2020-06-13 19:26:21 +00:00
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[[previous]](tutorial3.md) [[index]](tutorial-index.md) [[next]](tutorial5.md)
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# Playing a tone (Tutorial 4)
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In this tutorial we show how to use a stream to play a tone.
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Let's take a look at the code before we break it down:
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```c
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#include <math.h>
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#include <spa/param/audio/format-utils.h>
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#include <pipewire/pipewire.h>
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#define M_PI_M2 ( M_PI + M_PI )
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#define DEFAULT_RATE 44100
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#define DEFAULT_CHANNELS 2
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#define DEFAULT_VOLUME 0.7
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struct data {
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struct pw_main_loop *loop;
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struct pw_stream *stream;
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double accumulator;
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};
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static void on_process(void *userdata)
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{
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struct data *data = userdata;
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struct pw_buffer *b;
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struct spa_buffer *buf;
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int i, c, n_frames, stride;
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int16_t *dst, val;
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if ((b = pw_stream_dequeue_buffer(data->stream)) == NULL) {
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pw_log_warn("out of buffers: %m");
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return;
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}
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buf = b->buffer;
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if ((dst = buf->datas[0].data) == NULL)
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return;
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stride = sizeof(int16_t) * DEFAULT_CHANNELS;
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n_frames = buf->datas[0].maxsize / stride;
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for (i = 0; i < n_frames; i++) {
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data->accumulator += M_PI_M2 * 440 / DEFAULT_RATE;
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if (data->accumulator >= M_PI_M2)
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data->accumulator -= M_PI_M2;
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val = sin(data->accumulator) * DEFAULT_VOLUME * 16767.f;
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for (c = 0; c < DEFAULT_CHANNELS; c++)
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*dst++ = val;
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}
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buf->datas[0].chunk->offset = 0;
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buf->datas[0].chunk->stride = stride;
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buf->datas[0].chunk->size = n_frames * stride;
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pw_stream_queue_buffer(data->stream, b);
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}
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static const struct pw_stream_events stream_events = {
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PW_VERSION_STREAM_EVENTS,
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.process = on_process,
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};
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int main(int argc, char *argv[])
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{
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struct data data = { 0, };
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const struct spa_pod *params[1];
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uint8_t buffer[1024];
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struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buffer, sizeof(buffer));
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pw_init(&argc, &argv);
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data.loop = pw_main_loop_new(NULL);
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data.stream = pw_stream_new_simple(
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pw_main_loop_get_loop(data.loop),
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"audio-src",
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pw_properties_new(
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PW_KEY_MEDIA_TYPE, "Audio",
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PW_KEY_MEDIA_CATEGORY, "Playback",
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PW_KEY_MEDIA_ROLE, "Music",
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NULL),
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&stream_events,
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&data);
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params[0] = spa_format_audio_raw_build(&b, SPA_PARAM_EnumFormat,
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&SPA_AUDIO_INFO_RAW_INIT(
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.format = SPA_AUDIO_FORMAT_S16,
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.channels = DEFAULT_CHANNELS,
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.rate = DEFAULT_RATE ));
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pw_stream_connect(data.stream,
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PW_DIRECTION_OUTPUT,
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argc > 1 ? (uint32_t)atoi(argv[1]) : PW_ID_ANY,
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PW_STREAM_FLAG_AUTOCONNECT |
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PW_STREAM_FLAG_MAP_BUFFERS |
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PW_STREAM_FLAG_RT_PROCESS,
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params, 1);
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pw_main_loop_run(data.loop);
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pw_stream_destroy(data.stream);
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pw_main_loop_destroy(data.loop);
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return 0;
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}
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```
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Save as tutorial4.c and compile with:
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```
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gcc -Wall tutorial4.c -o tutorial4 -lm $(pkg-config --cflags --libs libpipewire-0.3)
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```
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We start with the usual boilerplate, `pw_init()` and a `pw_main_loop_new()`.
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We're going to store our objects in a structure so that we can pass them
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around in callbacks later.
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```c
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struct data {
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struct pw_main_loop *loop;
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struct pw_stream *stream;
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double accumulator;
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};
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int main(int argc, char *argv[])
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{
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struct data data = { 0, };
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pw_init(&argc, &argv);
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data.loop = pw_main_loop_new(NULL);
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```
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Next we create a stream object. It takes the mainloop as first argument and
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a stream name as the second. Next we provide some properties for the stream
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and a callback + data.
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```c
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data.stream = pw_stream_new_simple(
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pw_main_loop_get_loop(data.loop),
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"audio-src",
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pw_properties_new(
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PW_KEY_MEDIA_TYPE, "Audio",
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PW_KEY_MEDIA_CATEGORY, "Playback",
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PW_KEY_MEDIA_ROLE, "Music",
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NULL),
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&stream_events,
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&data);
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```
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2020-09-14 18:00:59 +00:00
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We are using `pw_stream_new_simple()` but there is also a `pw_stream_new()` that
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2020-07-22 18:54:06 +00:00
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takes an existing `struct pw_core` as the first argument and that requires you
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2020-06-13 19:26:21 +00:00
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to add the event handle manually, for more control. The `pw_stream_new_simple()`
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is, as the name implies, easier to use because it creates a `struct pw_context`
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and `struct pw_core` automatically.
