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io_service

Typedef for backwards compatibility.

typedef io_context io_service;
Types

Name

Description

executor_type

Executor used to submit functions to an io_context.

service

Base class for all io_context services.

strand

Provides serialised handler execution.

work

(Deprecated: Use executor_work_guard.) Class to inform the io_context when it has work to do.

count_type

The type used to count the number of handlers executed by the context.

fork_event

Fork-related event notifications.

Member Functions

Name

Description

dispatch

(Deprecated: Use boost::asio::dispatch().) Request the io_context to invoke the given handler.

get_executor

Obtains the executor associated with the io_context.

io_context

Constructor.

notify_fork

Notify the execution_context of a fork-related event.

poll

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

(Deprecated: Use non-error_code overload.) Run the io_context object's event processing loop to execute ready handlers.

poll_one

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

(Deprecated: Use non-error_code overload.) Run the io_context object's event processing loop to execute one ready handler.

post

(Deprecated: Use boost::asio::post().) Request the io_context to invoke the given handler and return immediately.

reset

(Deprecated: Use restart().) Reset the io_context in preparation for a subsequent run() invocation.

restart

Restart the io_context in preparation for a subsequent run() invocation.

run

Run the io_context object's event processing loop.

(Deprecated: Use non-error_code overload.) Run the io_context object's event processing loop.

run_for

Run the io_context object's event processing loop for a specified duration.

run_one

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

(Deprecated: Use non-error_code overlaod.) Run the io_context object's event processing loop to execute at most one handler.

run_one_for

Run the io_context object's event processing loop for a specified duration to execute at most one handler.

run_one_until

Run the io_context object's event processing loop until a specified time to execute at most one handler.

run_until

Run the io_context object's event processing loop until a specified time.

stop

Stop the io_context object's event processing loop.

stopped

Determine whether the io_context object has been stopped.

wrap

(Deprecated: Use boost::asio::bind_executor().) Create a new handler that automatically dispatches the wrapped handler on the io_context.

~io_context

Destructor.

Protected Member Functions

Name

Description

destroy

Destroys all services in the context.

shutdown

Shuts down all services in the context.

Friends

Name

Description

add_service

(Deprecated: Use make_service().) Add a service object to the execution_context.

has_service

Determine if an execution_context contains a specified service type.

make_service

Creates a service object and adds it to the execution_context.

use_service

Obtain the service object corresponding to the given type.

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

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

Thread Safety

Distinct objects: Safe.

Shared objects: Safe, with the specific exceptions of the restart() and notify_fork() functions. Calling restart() while there are unfinished run(), run_one(), run_for(), run_until(), poll() or poll_one() calls results in undefined behaviour. The notify_fork() function should not be called while any io_context function, or any function on an I/O object that is associated with the io_context, is being called in another thread.

Synchronous and asynchronous operations

Synchronous operations on I/O objects implicitly run the io_context object for an individual operation. The io_context functions run(), run_one(), run_for(), run_until(), poll() or poll_one() must be called for the io_context 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(), run_for(), run_until(), poll() or poll_one() for the io_context.

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(), run_for(), run_until(), 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(), run_for(), run_until(), poll() or poll_one() call may be restarted without the need for an intervening call to restart(). This allows the thread to rejoin the io_context object's thread pool without impacting any other threads in the pool.

For example:

boost::asio::io_context io_context;
...
for (;;)
{
  try
  {
    io_context.run();
    break; // run() exited normally
  }
  catch (my_exception& e)
  {
    // Deal with exception as appropriate.
  }
}
Submitting arbitrary tasks to the io_context

To submit functions to the io_context, use the dispatch , post or defer free functions.

For example:

void my_task()
{
  ...
}

...

boost::asio::io_context io_context;

// Submit a function to the io_context.
boost::asio::post(io_context, my_task);

// Submit a lambda object to the io_context.
boost::asio::post(io_context,
    []()
    {
      ...
    });

// Run the io_context until it runs out of work.
io_context.run();
Stopping the io_context from running out of work

Some applications may need to prevent an io_context object's run() call from returning when there is no more work to do. For example, the io_context 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 boost::asio::executor_work_guard<io_context::executor_type>:

boost::asio::io_context io_context;
boost::asio::executor_work_guard<boost::asio::io_context::executor_type>
  = boost::asio::make_work_guard(io_context);
...

To effect a shutdown, the application will then need to call the io_context object's stop() member function. This will cause the io_context 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 reset.

boost::asio::io_context io_context;
boost::asio::executor_work_guard<boost::asio::io_context::executor_type>
  = boost::asio::make_work_guard(io_context);
...
work.reset(); // Allow run() to exit.
Requirements

Header: boost/asio/io_service.hpp

Convenience header: boost/asio.hpp


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