flock() - Unix, Linux System Call
flock - apply or remove an advisory lock on an open file
int flock(int fd, int operation);
Apply or remove an advisory lock on the open file specified by
fd. The parameter operation is one of the following:
Place a shared lock.
More than one process may hold a shared lock for a given file
at a given time.
Place an exclusive lock.
Only one process may hold an exclusive lock for a given
file at a given time.
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
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,
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
(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.
flock() calls on an already locked file will convert an existing lock to the new
Locks created by
flock() are preserved across an
A shared or exclusive lock can be placed on a file regardless of the
mode in which the file was opened.
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
fd is not a not an open file descriptor.
While waiting to acquire a lock, the call was interrupted by
delivery of a signal caught by a handler.
operation is invalid.
The kernel ran out of memory for allocating lock records.
The file is locked and the
LOCK_NB flag was selected.
call first appeared in 4.2BSD).
A version of
possibly implemented in terms of
appears on most Unices.
does not lock files over NFS. Use
instead: that does work over NFS, given a sufficiently recent version of
Linux and a server which supports locking.
Since kernel 2.0,
is implemented as a system call in its own right rather
than being emulated in the GNU C library as a call to
This yields true BSD semantics:
there is no interaction between the types of lock
does not detect deadlock.
places advisory locks only; given suitable permissions on a file,
a process is free to ignore the use of
and perform I/O on the file.
locks have different semantics with respect to forked processes and
On systems that implement
fcntl(), 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 behaviour,
and occurs on many other implementations.)