lockf(2) lockf(2)
NAME [Toc] [Back]
lockf - provide semaphores and record locking on files
SYNOPSIS [Toc] [Back]
#include <unistd.h>
int lockf(int fildes, int function, off_t size);
DESCRIPTION [Toc] [Back]
The lockf() function allows regions of a file to be used as semaphores
(advisory locks) or restricts access to only the locking process
(enforcement-mode record locks). Other processes that attempt to
access the locked resource either return an error or sleep until the
resource becomes unlocked. All locks for a process are released upon
the first close of the file, even if the process still has the file
opened, and all locks held by a process are released when the process
terminates.
fildes is an open file descriptor. The file descriptor must have been
opened with write-only permission (O_WRONLY) or read-write permission
(O_RDWR) in order to establish a lock with this function call (see
open(2)).
If the calling process is a member of a group that has the
PRIV_LOCKRDONLY privilege (see getprivgrp(2)), it can also use lockf()
to lock files opened with read-only permission (O_RDONLY).
function is a control value that specifies the action to be taken.
Permissible values for function are defined in <unistd.h> as follows:
#define F_ULOCK 0 /* unlock a region */
#define F_LOCK 1 /* lock a region */
#define F_TLOCK 2 /* test and lock a region */
#define F_TEST 3 /* test region for lock */
All other values of function are reserved for future extensions and
result in an error return if not implemented.
F_TEST is used to detect whether a lock by another process is present
on the specified region. lockf() returns zero if the region is
accessible and -1 if it is not; in which case errno is set to
[EACCES]. F_LOCK and F_TLOCK both lock a region of a file if the
region is available. F_ULOCK removes locks from a region of the file.
size is the number of contiguous bytes to be locked or unlocked. The
resource to be locked starts at the current offset in the file, and
extends forward for a positive size, and backward for a negative size
(the preceding bytes up to but not including the current offset). If
size is zero, the region from the current offset through the end of
the largest possible file is locked (that is, from the current offset
through the present or any future end-of-file). An area need not be
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lockf(2) lockf(2)
allocated to the file in order to be locked, because such locks can
exist past the end of the file.
Regions locked with F_LOCK or F_TLOCK can, in whole or in part,
contain or be contained by a previously locked region for the same
process. When this occurs or if adjacent regions occur, the regions
are combined into a single region. If the request requires that a new
element be added to the table of active locks but the table is already
full, an error is returned, and the new region is not locked.
F_LOCK and F_TLOCK requests differ only by the action taken if the
resource is not available: F_LOCK causes the calling process to sleep
until the resource is available, whereas F_TLOCK returns an [EACCES]
error if the region is already locked by another process.
F_ULOCK requests can, in whole or part, release one or more locked
regions controlled by the process. When regions are not fully
released, the remaining regions are still locked by the process.
Releasing the center section of a locked region requires an additional
element in the table of active locks. If this table is full, an
[EDEADLK] error is returned, and the requested region is not released.
Regular files with the file mode of S_ENFMT, not having the group
execute bit set, will have an enforcement policy enabled. With
enforcement enabled, reads and writes that would access a locked
region sleep until the entire region is available if O_NDELAY is
clear, but return -1 with errno set if O_NDELAY is set. File access
by other system functions, such as exec(), are not subject to the
enforcement policy. Locks on directories, pipes, and special files
are advisory only; no enforcement policy is used.
A potential for deadlock occurs if a process controlling a locked
resource is put to sleep by accessing the locked resource of another
process. Thus, calls to fcntl(), lockf(), read(), or write() (see
fcntl(2), lockf(2), read(2), and write(2)) scan for a deadlock prior
to sleeping on a locked resource. Deadlock is not checked for the
wait() and pause() system calls (see wait(2) and pause(2)), so
potential for deadlock is not eliminated. A creat() call or an open()
call with the O_CREATE and O_TRUNC flags set on a regular file returns
error [EAGAIN] if another process has locked part of the file and the
file is currently in enforcement mode.
NETWORKING FEATURES [Toc] [Back]
NFS
The advisory record-locking capabilities of lockf() are implemented
throughout the network by the ``network lock daemon'' (see lockd(1M)).
If the file server crashes and is rebooted, the lock daemon attempts
to recover all locks associated with the crashed server. If a lock
cannot be reclaimed, the process that held the lock is issued a
SIGLOST signal.
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lockf(2) lockf(2)
Only advisory record locking is implemented for NFS files.
RETURN VALUE [Toc] [Back]
Upon successful completion, a value of 0 is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
ERRORS [Toc] [Back]
lockf() fails if any of the following occur:
[EACCES] function is F_TLOCK or F_TEST and the region is
already locked by another process.
[EBADF] fildes is not a valid, open file descriptor.
[EDEADLK] A deadlock would occur or the number of entries in
the system lock table would exceed a systemdependent
maximum. HP-UX guarantees this value to
be at least 50.
[EINTR] A signal was caught during the lockf() system
call.
[EINVAL] Either function is not one of the functions
specified above, or size plus current offset
produces a negative offset into the file.
[EINVAL] size plus current offset cannot be represented
correctly by an object of size off_t.
[ENOLCK] Either function is F_TLOCK or F_LOCK and the file
is an NFS file with access bits set for
enforcement mode, or the file is an NFS file and a
system error occurred on the remote node.
WARNINGS [Toc] [Back]
Deadlock conditions may arise when either the wait() or pause() system
calls are used in conjunction with enforced locking (see wait(2) and
pause(2) for details).
When a file descriptor is closed, all locks on the file from the
calling process are deleted, even if other file descriptors for that
file (obtained through dup() or open(), for example) still exist.
Unexpected results may occur in processes that use buffers in the user
address space. The process may later read or write data which is or
was locked. The standard I/O package, stdio(3S), is the most common
source of unexpected buffering.
In a hostile environment, locking can be misused by holding key public
resources locked. This is particularly true with public read files
that have enforcement enabled.
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lockf(2) lockf(2)
It is not recommended that the PRIV_LOCKRDONLY capability be used
because it is provided for backward compatibility only. This feature
may be modified or dropped from future HP-UX releases.
Locks default to advisory mode unless the setgid bit of the file
permissions is set.
Application Usage [Toc] [Back]
Because in the future the variable errno will be set to [EAGAIN]
rather than [EACCES] when a section of a file is already locked by
another process, portable application programs should expect and test
for either value. For example:
if (lockf(fd, F_TLOCK, siz) == -1)
if ((errno == EAGAIN) || (errno == EACCES))
/*
* section locked by another process
* check for either EAGAIN or EACCES
* due to different implementations
*/
else if ...
/*
* check for other errors
*/
SEE ALSO [Toc] [Back]
lockd(1M), statd(1M), chmod(2), close(2), creat(2), fcntl(2),
creat64(2), open(2), pause(2), read(2), stat(2), wait(2), write(2),
unistd(5).
STANDARDS CONFORMANCE [Toc] [Back]
lockf(): SVID2, SVID3, XPG2
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