Originální popis anglicky:
flock - apply or remove an advisory lock on an open file
Návod, kniha: Linux Programmer's Manual
#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
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.
On success, zero is returned. On error, -1 is returned, and
errno is set
appropriately.
- 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.
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.
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).
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.
