lockf(2)lockf(2)NAMElockf - provide semaphores and record locking on files
SYNOPSISDESCRIPTION
The function allows regions of a file to be used as semaphores (advi‐
sory locks) or restricts access to only the locking process (enforce‐
ment-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 or read-write permission 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 privilege
(see getprivgrp(2)), it can also use to lock files opened with read-
only permission
function is a control value that specifies the action to be taken.
Permissible values for function are defined in 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.
is used to detect whether a lock by another process is present on the
specified region. returns zero if the region is accessible and if it
is not; in which case is set to and both lock a region of a file if the
region is available. 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
allocated to the file in order to be locked, because such locks can
exist past the end of the file.
Regions locked with or can, in whole or in part, contain or be con‐
tained 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.
and requests differ only by the action taken if the resource is not
available: causes the calling process to sleep until the resource is
available, whereas returns an error if the region is already locked by
another process.
requests can, in whole or part, release one or more locked regions con‐
trolled by the process. When regions are not fully released, the
remaining regions are still locked by the process. Releasing the cen‐
ter section of a locked region requires an additional element in the
table of active locks. If this table is full, an error is returned,
and the requested region is not released.
Regular files with the file mode of 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 is clear, but return −1 with set if
is set. File access by other system functions, such as 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 or (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 and system calls (see wait(2) and
pause(2)), so potential for deadlock is not eliminated. A call or an
call with the and flags set on a regular file returns error if another
process has locked part of the file and the file is currently in
enforcement mode.
NETWORKING FEATURES
NFS
The advisory record-locking capabilities of 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 signal.
Only advisory record locking is implemented for NFS files.
RETURN VALUE
Upon successful completion, a value of is returned. Otherwise, a value
of is returned and is set to indicate the error.
ERRORS
fails if any of the following occur:
[EACCES] function is or 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 system-
dependent maximum. HP-UX guarantees this value
to be at least 50.
[EINTR] A signal was caught during the system call.
[EINVAL] Either function is not one of the functions spec‐
ified 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
[ENOLCK] Either function is or 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
Deadlock conditions may arise when either the or 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 call‐
ing process are deleted, even if other file descriptors for that file
(obtained through or 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.
It is not recommended that the capability be used because it is pro‐
vided for backward compatibility only. This feature may be modified or
dropped from future HP-UX releases.
Locks default to advisory mode unless the bit of the file permissions
is set.
Application Usage
Because in the future the variable will be set to rather than when a
section of a file is already locked by another process, portable appli‐
cation 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 ALSOlockd(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 CONFORMANCElockf(2)
