returns an error instead if the file is
98 Unlock the indicated section of the file.
99 This may cause a locked section to be split into two locked sections.
102 Test the lock: return 0 if the specified section
103 is unlocked or locked by this process; return \-1, set
108 on some other systems),
109 if another process holds a lock.
111 On success, zero is returned.
112 On error, \-1 is returned, and
114 is set appropriately.
117 .BR EACCES " or " EAGAIN
118 The file is locked and
122 was specified, or the operation is prohibited because the file has
123 been memory-mapped by another process.
127 is not an open file descriptor; or
135 is not a writable file descriptor.
140 and this lock operation would cause a deadlock.
143 An invalid operation was specified in
147 Too many segment locks open, lock table is full.
157 .I mandatory-locking.txt
158 in the kernel source directory
159 .IR Documentation/filesystems .
160 (On older kernels, these files are directly under the
163 .I mandatory-locking.txt