module Flock:sig
..end
flock(2)
.
See the man page for flock(2).
type
lock_operation =
| |
LOCK_SH |
| |
LOCK_EX |
| |
LOCK_UN |
val flock : ?nonblocking:bool -> Unix.file_descr -> lock_operation -> unit
flock ~nonblocking fd operation
applies or removes an advisory lock on
the open file specified by fd
. The argument 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.nonblocking
is false
(default), a call to flock
may block if an
incompatible lock is held by another process. Otherwise, if nonblocking
is true
then attempting to acquire a lock that is already held will
raises (Unix_error(EAGAIN,_,_)
or Unix_error(EWOULDBLOCK,_,_)
.
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 hold only 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.
A call to flock
could raise one of the following Unix.Unix_errors:
EBADF
: fd is 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; see signal(7).ENOLCK
:The kernel ran out of memory for allocating lock records.EWOULDBLOCK
or EAGAIN
:
The file is locked and the nonblocking
was true
.