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
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 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) .
