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thread-mainloop.h

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#ifndef foothreadmainloophfoo
#define foothreadmainloophfoo

/***
  This file is part of PulseAudio.

  Copyright 2006 Lennart Poettering
  Copyright 2006 Pierre Ossman <ossman@cendio.se> for Cendio AB

  PulseAudio is free software; you can redistribute it and/or modify
  it under the terms of the GNU Lesser General Public License as published
  by the Free Software Foundation; either version 2.1 of the License,
  or (at your option) any later version.

  PulseAudio is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with PulseAudio; if not, write to the Free Software
  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
  USA.
***/

#include <pulse/mainloop-api.h>
#include <pulse/cdecl.h>
#include <pulse/version.h>

PA_C_DECL_BEGIN

/** \page threaded_mainloop Threaded Main Loop
 *
 * \section overv_sec Overview
 *
 * The threaded main loop implementation is a special version of the primary
 * main loop implementation (see \ref mainloop). For the basic design, see
 * its documentation.
 *
 * The added feature in the threaded main loop is that it spawns a new thread
 * that runs the real main loop. This allows a synchronous application to use
 * the asynchronous API without risking to stall the PulseAudio library.
 *
 * \section creat_sec Creation
 *
 * A pa_threaded_mainloop object is created using pa_threaded_mainloop_new().
 * This will only allocate the required structures though, so to use it the
 * thread must also be started. This is done through
 * pa_threaded_mainloop_start(), after which you can start using the main loop.
 *
 * \section destr_sec Destruction
 *
 * When the PulseAudio connection has been terminated, the thread must be
 * stopped and the resources freed. Stopping the thread is done using
 * pa_threaded_mainloop_stop(), which must be called without the lock (see
 * below) held. When that function returns, the thread is stopped and the
 * pa_threaded_mainloop object can be freed using pa_threaded_mainloop_free().
 *
 * \section lock_sec Locking
 *
 * Since the PulseAudio API doesn't allow concurrent accesses to objects,
 * a locking scheme must be used to guarantee safe usage. The threaded main
 * loop API provides such a scheme through the functions
 * pa_threaded_mainloop_lock() and pa_threaded_mainloop_unlock().
 *
 * The lock is recursive, so it's safe to use it multiple times from the same
 * thread. Just make sure you call pa_threaded_mainloop_unlock() the same
 * number of times you called pa_threaded_mainloop_lock().
 *
 * The lock needs to be held whenever you call any PulseAudio function that
 * uses an object associated with this main loop. Make sure you do not hold
 * on to the lock more than necessary though, as the threaded main loop stops
 * while the lock is held.
 *
 * Example:
 *
 * \code
 * void my_check_stream_func(pa_threaded_mainloop *m, pa_stream *s) {
 *     pa_stream_state_t state;
 *
 *     pa_threaded_mainloop_lock(m);
 *
 *     state = pa_stream_get_state(s);
 *
 *     pa_threaded_mainloop_unlock(m);
 *
 *     if (state == PA_STREAM_READY)
 *         printf("Stream is ready!");
 *     else
 *         printf("Stream is not ready!");
 * }
 * \endcode
 *
 * \section cb_sec Callbacks
 *
 * Callbacks in PulseAudio are asynchronous, so they require extra care when
 * using them together with a threaded main loop.
 *
 * The easiest way to turn the callback based operations into synchronous
 * ones, is to simply wait for the callback to be called and continue from
 * there. This is the approach chosen in PulseAudio's threaded API.
 *
 * \subsection basic_subsec Basic callbacks
 *
 * For the basic case, where all that is required is to wait for the callback
 * to be invoked, the code should look something like this:
 *
 * Example:
 *
 * \code
 * static void my_drain_callback(pa_stream *s, int success, void *userdata) {
 *     pa_threaded_mainloop *m;
 *
 *     m = userdata;
 *     assert(m);
 *
 *     pa_threaded_mainloop_signal(m, 0);
 * }
 *
 * void my_drain_stream_func(pa_threaded_mainloop *m, pa_stream *s) {
