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flock

Manage locks from shell scripts


  1. flock.1.man
  2. flock.2.man


1. flock.1.man

Manpage of FLOCK

FLOCK

Section: H. Peter Anvin (1)
Updated: 4 Feb 2006
Index Return to Main Contents
 

NAME

flock - Manage locks from shell scripts  

SYNOPSIS

flock [-sxon] [-w timeout] lockfile [-c] command...

flock [-sxon] [-w timeout] lockdir [-c] command...

flock [-sxun] [-w timeout] fd  

DESCRIPTION

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

The first and second forms wraps the lock around the executing a command, in a manner similar to su(1) or newgrp(1). It locks a specified file or directory, which is created (assuming appropriate permissions), if it does not already exist.

The third form is convenient inside shell scripts, and is usually used the following manner:

(

  flock -n 9

  # ... commands executed under lock ...
) 9>/var/lock/mylockfile

The mode used to open the file doesn'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.

By default, if the lock cannot be immediately acquired, flock waits until the lock is available.  

OPTIONS

-s, --shared
Obtain a shared lock, sometimes called a read lock.
-x, -e, --exclusive
Obtain an exclusive lock, sometimes called a write lock. This is the default.
-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.
-n, --nb, --nonblock
Fail (with an exit code of 1) rather than wait if the lock cannot be immediately acquired.
-w, --wait, --timeout seconds
Fail (with an exit code of 1) if the lock cannot be acquired within seconds seconds. Decimal fractional values are allowed.
-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.
-c, --command command
Pass a single command to the shell with -c.
-h, --help
Print a help message.
 

AUTHOR

Written by H. Peter Anvin <hpa@zytor.com>.  

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 ftp://ftp.kernel.org/pub/linux/utils/util-linux/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
AUTHOR
COPYRIGHT
SEE ALSO
AVAILABILITY

This document was created by man2html using the manual pages.
Time: 17:30:58 GMT, October 23, 2013

2. flock.2.man

Manpage of FLOCK

FLOCK

Section: Linux Programmer's Manual (2)
Updated: 2009-07-25
Index Return to Main Contents
 

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 table entry. 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 descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate descriptors, or when all such descriptors have been closed.

If a process uses open(2) (or similar) to obtain more than one descriptor for the same file, these 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 descriptor.

A process may only hold 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

flock() does not lock files over NFS. Use fcntl(2) instead: that does work over NFS, given a sufficiently recent version of Linux and a server which supports locking.

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). This yields true BSD semantics: there is no interaction between the types of lock placed by flock() and fcntl(2), and flock() does not detect deadlock.

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.)  

SEE ALSO

close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)

See also Documentation/filesystem/locks.txt in the kernel source (Documentation/locks.txt in older kernels).  

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
SEE ALSO
COLOPHON

This document was created by man2html using the manual pages.
Time: 17:30:58 GMT, October 23, 2013

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