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### Threads

##### 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.

##### 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.