373 lines
8.2 KiB
HTML
373 lines
8.2 KiB
HTML
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
<HTML><HEAD><TITLE>Man page of FLOCK</TITLE>
|
|
</HEAD><BODY>
|
|
<H1>FLOCK</H1>
|
|
Section: Linux Programmer's Manual (2)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
|
|
<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
|
|
|
|
<A NAME="lbAB"> </A>
|
|
<H2>NAME</H2>
|
|
|
|
flock - apply or remove an advisory lock on an open file
|
|
<A NAME="lbAC"> </A>
|
|
<H2>SYNOPSIS</H2>
|
|
|
|
<B>#include <<A HREF="file:///usr/include/sys/file.h">sys/file.h</A>></B>
|
|
|
|
<P>
|
|
|
|
<B>int flock(int </B><I>fd</I><B>, int </B><I>operation</I><B>);</B>
|
|
|
|
<A NAME="lbAD"> </A>
|
|
<H2>DESCRIPTION</H2>
|
|
|
|
Apply or remove an advisory lock on the open file specified by
|
|
<I>fd</I>.
|
|
|
|
The argument
|
|
<I>operation</I>
|
|
|
|
is one of the following:
|
|
<DL COMPACT><DT id="1"><DD>
|
|
<DL COMPACT>
|
|
<DT id="2"><B>LOCK_SH</B>
|
|
|
|
<DD>
|
|
Place a shared lock.
|
|
More than one process may hold a shared lock for a given file
|
|
at a given time.
|
|
<DT id="3"><B>LOCK_EX</B>
|
|
|
|
<DD>
|
|
Place an exclusive lock.
|
|
Only one process may hold an exclusive lock for a given
|
|
file at a given time.
|
|
<DT id="4"><B>LOCK_UN</B>
|
|
|
|
<DD>
|
|
Remove an existing lock held by this process.
|
|
</DL>
|
|
</DL>
|
|
|
|
<P>
|
|
|
|
A call to
|
|
<B>flock</B>()
|
|
|
|
may block if an incompatible lock is held by another process.
|
|
To make a nonblocking request, include
|
|
<B>LOCK_NB</B>
|
|
|
|
(by ORing)
|
|
with any of the above operations.
|
|
<P>
|
|
|
|
A single file may not simultaneously have both shared and exclusive locks.
|
|
<P>
|
|
|
|
Locks created by
|
|
<B>flock</B>()
|
|
|
|
are associated with an open file description (see
|
|
<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)).
|
|
|
|
This means that duplicate file descriptors (created by, for example,
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fork">fork</A></B>(2)
|
|
|
|
or
|
|
<B><A HREF="/cgi-bin/man/man2html?2+dup">dup</A></B>(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
|
|
<B>LOCK_UN</B>
|
|
|
|
operation on any of these duplicate file descriptors, or when all
|
|
such file descriptors have been closed.
|
|
<P>
|
|
|
|
If a process uses
|
|
<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
|
|
|
|
(or similar) to obtain more than one file descriptor for the same file,
|
|
these file descriptors are treated independently by
|
|
<B>flock</B>().
|
|
|
|
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.
|
|
<P>
|
|
|
|
A process may hold only one type of lock (shared or exclusive)
|
|
on a file.
|
|
Subsequent
|
|
<B>flock</B>()
|
|
|
|
calls on an already locked file will convert an existing lock to the new
|
|
lock mode.
|
|
<P>
|
|
|
|
Locks created by
|
|
<B>flock</B>()
|
|
|
|
are preserved across an
|
|
<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2).
|
|
|
|
<P>
|
|
|
|
A shared or exclusive lock can be placed on a file regardless of the
|
|
mode in which the file was opened.
|
|
<A NAME="lbAE"> </A>
|
|
<H2>RETURN VALUE</H2>
|
|
|
|
On success, zero is returned.
|
|
On error, -1 is returned, and
|
|
<I>errno</I>
|
|
|
|
is set appropriately.
|
|
<A NAME="lbAF"> </A>
|
|
<H2>ERRORS</H2>
|
|
|
|
<DL COMPACT>
|
|
<DT id="5"><B>EBADF</B>
|
|
|
|
<DD>
|
|
<I>fd</I>
|
|
|
|
is not an open file descriptor.
|
|
<DT id="6"><B>EINTR</B>
|
|
|
|
<DD>
|
|
While waiting to acquire a lock, the call was interrupted by
|
|
delivery of a signal caught by a handler; see
|
|
<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7).
|
|
|
|
<DT id="7"><B>EINVAL</B>
|
|
|
|
<DD>
|
|
<I>operation</I>
|
|
|
|
is invalid.
|
|
<DT id="8"><B>ENOLCK</B>
|
|
|
|
<DD>
|
|
The kernel ran out of memory for allocating lock records.
|
|
<DT id="9"><B>EWOULDBLOCK</B>
|
|
|
|
<DD>
|
|
The file is locked and the
|
|
<B>LOCK_NB</B>
|
|
|
|
flag was selected.
|
|
</DL>
|
|
<A NAME="lbAG"> </A>
|
|
<H2>CONFORMING TO</H2>
|
|
|
|
4.4BSD (the
|
|
<B>flock</B>()
|
|
|
|
call first appeared in 4.2BSD).
|
|
A version of
|
|
<B>flock</B>(),
|
|
|
|
possibly implemented in terms of
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2),
|
|
|
|
appears on most UNIX systems.
