[/
 / Copyright (c) 2003-2008 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:threads Threads]

[heading Thread Safety]

In general, it is safe to make concurrent use of distinct objects, but unsafe
to make concurrent use of a single object. However, types such as `io_service`
provide a stronger guarantee that it is safe to use a single object
concurrently.

[heading Internal Threads]

The implementation of this library for a particular platform may make use of
one or more internal threads to emulate asynchronicity. As far as possible,
these threads must be invisible to the library user. In particular, the threads:

* must not call the user's code directly; and

* must block all signals.

This approach is complemented by the following guarantee:

* Asynchronous completion handlers will only be called from threads that are
  currently calling `io_service::run()`.

Consequently, it is the library user's responsibility to create and manage all
threads to which the notifications will be delivered.

The reasons for this approach include:

* By only calling `io_service::run()` from a single thread, the user's code can
  avoid the development complexity associated with synchronisation. For
  example, a library user can implement scalable servers that are
  single-threaded (from the user's point of view).

* A library user may need to perform initialisation in a thread shortly after
  the thread starts and before any other application code is executed. For
  example, users of Microsoft's COM must call `CoInitializeEx` before any other
  COM operations can be called from that thread.

* The library interface is decoupled from interfaces for thread creation and
  management, and permits implementations on platforms where threads are not
  available.

[endsect]
