1 .\" Copyright 1997 Nicolás Lichtmaier <nick@debian.org>
2 .\" Created Thu Aug 7 00:44:00 ART 1997
4 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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25 .\" Added section stuff, aeb, 2002-04-22.
26 .\" Corrected include file, drepper, 2003-06-15.
28 .TH LOCKF 3 2014-06-13 "GNU" "Linux Programmer's Manual"
30 lockf \- apply, test or remove a POSIX lock on an open file
32 .B #include <unistd.h>
34 .BI "int lockf(int " fd ", int " cmd ", off_t " len );
37 Feature Test Macro Requirements for glibc (see
38 .BR feature_test_macros (7)):
44 _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
45 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
49 Apply, test or remove a POSIX lock on a section of an open file.
50 The file is specified by
52 a file descriptor open for writing, the action by
54 and the section consists of byte positions
55 .IR pos .. pos + len \-1
59 .IR pos \- len .. pos \-1
64 is the current file position, and if
66 is zero, the section extends from the current file position to
67 infinity, encompassing the present and future end-of-file positions.
68 In all cases, the section may extend past current end-of-file.
72 is just an interface on top of
75 Many other systems implement
77 in this way, but note that POSIX.1-2001 leaves the relationship between
82 A portable application should probably avoid mixing calls
85 Valid operations are given below:
88 Set an exclusive lock on the specified section of the file.
89 If (part of) this section is already locked, the call
90 blocks until the previous lock is released.
91 If this section overlaps an earlier locked section,
93 File locks are released as soon as the process holding the locks
94 closes some file descriptor for the file.
95 A child process does not inherit these locks.
100 but the call never blocks and returns an error instead if the file is
104 Unlock the indicated section of the file.
105 This may cause a locked section to be split into two locked sections.
108 Test the lock: return 0 if the specified section
109 is unlocked or locked by this process; return \-1, set
114 on some other systems),
115 if another process holds a lock.
117 On success, zero is returned.
118 On error, \-1 is returned, and
120 is set appropriately.
123 .BR EACCES " or " EAGAIN
124 The file is locked and
128 was specified, or the operation is prohibited because the file has
129 been memory-mapped by another process.
133 is not an open file descriptor; or
141 is not a writable file descriptor.
146 and this lock operation would cause a deadlock.
149 An invalid operation was specified in
153 Too many segment locks open, lock table is full.
155 .SS Multithreading (see pthreads(7))
158 function is thread-safe.
167 .I mandatory-locking.txt
168 in the Linux kernel source directory
169 .IR Documentation/filesystems
170 (on older kernels, these files are directly under the
173 .I mandatory-locking.txt
177 This page is part of release 3.78 of the Linux
180 A description of the project,
181 information about reporting bugs,
182 and the latest version of this page,
184 \%http://www.kernel.org/doc/man\-pages/.