# Basic tutorial 7: Multithreading and Pad Availability

## Goal

GStreamer handles multithreading automatically, but, under some
circumstances, you might need to decouple threads manually. This
tutorial shows how to do this and, in addition, completes the exposition
about Pad Availability. More precisely, this document explains:

  - How to create new threads of execution for some parts of the
    pipeline

  - What is the Pad Availability

  - How to replicate streams

## Introduction

### Multithreading

GStreamer is a multithreaded framework. This means that, internally, it
creates and destroys threads as it needs them, for example, to decouple
streaming from the application thread. Moreover, plugins are also free
to create threads for their own processing, for example, a video decoder
could create 4 threads to take full advantage of a CPU with 4 cores.

On top of this, when building the pipeline an application can specify
explicitly that a *branch* (a part of the pipeline) runs on a different
thread (for example, to have the audio and video decoders executing
simultaneously).

This is accomplished using the `queue` element, which works as follows.
The sink pad just enqueues data and returns control. On a different
thread, data is dequeued and pushed downstream. This element is also
used for buffering, as seen later in the streaming tutorials. The size
of the queue can be controlled through properties.

### The example pipeline

This example builds the following pipeline:

![](images/tutorials/basic-tutorial-7.png)

The source is a synthetic audio signal (a continuous tone) which is
split using a `tee` element (it sends through its source pads everything
it receives through its sink pad). One branch then sends the signal to
the audio card, and the other renders a video of the waveform and sends
it to the screen.

As seen in the picture, queues create a new thread, so this pipeline
runs in 3 threads. Pipelines with more than one sink usually need to be
multithreaded, because, to be synchronized, sinks usually block
execution until all other sinks are ready, and they cannot get ready if
there is only one thread, being blocked by the first sink.

### Request pads

In [Basic tutorial 3: Dynamic
pipelines](tutorials/basic/dynamic-pipelines.md) we saw
an element (`uridecodebin`) which had no pads to begin with, and they
appeared as data started to flow and the element learned about the
media. These are called **Sometimes Pads**, and contrast with the
regular pads which are always available and are called **Always Pads**.

The third kind of pad is the **Request Pad**, which is created on
demand. The classical example is the `tee` element, which has one sink
pad and no initial source pads: they need to be requested and then
`tee` adds them. In this way, an input stream can be replicated any
number of times. The disadvantage is that linking elements with Request
Pads is not as automatic, as linking Always Pads, as the walkthrough for
this example will show.

Also, to request (or release) pads in the `PLAYING` or `PAUSED` states, you
need to take additional cautions (Pad blocking) which are not described
in this tutorial. It is safe to request (or release) pads in the `NULL` or
`READY` states, though.

Without further delay, let's see the code.

## Simple multithreaded example

Copy this code into a text file named `basic-tutorial-7.c` (or find it
in your GStreamer installation).

