Section (1) flock


Linux manual pages Section 1  

Name

flock — manage locks from shell scripts

Synopsis

flock [options] file | directory command [arguments]

flock [options] file | directory −c command

flock [options] number

DESCRIPTION

This utility manages flock(2) locks from within shell scripts or from the command line.

The first and second of the above forms wrap the lock around the execution of a command, in a manner similar to su(1) or newgrp(1). They lock a specified file or directory, which is created (assuming appropriate permissions) if it does not already exist. By default, if the lock cannot be immediately acquired, flock waits until the lock is available.

The third form uses an open file by its file descriptor number. See the examples below for how that can be used.

OPTIONS

−c, −−command command

Pass a single command, without arguments, to the shell with −c.

−E, −−conflict−exit−code number

The exit code used when the −n option is in use, and the conflicting lock exists, or the −w option is in use, and the timeout is reached. The default value is 1.

−F, −−no−fork

Do not fork before executing command. Upon execution the flock process is replaced by command which continues to hold the lock. This option is incompatible with −−close as there would otherwise be nothing left to hold the lock.

−e, −x, −−exclusive

Obtain an exclusive lock, sometimes called a write lock. This is the default.

−n, −−nb, −−nonblock

Fail rather than wait if the lock cannot be immediately acquired. See the −E option for the exit code used.

−o, −−close

Close the file descriptor on which the lock is held before executing command. This is useful if command spawns a child process which should not be holding the lock.

−s, −−shared

Obtain a shared lock, sometimes called a read lock.

−u, −−unlock

Drop a lock. This is usually not required, since a lock is automatically dropped when the file is closed. However, it may be required in special cases, for example if the enclosed command group may have forked a background process which should not be holding the lock.

−w, −−wait, −−timeout seconds

Fail if the lock cannot be acquired within seconds. Decimal fractional values are allowed. See the −E option for the exit code used. The zero number of seconds is interpreted as −−nonblock.

−−verbose

Report how long it took to acquire the lock, or why the lock could not be obtained.

−V, −−version

Display version information and exit.

−h, −−help

Display help text and exit.

EXAMPLES

shell1> flock /tmp -c cat
shell2> flock -w .007 /tmp -c echo; /bin/echo $?

Set exclusive lock to directory /tmp and the second command will fail.

shell1> flock -s /tmp -c cat
shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?

Set shared lock to directory /tmp and the second command will not fail. Notice that attempting to get exclusive lock with second command would fail.

shell> flock -x local-lock-file echo _zsingle_quotesz_a b c_zsingle_quotesz_

Grab the exclusive lock local-lock-file before running echo with _zsingle_quotesz_a b c_zsingle_quotesz_.

(
  flock -n 9 || exit 1
  # ... commands executed under lock ...
) 9>/var/lock/mylockfile

The form is convenient inside shell scripts. The mode used to open the file doesn_zsingle_quotesz_t matter to flock; using > or >> allows the lockfile to be created if it does not already exist, however, write permission is required. Using < requires that the file already exists but only read permission is required.

[ ${FLOCKER} != $0 ] && exec env FLOCKER=$0 flock -en $0 $0 [email protected] || :

This is useful boilerplate code for shell scripts. Put it at the top of the shell script you want to lock and it_zsingle_quotesz_ll automatically lock itself on the first run. If the env var $FLOCKER is not set to the shell script that is being run, then execute flock and grab an exclusive non-blocking lock (using the script itself as the lock file) before re-execing itself with the right arguments. It also sets the FLOCKER env var to the right value so it doesn_zsingle_quotesz_t run again.

EXIT STATUS

The command uses sysexits.h return values for everything, except when using either of the options −n or −w which report a failure to acquire the lock with a return value given by the −E option, or 1 by default.

When using the command variant, and executing the child worked, then the exit status is that of the child command.

AUTHOR

H. Peter Anvin

COPYRIGHT

Copyright © 2003−2006 H. Peter Anvin.

