flock — apply or remove an advisory lock on an open file
#include <sys/file.h>
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:
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 OR
ing) 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.
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.
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.
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 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 behaviour, and occurs on
many other implementations.)
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)
There are also locks.txt
and mandatory.txt
in /usr/src/linux/Documentation
.
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