flock(2) -- Linux man page
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 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
flock()
may block if an incompatible lock is held by another process.
To make a non-blocking 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 a file, or, more precisely, 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.
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
- EWOULDBLOCK
-
The file is locked and the
LOCK_NB
flag was selected.
- 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.
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 Unices.
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).
SEE ALSO
open(2),
close(2),
dup(2),
execve(2),
fcntl(2),
fork(2),
lockf(3)
There are also
locks.txt
and
mandatory.txt
in
/usr/src/linux/Documentation.
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