 *     pa_operation *o;
 *
 *     pa_threaded_mainloop_lock(m);
 *
 *     o = pa_stream_drain(s, my_drain_callback, m);
 *     assert(o);
 *
 *     while (pa_operation_get_state(o) == PA_OPERATION_RUNNING)
 *         pa_threaded_mainloop_wait(m);
 *
 *     pa_operation_unref(o);
 *
 *     pa_threaded_mainloop_unlock(m);
 * }
 * \endcode
 *
 * The main function, my_drain_stream_func(), will wait for the callback to
 * be called using pa_threaded_mainloop_wait().
 *
 * If your application is multi-threaded, then this waiting must be
 * done inside a while loop. The reason for this is that multiple
 * threads might be using pa_threaded_mainloop_wait() at the same
 * time. Each thread must therefore verify that it was its callback
 * that was invoked. Also the underlying OS synchronization primitives
 * are usually not free of spurious wake-ups, so a
 * pa_threaded_mainloop_wait() must be called within a loop even if
 * you have only one thread waiting.
 *
 * The callback, my_drain_callback(), indicates to the main function that it
 * has been called using pa_threaded_mainloop_signal().
 *
 * As you can see, pa_threaded_mainloop_wait() may only be called with
 * the lock held. The same thing is true for pa_threaded_mainloop_signal(),
 * but as the lock is held before the callback is invoked, you do not have to
 * deal with that.
 *
 * The functions will not dead lock because the wait function will release
 * the lock before waiting and then regrab it once it has been signaled.
 * For those of you familiar with threads, the behaviour is that of a
 * condition variable.
 *
 * \subsection data_subsec Data callbacks
 *
 * For many callbacks, simply knowing that they have been called is
 * insufficient. The callback also receives some data that is desired. To
 * access this data safely, we must extend our example a bit:
 *
 * \code
 * static int *drain_result;
 *
 * static void my_drain_callback(pa_stream*s, int success, void *userdata) {
 *     pa_threaded_mainloop *m;
 *
 *     m = userdata;
 *     assert(m);
 *
 *     drain_result = &success;
 *
 *     pa_threaded_mainloop_signal(m, 1);
 * }
 *
 * void my_drain_stream_func(pa_threaded_mainloop *m, pa_stream *s) {
 *     pa_operation *o;
 *
 *     pa_threaded_mainloop_lock(m);
 *
 *     o = pa_stream_drain(s, my_drain_callback, m);
 *     assert(o);
 *
 *     while (pa_operation_get_state(o) == PA_OPERATION_RUNNING)
 *         pa_threaded_mainloop_wait(m);
 *
 *     pa_operation_unref(o);
 *
 *     if (*drain_result)
 *         printf("Success!");
 *     else
 *         printf("Bitter defeat...");
 *
 *     pa_threaded_mainloop_accept(m);
 *
 *     pa_threaded_mainloop_unlock(m);
 * }
 * \endcode
 *
 * The example is a bit silly as it would probably have been easier to just
 * copy the contents of success, but for larger data structures this can be
 * wasteful.
 *
 * The difference here compared to the basic callback is the 1 sent to
 * pa_threaded_mainloop_signal() and the call to
 * pa_threaded_mainloop_accept(). What will happen is that
 * pa_threaded_mainloop_signal() will signal the main function and then stop.
 * The main function is then free to use the data in the callback until
 * pa_threaded_mainloop_accept() is called, which will allow the callback
 * to continue.
 *
 * Note that pa_threaded_mainloop_accept() must be called some time between
 * exiting the while loop and unlocking the main loop! Failure to do so will
 * result in a race condition. I.e. it is not ok to release the lock and
 * regrab it before calling pa_threaded_mainloop_accept().
 *
 * \subsection async_subsec Asynchronous callbacks
 *
 * PulseAudio also has callbacks that are completely asynchronous, meaning
 * that they can be called at any time. The threading main loop API provides
 * the locking mechanism to handle concurrent accesses, but nothing else.
 * Applications will have to handle communication from the callback to the
 * main program through some own system.
 *
 * The callbacks that are completely asynchronous are:
 *
 * \li State callbacks for contexts, streams, etc.
 * \li Subscription notifications
 */

