flock(1tcl)

flock -- set, clear, or return file options

funlock- remove lock created by flock

Synopsis

flock options fileId [start] [length] [origin]
funlock fileId [start] [length] [origin]

Description

flock places a lock on all or part of the file specified by fileId. funlock removes a lock from a file that was previously placed with flock.

The lock is either advisory or mandatory, depending on the mode bits of the file. The lock is placed beginning at relative byte offset start for length bytes. If start or length is omitted or empty, zero is assumed. If length is zero, then the lock always extends to end-of-file, even if the file grows. If origin is start, then the offset is relative to the beginning of the file. If it is current, it is relative to the current access position in the file. If it is end, then it is relative to the end-of-file (a negative is before the EOF, a positive is after). If origin is omitted, start is assumed.

The following options are recognized:

-read: Places a read lock on the file. Multiple processes can access a file with read-locks.
-write: Places a write lock on the file. Only one process can access a file with write-lock.
-nowait: If specified, then the process will not block if the lock can not be obtained. With this option, the command returns 1 if the lock is obtained and 0 if it is not.

See your system's fcntl system call documentation for full details of the behavior of file locking. If locking is being done on ranges of a file, it is best to use unbuffered file access (see the fcntl command).