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In the properties we need to give as much information about the stream as we
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can so that the session manager can make good decisions about how and where
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to route this stream. There are 3 important properties to configure:
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* `PW_KEY_MEDIA_TYPE` The media type, like Audio, Video, Midi
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* `pw_KEY_MEDIA_CATEGORY` The category, like Playback, Capture, Duplex, Monitor
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* `PW_KEY_MEDIA_ROLE` The media role, like Movie, Music, Camera, Screen,
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Communication, Game, Notification, DSP,
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Production, Accessibility, Test
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The properties are owned by the stream and freed when the stream is destroyed
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later.
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This is the event structure that we use to listen for events:
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```c
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static const struct pw_stream_events stream_events = {
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PW_VERSION_STREAM_EVENTS,
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.process = on_process,
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};
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```
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We are for the moment only interested now in the `process` event. This event
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2020-06-13 19:31:32 +00:00
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is called whenever we need to produce more data. We'll see how that function
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is implemented but first we need to setup the format of the stream:
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```c
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const struct spa_pod *params[1];
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uint8_t buffer[1024];
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struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buffer, sizeof(buffer));
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#define DEFAULT_RATE 44100
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#define DEFAULT_CHANNELS 2
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params[0] = spa_format_audio_raw_build(&b, SPA_PARAM_EnumFormat,
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&SPA_AUDIO_INFO_RAW_INIT(
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.format = SPA_AUDIO_FORMAT_S16,
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.channels = DEFAULT_CHANNELS,
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.rate = DEFAULT_RATE ));
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```
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This is using a `struct spa_pod_builder` to make a `struct spa_pod *` object
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in the buffer array on the stack. The parameter is of type `SPA_PARAM_EnumFormat`
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which means that it enumerates the possible formats for this stream. We have
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only one, a Signed 16 bit stereo format at 44.1KHz.
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We use `spa_format_audio_raw_build()` which is a helper function to make the param
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with the builder. See [SPA POD](spa/pod.md) for more information about how to
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make these POD objects.
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Now we're ready to connect the stream and run the main loop:
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```c
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pw_stream_connect(data.stream,
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PW_DIRECTION_OUTPUT,
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PW_ID_ANY,
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PW_STREAM_FLAG_AUTOCONNECT |
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PW_STREAM_FLAG_MAP_BUFFERS |
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PW_STREAM_FLAG_RT_PROCESS,
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params, 1);
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pw_main_loop_run(data.loop);
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```
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2020-09-14 18:00:59 +00:00
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To connect we specify that we have a `PW_DIRECTION_OUTPUT` stream. `PW_ID_ANY`
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2021-04-28 18:29:44 +00:00
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means that we are ok with connecting to any consumer. Next we set some flags:
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* `PW_STREAM_FLAG_AUTOCONNECT` automatically connect this stream. This instructs
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the session manager to link us to some consumer.
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* `PW_STREAM_FLAG_MAP_BUFFERS` mmap the buffers for us so we can access the
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memory. If you don't set these flags you have
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either work with the fd or mmap yourself.
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* `PW_STREAM_FLAG_RT_PROCESS` Run the process function in the realtime thread.
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Only use this if the process function only
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uses functions that are realtime safe, this means
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no allocation or file access or any locking.
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2020-06-13 19:31:32 +00:00
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And last we pass the extra parameters for our stream. Here we only have the
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allowed formats (`SPA_PARAM_EnumFormat`).
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Running the mainloop will then start processing and will result in our
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`process` callback to be called. Let's have a look at that function now.
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The main program flow of the process function is:
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* `pw_stream_dequeue_buffer()` to obtain a buffer to write into.
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* Get pointers in buffer memory to write to
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* write data into buffer
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* adjust buffer with number of written bytes, offset, stride,
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* `pw_stream_queue_buffer()` to queue the buffer for playback.
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```c
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static void on_process(void *userdata)
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{
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struct data *data = userdata;
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struct pw_buffer *b;
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struct spa_buffer *buf;
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int i, c, n_frames, stride;
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int16_t *dst, val;
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if ((b = pw_stream_dequeue_buffer(data->stream)) == NULL) {
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pw_log_warn("out of buffers: %m");
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return;
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}
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buf = b->buffer;
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if ((dst = buf->datas[0].data) == NULL)
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return;
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stride = sizeof(int16_t) * DEFAULT_CHANNELS;
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n_frames = buf->datas[0].maxsize / stride;
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for (i = 0; i < n_frames; i++) {
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data->accumulator += M_PI_M2 * 440 / DEFAULT_RATE;
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if (data->accumulator >= M_PI_M2)
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data->accumulator -= M_PI_M2;
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val = sin(data->accumulator) * DEFAULT_VOLUME * 16767.f;
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for (c = 0; c < DEFAULT_CHANNELS; c++)
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*dst++ = val;
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}
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buf->datas[0].chunk->offset = 0;
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buf->datas[0].chunk->stride = stride;
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buf->datas[0].chunk->size = n_frames * stride;
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pw_stream_queue_buffer(data->stream, b);
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}
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```
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Check out the docs for [buffers](spa/buffer.md) for more information
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about how to work with buffers.
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Try to change the number of channels, samplerate or format; the stream
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will automatically convert to the format on the server.
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[[previous]](tutorial3.md) [[index]](tutorial-index.md) [[next]](tutorial5.md)
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