package flock

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.

If 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.