1LOCKF(3P) POSIX Programmer's Manual LOCKF(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 lockf — record locking on files
13
15 #include <unistd.h>
16 int lockf(int fildes, int function, off_t size);
18
20 The lockf() function shall lock sections of a file with advisory-mode
21 locks. Calls to lockf() from threads in other processes which attempt
22 to lock the locked file section shall either return an error value or
23 block until the section becomes unlocked. All the locks for a process
24 are removed when the process terminates. Record locking with lockf()
25 shall be supported for regular files and may be supported for other
26 files.
27 The fildes argument is an open file descriptor. To establish a lock
29 with this function, the file descriptor shall be opened with write-only
30 permission (O_WRONLY) or with read/write permission (O_RDWR).
31 The function argument is a control value which specifies the action to
33 be taken. The permissible values for function are defined in <unistd.h>
34 as follows:
35 ┌─────────┬──────────────────────────────────────────────┐
37 │Function │ Description │
38 ├─────────┼──────────────────────────────────────────────┤
39 │F_ULOCK │ Unlock locked sections. │
40 │F_LOCK │ Lock a section for exclusive use. │
41 │F_TLOCK │ Test and lock a section for exclusive use. │
42 │F_TEST │ Test a section for locks by other processes. │
43 └─────────┴──────────────────────────────────────────────┘
44 F_TEST shall detect if a lock by another process is present on the
45 specified section.
46 F_LOCK and F_TLOCK shall both lock a section of a file if the section
48 is available.
49 F_ULOCK shall remove locks from a section of the file.
51 The size argument is the number of contiguous bytes to be locked or
53 unlocked. The section to be locked or unlocked starts at the current
54 offset in the file and extends forward for a positive size or backward
55 for a negative size (the preceding bytes up to but not including the
56 current offset). If size is 0, the section from the current offset
57 through the largest possible file offset shall be locked (that is, from
58 the current offset through the present or any future end-of-file). An
59 area need not be allocated to the file to be locked because locks may
60 exist past the end-of-file.
61 The sections locked with F_LOCK or F_TLOCK may, in whole or in part,
63 contain or be contained by a previously locked section for the same
64 process. When this occurs, or if adjacent locked sections would occur,
65 the sections shall be combined into a single locked section. If the
66 request would cause the number of locks to exceed a system-imposed
67 limit, the request shall fail.
68 F_LOCK and F_TLOCK requests differ only by the action taken if the sec‐
70 tion is not available. F_LOCK shall block the calling thread until the
71 section is available. F_TLOCK shall cause the function to fail if the
72 section is already locked by another process.
73 File locks shall be released on first close by the locking process of
75 any file descriptor for the file.
76 F_ULOCK requests may release (wholly or in part) one or more locked
78 sections controlled by the process. Locked sections shall be unlocked
79 starting at the current file offset through size bytes or to the end-
80 of-file if size is (off_t)0. When all of a locked section is not
81 released (that is, when the beginning or end of the area to be unlocked
82 falls within a locked section), the remaining portions of that section
83 shall remain locked by the process. Releasing the center portion of a
84 locked section shall cause the remaining locked beginning and end por‐
85 tions to become two separate locked sections. If the request would
86 cause the number of locks in the system to exceed a system-imposed
87 limit, the request shall fail.
88 A potential for deadlock occurs if the threads of a process controlling
90 a locked section are blocked by accessing a locked section of another
91 process. If the system detects that deadlock would occur, lockf() shall
92 fail with an [EDEADLK] error.
93 The interaction between fcntl() and lockf() locks is unspecified.
95 Blocking on a section shall be interrupted by any signal.
97 An F_ULOCK request in which size is non-zero and the offset of the last
99 byte of the requested section is the maximum value for an object of
100 type off_t, when the process has an existing lock in which size is 0
101 and which includes the last byte of the requested section, shall be
102 treated as a request to unlock from the start of the requested section
103 with a size equal to 0. Otherwise, an F_ULOCK request shall attempt to
104 unlock only the requested section.
105 Attempting to lock a section of a file that is associated with a
107 buffered stream produces unspecified results.
108
110 Upon successful completion, lockf() shall return 0. Otherwise, it shall
111 return -1, set errno to indicate an error, and existing locks shall not
112 be changed.
113
115 The lockf() function shall fail if:
116 EBADF The fildes argument is not a valid open file descriptor; or
118 function is F_LOCK or F_TLOCK and fildes is not a valid file
119 descriptor open for writing.
120 EACCES or EAGAIN
122 The function argument is F_TLOCK or F_TEST and the section is
123 already locked by another process.
124 EDEADLK
126 The function argument is F_LOCK and a deadlock is detected.
127 EINTR A signal was caught during execution of the function.
129 EINVAL The function argument is not one of F_LOCK, F_TLOCK, F_TEST, or
131 F_ULOCK; or size plus the current file offset is less than 0.
132 EOVERFLOW
134 The offset of the first, or if size is not 0 then the last, byte
135 in the requested section cannot be represented correctly in an
136 object of type off_t.
137 The lockf() function may fail if:
139 EAGAIN The function argument is F_LOCK or F_TLOCK and the file is
141 mapped with mmap().
142 EDEADLK or ENOLCK
144 The function argument is F_LOCK, F_TLOCK, or F_ULOCK, and the
145 request would cause the number of locks to exceed a system-
146 imposed limit.
147 EOPNOTSUPP or EINVAL
149 The implementation does not support the locking of files of the
150 type indicated by the fildes argument.
151 The following sections are informative.
153
155 Locking a Portion of a File
156 In the following example, a file named /home/cnd/mod1 is being modi‐
157 fied. Other processes that use locking are prevented from changing it
158 during this process. Only the first 10000 bytes are locked, and the
159 lock call fails if another process has any part of this area locked
160 already.
161 #include <fcntl.h>
164 #include <unistd.h>
165 int fildes;
167 int status;
168 ...
169 fildes = open("/home/cnd/mod1", O_RDWR);
170 status = lockf(fildes, F_TLOCK, (off_t)10000);
171
173 Record-locking should not be used in combination with the fopen(),
174 fread(), fwrite(), and other stdio functions. Instead, the more primi‐
175 tive, non-buffered functions (such as open()) should be used. Unex‐
176 pected results may occur in processes that do buffering in the user
177 address space. The process may later read/write data which is/was
178 locked. The stdio functions are the most common source of unexpected
179 buffering.
180 The alarm() function may be used to provide a timeout facility in
182 applications requiring it.
183
185 None.
186
188 None.
189
191 alarm(), chmod(), close(), creat(), fcntl(), fopen(), mmap(), open(),
192 read(), write()
193 The Base Definitions volume of POSIX.1‐2017, <unistd.h>
195
197 Portions of this text are reprinted and reproduced in electronic form
198 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
199 table Operating System Interface (POSIX), The Open Group Base Specifi‐
200 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
201 Electrical and Electronics Engineers, Inc and The Open Group. In the
202 event of any discrepancy between this version and the original IEEE and
203 The Open Group Standard, the original IEEE and The Open Group Standard
204 is the referee document. The original Standard can be obtained online
205 at http://www.opengroup.org/unix/online.html .
206 Any typographical or formatting errors that appear in this page are
208 most likely to have been introduced during the conversion of the source
209 files to man page format. To report such errors, see https://www.ker‐
210 nel.org/doc/man-pages/reporting_bugs.html .
211IEEE/The Open Group 2017 LOCKF(3P)