PipeWire  0.3.36
Tutorial - Part 4: Playing a tone

Tutorial - Part 3: Forcing a roundtrip | Index | Tutorial - Part 5: Capturing video frames

In this tutorial we show how to use a stream to play a tone.

Let's take a look at the code before we break it down:

#include <math.h>
#define M_PI_M2 ( M_PI + M_PI )
#define DEFAULT_RATE 44100
#define DEFAULT_CHANNELS 2
#define DEFAULT_VOLUME 0.7
struct data {
struct pw_main_loop *loop;
struct pw_stream *stream;
double accumulator;
};
static void on_process(void *userdata)
{
struct data *data = userdata;
struct pw_buffer *b;
struct spa_buffer *buf;
int i, c, n_frames, stride;
int16_t *dst, val;
if ((b = pw_stream_dequeue_buffer(data->stream)) == NULL) {
pw_log_warn("out of buffers: %m");
return;
}
buf = b->buffer;
if ((dst = buf->datas[0].data) == NULL)
return;
stride = sizeof(int16_t) * DEFAULT_CHANNELS;
n_frames = buf->datas[0].maxsize / stride;
for (i = 0; i < n_frames; i++) {
data->accumulator += M_PI_M2 * 440 / DEFAULT_RATE;
if (data->accumulator >= M_PI_M2)
data->accumulator -= M_PI_M2;
val = sin(data->accumulator) * DEFAULT_VOLUME * 16767.f;
for (c = 0; c < DEFAULT_CHANNELS; c++)
*dst++ = val;
}
buf->datas[0].chunk->offset = 0;
buf->datas[0].chunk->stride = stride;
buf->datas[0].chunk->size = n_frames * stride;
}
static const struct pw_stream_events stream_events = {
.process = on_process,
};
int main(int argc, char *argv[])
{
struct data data = { 0, };
const struct spa_pod *params[1];
uint8_t buffer[1024];
pw_init(&argc, &argv);
data.loop = pw_main_loop_new(NULL);
"audio-src",
PW_KEY_MEDIA_CATEGORY, "Playback",
NULL),
&stream_events,
&data);
.channels = DEFAULT_CHANNELS,
.rate = DEFAULT_RATE ));
argc > 1 ? (uint32_t)atoi(argv[1]) : PW_ID_ANY,
params, 1);
return 0;
}
#define PW_ID_ANY
Definition: core.h:69
#define PW_KEY_MEDIA_TYPE
Media.
Definition: src/pipewire/keys.h:277
#define PW_KEY_MEDIA_ROLE
Role: Movie, Music, Camera, Screen, Communication, Game, Notification, DSP, Production,...
Definition: src/pipewire/keys.h:281
#define PW_KEY_MEDIA_CATEGORY
Media Category: Playback, Capture, Duplex, Monitor, Manager.
Definition: src/pipewire/keys.h:279
#define pw_log_warn(...)
Definition: src/pipewire/log.h:87
struct pw_main_loop * pw_main_loop_new(const struct spa_dict *props)
Create a new main loop.
Definition: main-loop.c:86
void pw_main_loop_destroy(struct pw_main_loop *loop)
Destroy a loop.
Definition: main-loop.c:96
int pw_main_loop_run(struct pw_main_loop *loop)
Run a main loop.
Definition: main-loop.c:145
struct pw_loop * pw_main_loop_get_loop(struct pw_main_loop *loop)
Get the loop implementation.
Definition: main-loop.c:119
void pw_init(int *argc, char **argv[])
Initialize PipeWire.
Definition: pipewire.c:533
#define PW_DIRECTION_OUTPUT
Definition: port.h:58
