Tasks in GStreamer

stay gstreamer in , You can easily create one when you need it task As an independent thread ,task It can be ordinary task, It can also be pad task, This needs to be determined according to specific needs , So let's see v4l2 and avdemux Inside task Code for .

task The creation of

gst-plugins-good/sys/v4l2/gstv4l2videodec.c

adopt gst_pad_start_task Start a pad task,task The function is gst_v4l2_video_dec_loop. The function is to activate v4l2 The decoding task, And when the task Exit time , Input processing will be disabled in drain when unlock Input , To prevent blocking ：

/* start task */
if (!gst_pad_start_task (decoder->srcpad, (GstTaskFunction) gst_v4l2_video_dec_loop, self, NULL))
    goto start_task_failed;

gst-libav/ext/libav/gstavdemux.c

Create a demux->task,task The function is gst_ffmpegdemux_loop,gst_task_init Will pass gst_task_new Indirect invocation , stay init Call in function ensure_klass_pool Make sure _global_task_pool Thread pool available ,start_task Will be task adopt gst_task_pool_push Put it in task In the thread pool of , below gst_pad_start_task You can see the call stack of .

/* static GstTaskPool *_global_task_pool = NULL; */

/* new task */
demux->task = gst_task_new ((GstTaskFunction) gst_ffmpegdemux_loop, demux, NULL);
g_rec_mutex_init (&demux->task_lock);

/* set lock */
gst_task_set_lock (demux->task, &demux->task_lock);

gst_ffmpegdemux_loop and gst_v4l2_video_dec_loop The two functions are task Of loop function , stay gstreamer in ,task Unlike pthread Of task, There needs to be a loop to ensure permanent task Always exist . Instead, it runs through the previous thread pool .

task Operation of

see gst_pad_start_task You can clearly see the code and stack of task Of new,init,start It's all in gst_pad_start_task It is realized in , These are in if not through gst_pad_start_task Starting up task, Both need to be called manually .

1   gst_task_init                      gsttask.c     249  0x7ffff7b49fa3 
2   g_type_create_instance                                0x7ffff75559c5 
4   g_object_new_with_properties                          0x7ffff7537ee5 
5   g_object_new                                          0x7ffff7538961 
6   gst_task_new                       gsttask.c     482  0x7ffff7b4a5ad 
7   gst_pad_start_task                 gstpad.c      6287 0x7ffff7b1d7fe

call start_task The stack ：

1   gst_task_pool_push                 gsttaskpool.c 252  0x7ffff7b4b820 
2   start_task                         gsttask.c     712  0x7ffff7b49f19 
3   gst_task_set_state_unlocked        gsttask.c     744  0x7ffff7b4ad3e 
4   gst_task_set_state                 gsttask.c     792  0x7ffff7b4ad3e                                                          
7   gst_pad_start_task                 gstpad.c      6287 0x7ffff7b1d7fe 

gst_task_pool_push Is in start_task Invocation in function , take task Put it in the thread pool , Appoint taskpool Function is gst_task_func:

  /* push on the thread pool, we remember the original pool because the user * could change it later on and then we join to the wrong pool. */
  priv->pool_id = gst_object_ref (priv->pool);
  priv->id =
      gst_task_pool_push (priv->pool_id, (GstTaskPoolFunction) gst_task_func,
      task, &error);

gst_task_pool_push Continue call in GstTaskPoolClass Of push function default_push, stay default_push The function is called g_thread_pool_push The function is placed in g_thread_pool Inside , Back task The operation of is GLib Of g_thread_pool Relevant implementation .

gpointer
gst_task_pool_push (GstTaskPool * pool, GstTaskPoolFunction func,
                        gpointer user_data, GError ** error)
{
    
    GstTaskPoolClass *klass;
    g_return_val_if_fail (GST_IS_TASK_POOL (pool), NULL);

    klass = GST_TASK_POOL_GET_CLASS (pool);
    if (klass->push == NULL)
        goto not_supported;

    /* gsttaskpool_class->push = default_push; */
    return klass->push (pool, func, user_data, error);
}

Be careful ：task Not allowed no lock While using , Otherwise, an error will be reported ：

task without a lock can't be set to state 0

adopt gst_task_set_lock Function prototypes can be seen , here task Needed lock yes GRecMutex type .

The following code is curlhttpsrc Code for , You can have a preliminary understanding ：

subprojects/gst-plugins-bad/ext/curl/gstcurlhttpsrc.c

/* init RecMutex */
g_rec_mutex_init (&klass->multi_task_context.task_rec_mutex);

klass->multi_task_context.state = GSTCURL_MULTI_LOOP_STATE_RUNNING;

/* new task */
klass->multi_task_context.task = gst_task_new (
    (GstTaskFunction) gst_curl_http_src_curl_multi_loop,
    (gpointer) & klass->multi_task_context, NULL);

/* set lock */
gst_task_set_lock (klass->multi_task_context.task,
                   &klass->multi_task_context.task_rec_mutex);

/* start task */
if (gst_task_start (klass->multi_task_context.task) == FALSE) {
    
    GSTCURL_ERROR_PRINT ("Couldn't start curl_multi task! Aborting.");
    abort ();
}

GMutex

GMutex Structure is an opaque data structure , Represents a mutex （mutex）, It can be used to protect data from shared access .

GRecMutex

GRecMutex Structure is an opaque data structure , Represents a recursive mutual exclusion . It is similar to GMutex, The difference is , It can be locked multiple times in the same thread GRecMutex Without causing deadlock . In doing so , You must pay attention to unlocking the recursive mutex when it is locked .