**basic-tutorial-7.c**

``` c
#include <gst/gst.h>

int main(int argc, char *argv[]) {
  GstElement *pipeline, *audio_source, *tee, *audio_queue, *audio_convert, *audio_resample, *audio_sink;
  GstElement *video_queue, *visual, *video_convert, *video_sink;
  GstBus *bus;
  GstMessage *msg;
  GstPad *tee_audio_pad, *tee_video_pad;
  GstPad *queue_audio_pad, *queue_video_pad;

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Create the elements */
  audio_source = gst_element_factory_make ("audiotestsrc", "audio_source");
  tee = gst_element_factory_make ("tee", "tee");
  audio_queue = gst_element_factory_make ("queue", "audio_queue");
  audio_convert = gst_element_factory_make ("audioconvert", "audio_convert");
  audio_resample = gst_element_factory_make ("audioresample", "audio_resample");
  audio_sink = gst_element_factory_make ("autoaudiosink", "audio_sink");
  video_queue = gst_element_factory_make ("queue", "video_queue");
  visual = gst_element_factory_make ("wavescope", "visual");
  video_convert = gst_element_factory_make ("videoconvert", "csp");
  video_sink = gst_element_factory_make ("autovideosink", "video_sink");

  /* Create the empty pipeline */
  pipeline = gst_pipeline_new ("test-pipeline");

  if (!pipeline || !audio_source || !tee || !audio_queue || !audio_convert || !audio_resample || !audio_sink ||
      !video_queue || !visual || !video_convert || !video_sink) {
    g_printerr ("Not all elements could be created.\n");
    return -1;
  }

  /* Configure elements */
  g_object_set (audio_source, "freq", 215.0f, NULL);
  g_object_set (visual, "shader", 0, "style", 1, NULL);

  /* Link all elements that can be automatically linked because they have "Always" pads */
  gst_bin_add_many (GST_BIN (pipeline), audio_source, tee, audio_queue, audio_convert, audio_resample, audio_sink,
      video_queue, visual, video_convert, video_sink, NULL);
  if (gst_element_link_many (audio_source, tee, NULL) != TRUE ||
      gst_element_link_many (audio_queue, audio_convert, audio_resample, audio_sink, NULL) != TRUE ||
      gst_element_link_many (video_queue, visual, video_convert, video_sink, NULL) != TRUE) {
    g_printerr ("Elements could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }

  /* Manually link the Tee, which has "Request" pads */
  tee_audio_pad = gst_element_get_request_pad (tee, "src_%u");
  g_print ("Obtained request pad %s for audio branch.\n", gst_pad_get_name (tee_audio_pad));
  queue_audio_pad = gst_element_get_static_pad (audio_queue, "sink");
  tee_video_pad = gst_element_get_request_pad (tee, "src_%u");
  g_print ("Obtained request pad %s for video branch.\n", gst_pad_get_name (tee_video_pad));
  queue_video_pad = gst_element_get_static_pad (video_queue, "sink");
  if (gst_pad_link (tee_audio_pad, queue_audio_pad) != GST_PAD_LINK_OK ||
      gst_pad_link (tee_video_pad, queue_video_pad) != GST_PAD_LINK_OK) {
    g_printerr ("Tee could not be linked.\n");
    gst_object_unref (pipeline);
    return -1;
  }
  gst_object_unref (queue_audio_pad);
  gst_object_unref (queue_video_pad);

  /* Start playing the pipeline */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);

  /* Wait until error or EOS */
  bus = gst_element_get_bus (pipeline);
  msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS);

  /* Release the request pads from the Tee, and unref them */
  gst_element_release_request_pad (tee, tee_audio_pad);
  gst_element_release_request_pad (tee, tee_video_pad);
  gst_object_unref (tee_audio_pad);
  gst_object_unref (tee_video_pad);

  /* Free resources */
  if (msg != NULL)
    gst_message_unref (msg);
  gst_object_unref (bus);
  gst_element_set_state (pipeline, GST_STATE_NULL);

  gst_object_unref (pipeline);
  return 0;
}
```

> ![Information](images/icons/emoticons/information.png)
> Need help?
>
> If you need help to compile this code, refer to the **Building the tutorials**  section for your platform: [Linux](installing/on-linux.md#InstallingonLinux-Build), [Mac OS X](installing/on-mac-osx.md#InstallingonMacOSX-Build) or [Windows](installing/on-windows.md#InstallingonWindows-Build), or use this specific command on Linux:
>
> ``gcc basic-tutorial-7.c -o basic-tutorial-7 `pkg-config --cflags --libs gstreamer-1.0` ``
>
>If you need help to run this code, refer to the **Running the tutorials** section for your platform: [Linux](installing/on-linux.md#InstallingonLinux-Run), [Mac OS X](installing/on-mac-osx.md#InstallingonMacOSX-Run) or [Windows](installing/on-windows.md#InstallingonWindows-Run).
>
> This tutorial plays an audible tone through the audio card and opens a window with a waveform representation of the tone. The waveform should be a sinusoid, but due to the refreshing of the window might not appear so.
>
> Required libraries: `gstreamer-1.0`

## Walkthrough

``` c
/* Create the elements */
audio_source = gst_element_factory_make ("audiotestsrc", "audio_source");
tee = gst_element_factory_make ("tee", "tee");
audio_queue = gst_element_factory_make ("queue", "audio_queue");
audio_convert = gst_element_factory_make ("audioconvert", "audio_convert");
  audio_resample = gst_element_factory_make ("audioresample", "audio_resample");
audio_sink = gst_element_factory_make ("autoaudiosink", "audio_sink");
video_queue = gst_element_factory_make ("queue", "video_queue");
visual = gst_element_factory_make ("wavescope", "visual");
video_convert = gst_element_factory_make ("videoconvert", "video_convert");
video_sink = gst_element_factory_make ("autovideosink", "video_sink");
```

All the elements in the above picture are instantiated here:

`audiotestsrc` produces a synthetic tone. `wavescope` consumes an audio
signal and renders a waveform as if it was an (admittedly cheap)
oscilloscope. We have already worked with the `autoaudiosink` and
`autovideosink`.

