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#### Strands: Use Threads Without Explicit Locking

A strand is defined as a strictly sequential invocation of event handlers (i.e. no concurrent invocation). Use of strands allows execution of code in a multithreaded program without the need for explicit locking (e.g. using mutexes).

Strands may be either implicit or explicit, as illustrated by the following alternative approaches:

- Calling io_service::run() from only one thread means all event handlers execute in an implicit strand, due to the io_service's guarantee that handlers are only invoked from inside run().
- Where there is a single chain of asynchronous operations associated with a connection (e.g. in a half duplex protocol implementation like HTTP) there is no possibility of concurrent execution of the handlers. This is an implicit strand.
- An explicit strand is an instance of `io_service::strand`. All event handler function objects need to be wrapped using `io_service::strand::wrap()` or otherwise posted/dispatched through the `io_service::strand` object.

In the case of composed asynchronous operations, such as `async_read()` or `async_read_until()`, if a completion handler goes through a strand, then all intermediate handlers should also go through the same strand. This is needed to ensure thread safe access for any objects that are shared between the caller and the composed operation (in the case of `async_read()` it's the socket, which the caller can close() to cancel the operation). This is done by having hook functions for all intermediate handlers which forward the calls to the customisable hook associated with the final handler:

    struct my_handler
    {
      void operator()() { ... }
    };

    template<class F>
    void asio_handler_invoke(F f, my_handler*)
    {
      // Do custom invocation here.
      // Default implementation calls f();
    }

The `io_service::strand::wrap()` function creates a new completion handler that defines `asio_handler_invoke` so that the function object is executed through the strand.

###### See Also

io_service::strand, tutorial Timer.5, HTTP server 3 example.