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.
false (default), a call to
flock may block if an incompatible lock is held by another process. Otherwise, if
true then attempting to acquire a lock that is already held will raises
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,
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
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.
EAGAIN: The file is locked and the