The conversion elements (`audioconvert`, `audioresample` and
`videoconvert`) are necessary to guarantee that the pipeline can be
linked. Indeed, the Capabilities of the audio and video sinks depend on
the hardware, and you do not know at design time if they will match the
Caps produced by the `audiotestsrc` and `wavescope`. If the Caps
matched, though, these elements act in “pass-through” mode and do not
modify the signal, having negligible impact on performance.

``` c
/* Configure elements */
g_object_set (audio_source, "freq", 215.0f, NULL);
g_object_set (visual, "shader", 0, "style", 1, NULL);
```

Small adjustments for better demonstration: The “freq” property of
`audiotestsrc` controls the frequency of the wave (215Hz makes the wave
appear almost stationary in the window), and this style and shader for
`wavescope` make the wave continuous. Use the `gst-inspect-1.0` tool
described in [Basic tutorial 10: GStreamer
tools](tutorials/basic/gstreamer-tools.md) to learn all
the properties of these
elements.

``` c
/* Link all elements that can be automatically linked because they have "Always" pads */
gst_bin_add_many (GST_BIN (pipeline), audio_source, tee, audio_queue, audio_convert, audio_sink,
    video_queue, visual, video_convert, video_sink, NULL);
if (gst_element_link_many (audio_source, tee, NULL) != TRUE ||
    gst_element_link_many (audio_queue, audio_convert, audio_sink, NULL) != TRUE ||
    gst_element_link_many (video_queue, visual, video_convert, video_sink, NULL) != TRUE) {
  g_printerr ("Elements could not be linked.\n");
  gst_object_unref (pipeline);
  return -1;
}
```

This code block adds all elements to the pipeline and then links the
ones that can be automatically linked (the ones with Always Pads, as the
comment says).

> ![Warning](images/icons/emoticons/warning.png)
> `gst_element_link_many()` can actually link elements with Request Pads. It internally requests the Pads so you do not have worry about the elements being linked having Always or Request Pads. Strange as it might seem, this is actually inconvenient, because you still need to release the requested Pads afterwards, and, if the Pad was requested automatically by `gst_element_link_many()`, it is easy to forget. Stay out of trouble by always requesting Request Pads manually, as shown in the next code block.

``` c
/* Manually link the Tee, which has "Request" pads */
tee_audio_pad = gst_element_get_request_pad (tee, "src_%u");
g_print ("Obtained request pad %s for audio branch.\n", gst_pad_get_name (tee_audio_pad));
queue_audio_pad = gst_element_get_static_pad (audio_queue, "sink");
tee_video_pad = gst_element_get_request_pad (tee, "src_%u");
g_print ("Obtained request pad %s for video branch.\n", gst_pad_get_name (tee_video_pad));
queue_video_pad = gst_element_get_static_pad (video_queue, "sink");
if (gst_pad_link (tee_audio_pad, queue_audio_pad) != GST_PAD_LINK_OK ||
    gst_pad_link (tee_video_pad, queue_video_pad) != GST_PAD_LINK_OK) {
  g_printerr ("Tee could not be linked.\n");
  gst_object_unref (pipeline);
  return -1;
}
gst_object_unref (queue_audio_pad);
gst_object_unref (queue_video_pad);
```

To link Request Pads, they need to be obtained by “requesting” them to
the element. An element might be able to produce different kinds of
Request Pads, so, when requesting them, the desired Pad Template name must be
provided.
In the documentation for the `tee` element we see that it has two pad
templates named “sink” (for its sink Pads) and “src_%u” (for the Request
Pads). We request two Pads from the tee (for the
audio and video branches) with `gst_element_get_request_pad()`.

We then obtain the Pads from the downstream elements to which these
Request Pads need to be linked. These are normal Always Pads, so we
obtain them with `gst_element_get_static_pad()`.

Finally, we link the pads with `gst_pad_link()`. This is the function
that `gst_element_link()` and `gst_element_link_many()` use internally.

The sink Pads we have obtained need to be released with
`gst_object_unref()`. The Request Pads will be released when we no
longer need them, at the end of the program.

We then set the pipeline to playing as usual, and wait until an error
message or an EOS is produced. The only thing left to so is cleanup the
requested Pads:

``` c
/* Release the request pads from the Tee, and unref them */
gst_element_release_request_pad (tee, tee_audio_pad);
gst_element_release_request_pad (tee, tee_video_pad);
gst_object_unref (tee_audio_pad);
gst_object_unref (tee_video_pad);
```

`gst_element_release_request_pad()` releases the pad from the `tee`, but
it still needs to be unreferenced (freed) with `gst_object_unref()`.

## Conclusion

 This tutorial has shown:

  - How to make parts of a pipeline run on a different thread by using
    `queue` elements.

  - What is a Request Pad and how to link elements with request pads,
    with `gst_element_get_request_pad()`, `gst_pad_link()` and
     `gst_element_release_request_pad()`.

  - How to have the same stream available in different branches by using
    `tee` elements.

The next tutorial builds on top of this one to show how data can be
manually injected into and extracted from a running pipeline.

It has been a pleasure having you here, and see you soon!
