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lockf - Lock and unlocks regions of open file descriptors
off_t size );
Interfaces documented on this reference page conform to industry standards
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
Specifies the file to which the lock is to be applied or removed. The
file descriptor is returned by a successful open() or fcntl() function.
Specifies one of the following constants for the lockf() function:
Unlocks a previously locked region in the file.
Locks the region for exclusive use. This request causes the calling
process to sleep if the region overlaps a locked region, and to resume
when it is granted the lock.
Same as F_LOCK, except that the request returns an error if the region
overlaps a locked region.
Tests to see if another process has already locked a region. The
lockf() function returns 0 (zero) if the region is unlocked. If the
region is locked, then -1 is returned and errno is set to [EACCES].
The number of bytes to be locked or unlocked for the lockf() function.
The region starts at the current location in the open file and extends
forward if size is positive and backward ifsize is negative. If the
size parameter is 0 (zero), the region starts at the current location
and extends forward to the maximum possible file size, including the
unallocated space after the end of the file.
The lockf() function locks and unlocks sections of an open file. Unlike the
fcntl() function, however, its interface is limited to setting only write
Although the lockf() and fcntl() functions are different, the
implementations are fully integrated. Therefore, locks obtained from one
function are honored and enforced by the other lock function.
Each lock is either an enforced lock or an advisory lock, and must also be
either a read lock or a write lock.
Locks on a file are advisory or enforced depending on the mode of the file
(see the chmod() function.) A given file can have advisory or enforced
locks, but not both. See the sys/mode.h header file for a description of
When a process holds an enforced exclusive lock on a section of a file, no
other process can access that section of the file with theread() or write()
functions. In addition, the open(), truncate(), and ftruncate() functions
cannot truncate the locked section of the file. If another process attempts
to read or modify the locked section of the file, it sleeps until the
section is unlocked or returns with an error indication.
The file descriptor on which an exclusive lock is being placed must have
been opened with write access.
Some general rules about file locking include the following:
· Changing or unlocking part of a file in the middle of a locked section
leaves two smaller sections locked at each end of the originally
· All locks associated with a file for a given process are removed when
the process closes any file descriptor for that file.
· Locks are not inherited by a child process after running a fork()
Locks can start and extend beyond the current end of a file, but cannot be
negative relative to the beginning of the file. A lock can be set to extend
to the end of the file by setting the l_len field to 0 (zero). If a lock is
specified with the l_start field set to 0 and the l_whence field set to
SEEK_SET, the whole file is locked.
Advisory file region locking is supported over NFS, provided the locking
daemon (rpc.lockd) and status monitor daemon (rpc.statd) are running.
Buffered I/O does not work properly when used with file locking. Do not use
the standard I/O package routines on files that will be locked.
Deadlocks due to file locks in a distributed system are not always
detected. When such deadlocks are possible, the programs requesting the
locks should set time-out timers.
Upon successful completion, a value of 0 (zero) is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
If the lockf() function fails, errno may be set to one of the following
The file region is locked and F_TEST was specified; or the file region
is locked and F_TLOCK was specified.
The filedes parameter is not a valid open file descriptor; or the
request parameter is F_LOCK or F_TLOCK and filedes is not a valid file
descriptor open for writing.
The lock is blocked by some lock from another process. Putting the
calling process to sleep while waiting for that lock to become free
would cause a deadlock.
The request parameter is F_TLOCK and the lockf() function was
interrupted by a signal which was caught.
The request parameter is not valid or size plus the current file offset
is less than 0 (zero).
The request parameter is F_LOCK, F_TLOCK, or F_UNLOCK and satisfying
the lock or unlock request would exceed the configurable system limit
[Tru64 UNIX] If using NFS, the server is out of resources or the file
handle is stale.
Functions: chmod(2), close(2), exec(2), fcntl(2), flock(2), fork(2),
open(2), read(2), write(2)
Commands: rpc.lockd(8), rpc.statd(8)
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