NAME
flock - apply or remove an advisory lock on an open file
SYNOPSIS
#include <sys/file.h>
I int flock(int fd , int operation );
DESCRIPTION
Apply or remove an advisory lock on the open file specified by
R fd .
The parameter
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
R flock ()
may block if an incompatible lock is held by another process.
To make a non-blocking request, include
LOCK_NB
(by
R OR ing)
with any of the above operations.
A single file may not simultaneously have both shared and exclusive locks.
Locks created by
R 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
R 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
R flock ()
calls on an already locked file will convert an existing lock to the new
lock mode.
Locks created by
R 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 a 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.
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(2)
call first appeared in 4.2BSD).
A version of
flock(2),
possibly implemented in terms of
fcntl(2),
appears on most Unix systems.
NOTES
flock(2)
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(2)
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(2)
and
fcntl(2),
and
flock(2)
does not detect deadlock.
flock(2)
places advisory locks only; given suitable permissions on a file,
a process is free to ignore the use of
flock(2)
and perform I/O on the file.
flock(2)
and
fcntl(2)
locks have different semantics with respect to forked processes and
dup(2).
On systems that implement
R flock ()
using
fcntl(2),
the semantics of
R 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
