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Provides core I/O functionality.

class io_service :




Class used to uniquely identify a service.


Base class for all io_service services.


Provides serialised handler execution.


Class to inform the io_service when it has work to do.

Member Functions




Request the io_service to invoke the given handler.




Run the io_service object's event processing loop to execute ready handlers.


Run the io_service object's event processing loop to execute one ready handler.


Request the io_service to invoke the given handler and return immediately.


Reset the io_service in preparation for a subsequent run() invocation.


Run the io_service object's event processing loop.


Run the io_service object's event processing loop to execute at most one handler.


Stop the io_service object's event processing loop.


Create a new handler that automatically dispatches the wrapped handler on the io_service.







Add a service object to the io_service.


Determine if an io_service contains a specified service type.


Obtain the service object corresponding to the given type.

The io_service class provides the core I/O functionality for users of the asynchronous I/O objects, including:

The io_service class also includes facilities intended for developers of custom asynchronous services.

Thread Safety

Distinct objects: Safe.

Shared objects: Safe, with the exception that calling reset() while there are unfinished run(), run_one(), poll() or poll_one() calls results in undefined behaviour.

Synchronous and asynchronous operations

Synchronous operations on I/O objects implicitly run the io_service object for an individual operation. The io_service functions run(), run_one(), poll() or poll_one() must be called for the io_service to perform asynchronous operations on behalf of a C++ program. Notification that an asynchronous operation has completed is delivered by invocation of the associated handler. Handlers are invoked only by a thread that is currently calling any overload of run(), run_one(), poll() or poll_one() for the io_service.

Effect of exceptions thrown from handlers

If an exception is thrown from a handler, the exception is allowed to propagate through the throwing thread's invocation of run(), run_one(), poll() or poll_one(). No other threads that are calling any of these functions are affected. It is then the responsibility of the application to catch the exception.

After the exception has been caught, the run(), run_one(), poll() or poll_one() call may be restarted without the need for an intervening call to reset(). This allows the thread to rejoin the io_service object's thread pool without impacting any other threads in the pool.

For example:

boost::asio::io_service io_service;
for (;;)
    break; // run() exited normally
  catch (my_exception& e)
    // Deal with exception as appropriate.
Stopping the io_service from running out of work

Some applications may need to prevent an io_service object's run() call from returning when there is no more work to do. For example, the io_service may be being run in a background thread that is launched prior to the application's asynchronous operations. The run() call may be kept running by creating an object of type io_service::work:

boost::asio::io_service io_service;
boost::asio::io_service::work work(io_service);

To effect a shutdown, the application will then need to call the io_service object's stop() member function. This will cause the io_service run() call to return as soon as possible, abandoning unfinished operations and without permitting ready handlers to be dispatched.

Alternatively, if the application requires that all operations and handlers be allowed to finish normally, the work object may be explicitly destroyed.

boost::asio::io_service io_service;
auto_ptr<boost::asio::io_service::work> work(
    new boost::asio::io_service::work(io_service));
work.reset(); // Allow run() to exit. 
The io_service class and I/O services

Class io_service implements an extensible, type-safe, polymorphic set of I/O services, indexed by service type. An object of class io_service must be initialised before I/O objects such as sockets, resolvers and timers can be used. These I/O objects are distinguished by having constructors that accept an io_service& parameter.

I/O services exist to manage the logical interface to the operating system on behalf of the I/O objects. In particular, there are resources that are shared across a class of I/O objects. For example, timers may be implemented in terms of a single timer queue. The I/O services manage these shared resources.

Access to the services of an io_service is via three function templates, use_service(), add_service() and has_service().

In a call to use_service<Service>(), the type argument chooses a service, making available all members of the named type. If Service is not present in an io_service, an object of type Service is created and added to the io_service. A C++ program can check if an io_service implements a particular service with the function template has_service<Service>().

Service objects may be explicitly added to an io_service using the function template add_service<Service>(). If the Service is already present, the service_already_exists exception is thrown. If the owner of the service is not the same object as the io_service parameter, the invalid_service_owner exception is thrown.

Once a service reference is obtained from an io_service object by calling use_service(), that reference remains usable as long as the owning io_service object exists.

All I/O service implementations have io_service::service as a public base class. Custom I/O services may be implemented by deriving from this class and then added to an io_service using the facilities described above.


Header: boost/asio/io_service.hpp

Convenience header: boost/asio.hpp