|
|
<A NAME="lbAH"> </A>
|
|
<H2>NOTES</H2>
|
|
|
|
Since kernel 2.0,
|
|
<B>flock</B>()
|
|
|
|
is implemented as a system call in its own right rather
|
|
than being emulated in the GNU C library as a call to
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2).
|
|
|
|
With this implementation,
|
|
there is no interaction between the types of lock
|
|
placed by
|
|
<B>flock</B>()
|
|
|
|
and
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2),
|
|
|
|
and
|
|
<B>flock</B>()
|
|
|
|
does not detect deadlock.
|
|
(Note, however, that on some systems, such as the modern BSDs,
|
|
|
|
<B>flock</B>()
|
|
|
|
and
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
|
|
|
|
locks
|
|
<I>do</I>
|
|
|
|
interact with one another.)
|
|
<P>
|
|
|
|
<B>flock</B>()
|
|
|
|
places advisory locks only; given suitable permissions on a file,
|
|
a process is free to ignore the use of
|
|
<B>flock</B>()
|
|
|
|
and perform I/O on the file.
|
|
<P>
|
|
|
|
<B>flock</B>()
|
|
|
|
and
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
|
|
|
|
locks have different semantics with respect to forked processes and
|
|
<B><A HREF="/cgi-bin/man/man2html?2+dup">dup</A></B>(2).
|
|
|
|
On systems that implement
|
|
<B>flock</B>()
|
|
|
|
using
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2),
|
|
|
|
the semantics of
|
|
<B>flock</B>()
|
|
|
|
will be different from those described in this manual page.
|
|
<P>
|
|
|
|
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
|
|
<B>LOCK_NB</B>
|
|
|
|
was specified.
|
|
(This is the original BSD behavior,
|
|
and occurs on many other implementations.)
|
|
|
|
|
|
<A NAME="lbAI"> </A>
|
|
<H3>NFS details</H3>
|
|
|
|
In Linux kernels up to 2.6.11,
|
|
<B>flock</B>()
|
|
|
|
does not lock files over NFS
|
|
(i.e., the scope of locks was limited to the local system).
|
|
Instead, one could use
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
|
|
|
|
byte-range locking, which does work over NFS,
|
|
given a sufficiently recent version of
|
|
Linux and a server which supports locking.
|
|
<P>
|
|
|
|
Since Linux 2.6.12, NFS clients support
|
|
<B>flock</B>()
|
|
|
|
locks by emulating them as
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
|
|
|
|
byte-range locks on the entire file.
|
|
This means that
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
|
|
|
|
and
|
|
<B>flock</B>()
|
|
|
|
locks
|
|
<I>do</I>
|
|
|
|
interact with one another over NFS.
|
|
It also means that in order to place an exclusive lock,
|
|
the file must be opened for writing.
|
|
<P>
|
|
|
|
Since Linux 2.6.37,
|
|
|
|
the kernel supports a compatibility mode that allows
|
|
<B>flock</B>()
|
|
|
|
locks (and also
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
|
|
|
|
byte region locks) to be treated as local;
|
|
see the discussion of the
|
|
<I>local_lock</I>
|
|
|
|
option in
|
|
<B><A HREF="/cgi-bin/man/man2html?5+nfs">nfs</A></B>(5).
|
|
|
|
<A NAME="lbAJ"> </A>
|
|
<H2>SEE ALSO</H2>
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?1+flock">flock</A></B>(1),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+close">close</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+dup">dup</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fork">fork</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?3+lockf">lockf</A></B>(3),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?8+lslocks">lslocks</A></B>(8)
|
|
|
|
<P>
|
|
|
|
<I>Documentation/filesystems/locks.txt</I>
|
|
|
|
in the Linux kernel source tree
|
|
(<I>Documentation/locks.txt</I>
|
|
|
|
in older kernels)
|
|
<A NAME="lbAK"> </A>
|
|
<H2>COLOPHON</H2>
|
|
|
|
This page is part of release 5.05 of the Linux
|
|
<I>man-pages</I>
|
|
|
|
project.
|
|
A description of the project,
|
|
information about reporting bugs,
|
|
and the latest version of this page,
|
|
can be found at
|
|
<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
|
|
<P>
|
|
|
|
<HR>
|
|
<A NAME="index"> </A><H2>Index</H2>
|
|
<DL>
|
|
<DT id="10"><A HREF="#lbAB">NAME</A><DD>
|
|
<DT id="11"><A HREF="#lbAC">SYNOPSIS</A><DD>
|
|
<DT id="12"><A HREF="#lbAD">DESCRIPTION</A><DD>
|
|
<DT id="13"><A HREF="#lbAE">RETURN VALUE</A><DD>
|
|
<DT id="14"><A HREF="#lbAF">ERRORS</A><DD>
|
|
<DT id="15"><A HREF="#lbAG">CONFORMING TO</A><DD>
|
|
<DT id="16"><A HREF="#lbAH">NOTES</A><DD>
|
|
<DL>
|
|
<DT id="17"><A HREF="#lbAI">NFS details</A><DD>
|
|
</DL>
|
|
<DT id="18"><A HREF="#lbAJ">SEE ALSO</A><DD>
|
|
<DT id="19"><A HREF="#lbAK">COLOPHON</A><DD>
|
|
</DL>
|
|
<HR>
|
|
This document was created by
|
|
<A HREF="/cgi-bin/man/man2html">man2html</A>,
|
|
using the manual pages.<BR>
|
|
Time: 00:05:32 GMT, March 31, 2021
|
|
</BODY>
|
|
</HTML>
|