/** \file
 *
 * A thread based event loop implementation based on pa_mainloop. The
 * event loop is run in a helper thread in the background. A few
 * synchronization primitives are available to access the objects
 * attached to the event loop safely.
 *
 * See also \subpage threaded_mainloop
 */

/** An opaque threaded main loop object */
00248 typedef struct pa_threaded_mainloop pa_threaded_mainloop;

/** Allocate a new threaded main loop object. You have to call
 * pa_threaded_mainloop_start() before the event loop thread starts
 * running. */
pa_threaded_mainloop *pa_threaded_mainloop_new(void);

/** Free a threaded main loop object. If the event loop thread is
 * still running, terminate it with pa_threaded_mainloop_stop()
 * first. */
void pa_threaded_mainloop_free(pa_threaded_mainloop* m);

/** Start the event loop thread. */
int pa_threaded_mainloop_start(pa_threaded_mainloop *m);

/** Terminate the event loop thread cleanly. Make sure to unlock the
 * mainloop object before calling this function. */
void pa_threaded_mainloop_stop(pa_threaded_mainloop *m);

/** Lock the event loop object, effectively blocking the event loop
 * thread from processing events. You can use this to enforce
 * exclusive access to all objects attached to the event loop. This
 * lock is recursive. This function may not be called inside the event
 * loop thread. Events that are dispatched from the event loop thread
 * are executed with this lock held. */
void pa_threaded_mainloop_lock(pa_threaded_mainloop *m);

/** Unlock the event loop object, inverse of pa_threaded_mainloop_lock() */
void pa_threaded_mainloop_unlock(pa_threaded_mainloop *m);

/** Wait for an event to be signalled by the event loop thread. You
 * can use this to pass data from the event loop thread to the main
 * thread in synchronized fashion. This function may not be called
 * inside the event loop thread. Prior to this call the event loop
 * object needs to be locked using pa_threaded_mainloop_lock(). While
 * waiting the lock will be released, immediately before returning it
 * will be acquired again. This function may spuriously wake up even
 * without _signal() being called. You need to make sure to handle
 * that! */
void pa_threaded_mainloop_wait(pa_threaded_mainloop *m);

/** Signal all threads waiting for a signalling event in
 * pa_threaded_mainloop_wait(). If wait_for_release is non-zero, do
 * not return before the signal was accepted by a
 * pa_threaded_mainloop_accept() call. While waiting for that condition
 * the event loop object is unlocked. */
void pa_threaded_mainloop_signal(pa_threaded_mainloop *m, int wait_for_accept);

/** Accept a signal from the event thread issued with
 * pa_threaded_mainloop_signal(). This call should only be used in
 * conjunction with pa_threaded_mainloop_signal() with a non-zero
 * wait_for_accept value.  */
void pa_threaded_mainloop_accept(pa_threaded_mainloop *m);

/** Return the return value as specified with the main loop's quit() routine. */
int pa_threaded_mainloop_get_retval(pa_threaded_mainloop *m);

/** Return the abstract main loop abstraction layer vtable for this
    main loop. No need to free the API as it is owned by the loop
    and is destroyed when the loop is freed. */
pa_mainloop_api* pa_threaded_mainloop_get_api(pa_threaded_mainloop*m);

/** Returns non-zero when called from withing the event loop thread. \since 0.9.7 */
int pa_threaded_mainloop_in_thread(pa_threaded_mainloop *m);

PA_C_DECL_END

#endif

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