This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

SEE ALSO

flock(2)

AVAILABILITY

The flock command is part of the util-linux package and is available from Linux Kernel Archive


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Section (2) flock


Linux manual pages Section 2  

Name

flock — apply or remove an advisory lock on an open file

Synopsis

#include <sys/file.h>
int flock( int fd,
  int operation);
 

DESCRIPTION

Apply or remove an advisory lock on the open file specified by fd. The argument operation is one of the following:

LOCK_SH

Place a shared lock. More than one process may hold a shared lock for a given file at a given time.

LOCK_EX

Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.

LOCK_UN

Remove an existing lock held by this process.

A call to flock() may block if an incompatible lock is held by another process. To make a nonblocking request, include LOCK_NB (by ORing) with any of the above operations.

A single file may not simultaneously have both shared and exclusive locks.

Locks created by flock() are associated with an open file description (see open(2)). This means that duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the same lock, and this lock may be modified or released using any of these file descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate file descriptors, or when all such file descriptors have been closed.

If a process uses open(2) (or similar) to obtain more than one file descriptor for the same file, these file descriptors are treated independently by flock(). An attempt to lock the file using one of these file descriptors may be denied by a lock that the calling process has already placed via another file descriptor.

A process may hold only one type of lock (shared or exclusive) on a file. Subsequent flock() calls on an already locked file will convert an existing lock to the new lock mode.

Locks created by flock() are preserved across an execve(2).

A shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened.

RETURN VALUE

On success, zero is returned. On error, −1 is returned, and errno is set appropriately.

ERRORS

EBADF

fd is not an open file descriptor.

EINTR

While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).

EINVAL

operation is invalid.

ENOLCK

The kernel ran out of memory for allocating lock records.

EWOULDBLOCK

The file is locked and the LOCK_NB flag was selected.

CONFORMING TO

4.4BSD (the flock() call first appeared in 4.2BSD). A version of flock(), possibly implemented in terms of fcntl(2), appears on most UNIX systems.

NOTES

Since kernel 2.0, flock() is implemented as a system call in its own right rather than being emulated in the GNU C library as a call to fcntl(2). With this implementation, there is no interaction between the types of lock placed by flock() and fcntl(2), and flock() does not detect deadlock. (Note, however, that on some systems, such as the modern BSDs, flock() and fcntl(2) locks do interact with one another.)

flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file.

flock() and fcntl(2) locks have different semantics with respect to forked processes and dup(2). On systems that implement flock() using fcntl(2), the semantics of flock() will be different from those described in this manual page.

Converting a lock (shared to exclusive, or vice versa) is not guaranteed to be atomic: the existing lock is first removed, and then a new lock is established. Between these two steps, a pending lock request by another process may be granted, with the result that the conversion either blocks, or fails if LOCK_NB was specified. (This is the original BSD behavior, and occurs on many other implementations.)

NFS details

In Linux kernels up to 2.6.11, flock() does not lock files over NFS (i.e., the scope of locks was limited to the local system). Instead, one could use fcntl(2) byte-range locking, which does work over NFS, given a sufficiently recent version of Linux and a server which supports locking.

Since Linux 2.6.12, NFS clients support flock() locks by emulating them as fcntl(2) byte-range locks on the entire file. This means that fcntl(2) and flock() locks do interact with one another over NFS. It also means that in order to place an exclusive lock, the file must be opened for writing.

Since Linux 2.6.37, the kernel supports a compatibility mode that allows flock() locks (and also fcntl(2) byte region locks) to be treated as local; see the discussion of the local_lock option in nfs(5).

SEE ALSO

flock(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3), lslocks(8)

Documentation/filesystems/locks.txt in the Linux kernel source tree (Documentation/locks.txt in older kernels)

COLOPHON

This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright 1993 Rickard E. Faith (faithcs.unc.edu) and
and Copyright 2002 Michael Kerrisk

%%%LICENSE_START(VERBATIM)
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Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esrthyrsus.com>
Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamieimbolc.ucc.ie>
Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpagesgmail.com>
Substantial rewrites and additions
2005-05-10 mtk, noted that lock conversions are not atomic.

FIXME Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE
which only have effect for SAMBA.