struct pw_properties * pw_properties_new(const char *key,...) 1
Make a new properties object.
Definition: properties.c:98
int pw_stream_connect(struct pw_stream *stream, enum pw_direction direction, uint32_t target_id, enum pw_stream_flags flags, const struct spa_pod **params, uint32_t n_params)
Connect a stream for input or output on port_path.
Definition: stream.c:1603
struct pw_stream * pw_stream_new_simple(struct pw_loop *loop, const char *name, struct pw_properties *props, const struct pw_stream_events *events, void *data)
Definition: stream.c:1357
struct pw_buffer * pw_stream_dequeue_buffer(struct pw_stream *stream)
Get a buffer that can be filled for playback streams or consumed for capture streams.
Definition: stream.c:2020
int pw_stream_queue_buffer(struct pw_stream *stream, struct pw_buffer *buffer)
Submit a buffer for playback or recycle a buffer for capture.
Definition: stream.c:2047
void pw_stream_destroy(struct pw_stream *stream)
Destroy a stream.
Definition: stream.c:1419
@ PW_STREAM_FLAG_MAP_BUFFERS
mmap the buffers except DmaBuf
Definition: stream.h:254
@ PW_STREAM_FLAG_AUTOCONNECT
try to automatically connect this stream
Definition: stream.h:249
@ PW_STREAM_FLAG_RT_PROCESS
call process from the realtime thread.
Definition: stream.h:256
#define SPA_AUDIO_INFO_RAW_INIT(...)
Definition: audio/raw.h:296
struct spa_pod * spa_format_audio_raw_build(struct spa_pod_builder *builder, uint32_t id, struct spa_audio_info_raw *info)
Definition: audio/format-utils.h:103
@ SPA_PARAM_EnumFormat
available formats as SPA_TYPE_OBJECT_Format
Definition: param.h:47
@ SPA_AUDIO_FORMAT_S16
Definition: audio/raw.h:108
#define SPA_POD_BUILDER_INIT(buffer, size)
Definition: builder.h:71
int main(int argc, char *argv[])
Definition: media-session.c:2433
#define DEFAULT_CHANNELS
Definition: module-example-sink.c:58
#define DEFAULT_RATE
Definition: module-example-sink.c:57
#define PW_VERSION_STREAM_EVENTS
Definition: stream.h:212
Definition: filter.c:59
user data to add to an object
Definition: filter.c:75
Definition: stream.h:175
struct spa_buffer * buffer
the spa buffer
Definition: stream.h:176
Events for a stream.
Definition: stream.h:211
A Buffer.
Definition: buffer/buffer.h:93
struct spa_data * datas
array of data members
Definition: buffer/buffer.h:97
int32_t stride
stride of valid data
Definition: buffer/buffer.h:63
uint32_t size
size of valid data.
Definition: buffer/buffer.h:61
uint32_t offset
offset of valid data.
Definition: buffer/buffer.h:58
struct spa_chunk * chunk
valid chunk of memory
Definition: buffer/buffer.h:89
void * data
optional data pointer
Definition: buffer/buffer.h:88
uint32_t maxsize
max size of data
Definition: buffer/buffer.h:87
Definition: builder.h:63
Definition: pod/pod.h:50
Definition: stream.c:97

