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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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LOCKF(3P) POSIX Programmer's Manual LOCKF(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
lockf — record locking on files
#include <unistd.h>
int lockf(int fildes, int function, off_t size);
The lockf() function shall lock sections of a file with advisory-
mode locks. Calls to lockf() from threads in other processes which
attempt to lock the locked file section shall either return an
error value or block until the section becomes unlocked. All the
locks for a process are removed when the process terminates.
Record locking with lockf() shall be supported for regular files
and may be supported for other files.
The fildes argument is an open file descriptor. To establish a
lock with this function, the file descriptor shall be opened with
write-only permission (O_WRONLY) or with read/write permission
(O_RDWR).
The function argument is a control value which specifies the
action to be taken. The permissible values for function are
defined in <unistd.h> as follows:
┌──────────┬──────────────────────────────────────────────┐
│ Function │ Description │
├──────────┼──────────────────────────────────────────────┤
│ F_ULOCK │ Unlock locked sections. │
│ F_LOCK │ Lock a section for exclusive use. │
│ F_TLOCK │ Test and lock a section for exclusive use. │
│ F_TEST │ Test a section for locks by other processes. │
└──────────┴──────────────────────────────────────────────┘
F_TEST shall detect if a lock by another process is present on the
specified section.
F_LOCK and F_TLOCK shall both lock a section of a file if the
section is available.
F_ULOCK shall remove locks from a section of the file.
The size argument is the number of contiguous bytes to be locked
or unlocked. The section to be locked or unlocked starts at the
current offset in the file and extends forward for a positive size
or backward for a negative size (the preceding bytes up to but not
including the current offset). If size is 0, the section from the
current offset through the largest possible file offset shall be
locked (that is, from the current offset through the present or
any future end-of-file). An area need not be allocated to the file
to be locked because locks may exist past the end-of-file.
The sections locked with F_LOCK or F_TLOCK may, in whole or in
part, contain or be contained by a previously locked section for
the same process. When this occurs, or if adjacent locked sections
would occur, the sections shall be combined into a single locked
section. If the request would cause the number of locks to exceed
a system-imposed limit, the request shall fail.
F_LOCK and F_TLOCK requests differ only by the action taken if the
section is not available. F_LOCK shall block the calling thread
until the section is available. F_TLOCK shall cause the function
to fail if the section is already locked by another process.
File locks shall be released on first close by the locking process
of any file descriptor for the file.
F_ULOCK requests may release (wholly or in part) one or more
locked sections controlled by the process. Locked sections shall
be unlocked starting at the current file offset through size bytes
or to the end-of-file if size is (off_t)0. When all of a locked
section is not released (that is, when the beginning or end of the
area to be unlocked falls within a locked section), the remaining
portions of that section shall remain locked by the process.
Releasing the center portion of a locked section shall cause the
remaining locked beginning and end portions to become two separate
locked sections. If the request would cause the number of locks in
the system to exceed a system-imposed limit, the request shall
fail.
A potential for deadlock occurs if the threads of a process
controlling a locked section are blocked by accessing a locked
section of another process. If the system detects that deadlock
would occur, lockf() shall fail with an [EDEADLK] error.
The interaction between fcntl() and lockf() locks is unspecified.
Blocking on a section shall be interrupted by any signal.
An F_ULOCK request in which size is non-zero and the offset of the
last byte of the requested section is the maximum value for an
object of type off_t, when the process has an existing lock in
which size is 0 and which includes the last byte of the requested
section, shall be treated as a request to unlock from the start of
the requested section with a size equal to 0. Otherwise, an
F_ULOCK request shall attempt to unlock only the requested
section.
Attempting to lock a section of a file that is associated with a
buffered stream produces unspecified results.
Upon successful completion, lockf() shall return 0. Otherwise, it
shall return -1, set errno to indicate an error, and existing
locks shall not be changed.
The lockf() function shall fail if:
EBADF The fildes argument is not a valid open file descriptor; or
function is F_LOCK or F_TLOCK and fildes is not a valid
file descriptor open for writing.
EACCES or EAGAIN
The function argument is F_TLOCK or F_TEST and the section
is already locked by another process.
EDEADLK
The function argument is F_LOCK and a deadlock is detected.
EINTR A signal was caught during execution of the function.
EINVAL The function argument is not one of F_LOCK, F_TLOCK,
F_TEST, or F_ULOCK; or size plus the current file offset is
less than 0.
EOVERFLOW
The offset of the first, or if size is not 0 then the last,
byte in the requested section cannot be represented
correctly in an object of type off_t.
The lockf() function may fail if:
EAGAIN The function argument is F_LOCK or F_TLOCK and the file is
mapped with mmap().
EDEADLK or ENOLCK
The function argument is F_LOCK, F_TLOCK, or F_ULOCK, and
the request would cause the number of locks to exceed a
system-imposed limit.
EOPNOTSUPP or EINVAL
The implementation does not support the locking of files of
the type indicated by the fildes argument.
The following sections are informative.
Locking a Portion of a File
In the following example, a file named /home/cnd/mod1 is being
modified. Other processes that use locking are prevented from
changing it during this process. Only the first 10000 bytes are
locked, and the lock call fails if another process has any part of
this area locked already.
#include <fcntl.h>
#include <unistd.h>
int fildes;
int status;
...
fildes = open("/home/cnd/mod1", O_RDWR);
status = lockf(fildes, F_TLOCK, (off_t)10000);
Record-locking should not be used in combination with the fopen(),
fread(), fwrite(), and other stdio functions. Instead, the more
primitive, non-buffered functions (such as open()) should be used.
Unexpected results may occur in processes that do buffering in the
user address space. The process may later read/write data which
is/was locked. The stdio functions are the most common source of
unexpected buffering.
The alarm() function may be used to provide a timeout facility in
applications requiring it.
None.
None.
alarm(3p), chmod(3p), close(3p), creat(3p), fcntl(3p), fopen(3p),
mmap(3p), open(3p), read(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 LOCKF(3P)
Pages that refer to this page: unistd.h(0p), mmap(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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