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
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19 .\" Added section stuff, aeb, 2002-04-22.
20 .\" Corrected include file, drepper, 2003-06-15.
22 .TH LOCKF 3 2009-07-25 "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)):
36 _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500
38 Apply, test or remove a POSIX lock on a section of an open file.
39 The file is specified by
41 a file descriptor open for writing, the action by
43 and the section consists of byte positions
44 .IR pos .. pos + len \-1
48 .IR pos \- len .. pos \-1
53 is the current file position, and if
55 is zero, the section extends from the current file position to
56 infinity, encompassing the present and future end-of-file positions.
57 In all cases, the section may extend past current end-of-file.
61 is just an interface on top of
64 Many other systems implement
66 in this way, but note that POSIX.1-2001 leaves the relationship between
71 A portable application should probably avoid mixing calls
74 Valid operations are given below:
77 Set an exclusive lock on the specified section of the file.
78 If (part of) this section is already locked, the call
79 blocks until the previous lock is released.
80 If this section overlaps an earlier locked section,
82 File locks are released as soon as the process holding the locks
83 closes some file descriptor for the file.
84 A child process does not inherit these locks.
89 but the call never blocks and returns an error instead if the file is
93 Unlock the indicated section of the file.
94 This may cause a locked section to be split into two locked sections.
97 Test the lock: return 0 if the specified section
98 is unlocked or locked by this process; return \-1, set
103 on some other systems),
104 if another process holds a lock.
106 On success, zero is returned.
107 On error, \-1 is returned, and
109 is set appropriately.
112 .BR EACCES " or " EAGAIN
113 The file is locked and
117 was specified, or the operation is prohibited because the file has
118 been memory-mapped by another process.
122 is not an open file descriptor.
127 and this lock operation would cause a deadlock.
130 An invalid operation was specified in
134 Too many segment locks open, lock table is full.
144 .I mandatory-locking.txt
145 in the kernel source directory
146 .IR Documentation/filesystems .
147 (On older kernels, these files are directly under the
150 .I mandatory-locking.txt