lockf(3) Library Functions Manual lockf(3)
lockf - Locks and unlocks regions of open file descriptors
int lockf( int filedes, int request, off_t size );
Interfaces documented on this reference page conform to industry standards as follows:
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() func-
tion. 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 if size 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 (exclusive) locks.
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 file attributes.
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 the
read() 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
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 locked section. 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() function.
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 run-
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 values: 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 of NLOCK_RECORD.
[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)
Standards: standards(5) delim off