Save as tutorial4.c and compile with:

gcc -Wall tutorial4.c -o tutorial4 -lm $(pkg-config --cflags --libs libpipewire-0.3)

We start with the usual boilerplate, pw_init() and a pw_main_loop_new(). We're going to store our objects in a structure so that we can pass them around in callbacks later.

struct data {
struct pw_main_loop *loop;
struct pw_stream *stream;
double accumulator;
};
int main(int argc, char *argv[])
{
struct data data = { 0, };
pw_init(&argc, &argv);
data.loop = pw_main_loop_new(NULL);

Next we create a stream object. It takes the mainloop as first argument and a stream name as the second. Next we provide some properties for the stream and a callback + data.

"audio-src",
PW_KEY_MEDIA_CATEGORY, "Playback",
NULL),
&stream_events,
&data);

We are using pw_stream_new_simple() but there is also a pw_stream_new() that takes an existing struct pw_core as the first argument and that requires you to add the event handle manually, for more control. The pw_stream_new_simple() is, as the name implies, easier to use because it creates a struct pw_context and struct pw_core automatically.

In the properties we need to give as much information about the stream as we can so that the session manager can make good decisions about how and where to route this stream. There are 3 important properties to configure:

PW_KEY_MEDIA_TYPE The media type, like Audio, Video, Midi pw_KEY_MEDIA_CATEGORY The category, like Playback, Capture, Duplex, Monitor PW_KEY_MEDIA_ROLE The media role, like Movie, Music, Camera, Screen, Communication, Game, Notification, DSP, Production, Accessibility, Test

The properties are owned by the stream and freed when the stream is destroyed later.

This is the event structure that we use to listen for events:

static const struct pw_stream_events stream_events = {
.process = on_process,
};

We are for the moment only interested now in the process event. This event is called whenever we need to produce more data. We'll see how that function is implemented but first we need to setup the format of the stream:

const struct spa_pod *params[1];
uint8_t buffer[1024];
#define DEFAULT_RATE 44100
#define DEFAULT_CHANNELS 2
.channels = DEFAULT_CHANNELS,
.rate = DEFAULT_RATE ));

This is using a struct spa_pod_builder to make a struct spa_pod * object in the buffer array on the stack. The parameter is of type SPA_PARAM_EnumFormat which means that it enumerates the possible formats for this stream. We have only one, a Signed 16 bit stereo format at 44.1KHz.

We use spa_format_audio_raw_build() which is a helper function to make the param with the builder. See SPA POD for more information about how to make these POD objects.

Now we're ready to connect the stream and run the main loop:

To connect we specify that we have a PW_DIRECTION_OUTPUT stream. PW_ID_ANY means that we are ok with connecting to any consumer. Next we set some flags:

PW_STREAM_FLAG_AUTOCONNECT automatically connect this stream. This instructs the session manager to link us to some consumer. PW_STREAM_FLAG_MAP_BUFFERS mmap the buffers for us so we can access the memory. If you don't set these flags you have either work with the fd or mmap yourself. PW_STREAM_FLAG_RT_PROCESS Run the process function in the realtime thread. Only use this if the process function only uses functions that are realtime safe, this means no allocation or file access or any locking.

And last we pass the extra parameters for our stream. Here we only have the allowed formats (SPA_PARAM_EnumFormat).

Running the mainloop will then start processing and will result in our process callback to be called. Let's have a look at that function now.

The main program flow of the process function is:

pw_stream_dequeue_buffer() to obtain a buffer to write into. Get pointers in buffer memory to write to write data into buffer adjust buffer with number of written bytes, offset, stride, pw_stream_queue_buffer() to queue the buffer for playback.

static void on_process(void *userdata)
{
struct data *data = userdata;
struct pw_buffer *b;
struct spa_buffer *buf;
int i, c, n_frames, stride;
int16_t *dst, val;
if ((b = pw_stream_dequeue_buffer(data->stream)) == NULL) {
pw_log_warn("out of buffers: %m");
return;
}
buf = b->buffer;
if ((dst = buf->datas[0].data) == NULL)
return;
stride = sizeof(int16_t) * DEFAULT_CHANNELS;
n_frames = buf->datas[0].maxsize / stride;
for (i = 0; i < n_frames; i++) {
data->accumulator += M_PI_M2 * 440 / DEFAULT_RATE;
if (data->accumulator >= M_PI_M2)
data->accumulator -= M_PI_M2;
val = sin(data->accumulator) * DEFAULT_VOLUME * 16767.f;
for (c = 0; c < DEFAULT_CHANNELS; c++)
*dst++ = val;
}
buf->datas[0].chunk->offset = 0;
buf->datas[0].chunk->stride = stride;
buf->datas[0].chunk->size = n_frames * stride;
}

Check out the docs for SPA Buffers for more information about how to work with buffers.

Try to change the number of channels, samplerate or format; the stream will automatically convert to the format on the server.

Tutorial - Part 3: Forcing a roundtrip | Index | Tutorial - Part 5: Capturing video frames