Class Thread.Condition

Description

Implementation of condition variables.

Condition variables are used by threaded programs to wait for events happening in other threads.

In order to prevent races (which is the whole point of condition variables), the modification of a shared resource and the signal notifying modification of the resource must be performed inside the same mutex lock, and the examining of the resource and waiting for a signal that the resource has changed based on that examination must also happen inside the same mutex lock.

Typical wait operation:

1. Take mutex lock

2. Read/write shared resource

3. Wait for the signal with the mutex lock in released state

4. Reacquire mutex lock

5. If needed, jump back to step 2 again

6. Release mutex lock

Typical signal operation:

1. Take mutex lock

2. Read/write shared resource

3. Send signal

4. Release mutex lock

Example

You have some resource that multiple treads want to use.  To
   protect this resource for simultaneous access, you create a shared mutex.
   Before you read or write the resource, you take the mutex so that you
   get a consistent and private view of / control over it.  When you decide
   that the resource is not in the state you want it, and that you need
   to wait for some other thread to modify the state for you before you
   can continue, you wait on the conditional variable, which will
   temporarily relinquish the mutex during the wait.  This way a
   different thread can take the mutex, update the state of the resource,
   and then signal the condition (which does not in itself release the
   mutex, but the signalled thread will be next in line once the mutex is
   released).

Note

Condition variables are only available on systems with thread support. The Condition class is not simulated otherwise, since that can't be done accurately without continuations.

See also

Mutex