PipeWire  0.3.30
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;
}

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

spa_data::maxsize
uint32_t maxsize
max size of data
Definition: buffer/buffer.h:87
pw_stream_new_simple
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:1338
format-utils.h
pw_buffer
Definition: stream.h:175
pw_init
void pw_init(int *argc, char **argv[])
Initialize PipeWire.
Definition: pipewire.c:479
PW_STREAM_FLAG_RT_PROCESS
@ PW_STREAM_FLAG_RT_PROCESS
call process from the realtime thread.
Definition: stream.h:257
data
Definition: filter.c:75
spa_chunk::stride
int32_t stride
stride of valid data
Definition: buffer/buffer.h:63
spa_format_audio_raw_build
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:72
PW_VERSION_STREAM_EVENTS
#define PW_VERSION_STREAM_EVENTS
Definition: stream.h:212
pw_main_loop_new
struct pw_main_loop * pw_main_loop_new(const struct spa_dict *props)
Create a new main loop.
Definition: main-loop.c:86
PW_STREAM_FLAG_AUTOCONNECT
@ PW_STREAM_FLAG_AUTOCONNECT
try to automatically connect this stream
Definition: stream.h:250
SPA_POD_BUILDER_INIT
#define SPA_POD_BUILDER_INIT(buffer, size)
Definition: builder.h:71
stream
Definition: stream.c:97
spa_pod
Definition: pod/pod.h:50
PW_KEY_MEDIA_CATEGORY
#define PW_KEY_MEDIA_CATEGORY
Media Category: Playback, Capture, Duplex, Monitor, Manager.
Definition: src/pipewire/keys.h:262
pw_main_loop_get_loop
struct pw_loop * pw_main_loop_get_loop(struct pw_main_loop *loop)
Get the loop implementation.
Definition: main-loop.c:119
spa_chunk::offset
uint32_t offset
offset of valid data.
Definition: buffer/buffer.h:58
PW_DIRECTION_OUTPUT
#define PW_DIRECTION_OUTPUT
Definition: port.h:58
pw_stream_dequeue_buffer
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:2007
pw_main_loop_run
int pw_main_loop_run(struct pw_main_loop *loop)
Run a main loop.
Definition: main-loop.c:145
pw_properties_new
struct pw_properties * pw_properties_new(const char *key,...) SPA_SENTINEL
Make a new properties object.
Definition: properties.c:98
spa_buffer
A Buffer.
Definition: buffer/buffer.h:93
pw_stream_destroy
void pw_stream_destroy(struct pw_stream *stream)
Destroy a stream.
Definition: stream.c:1400
pw_stream_connect
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:1583
buffer
Definition: filter.c:59
spa_data::chunk
struct spa_chunk * chunk
valid chunk of memory
Definition: buffer/buffer.h:89
SPA_PARAM_EnumFormat
@ SPA_PARAM_EnumFormat
available formats as SPA_TYPE_OBJECT_Format
Definition: param.h:47
SPA_AUDIO_INFO_RAW_INIT
#define SPA_AUDIO_INFO_RAW_INIT(...)
Definition: audio/raw.h:225
SPA_AUDIO_FORMAT_S16
@ SPA_AUDIO_FORMAT_S16
Definition: audio/raw.h:105
PW_ID_ANY
#define PW_ID_ANY
Definition: core.h:64
spa_chunk::size
uint32_t size
size of valid data.
Definition: buffer/buffer.h:61
PW_KEY_MEDIA_ROLE
#define PW_KEY_MEDIA_ROLE
Role: Movie, Music, Camera, Screen, Communication, Game, Notification, DSP, Production,...
Definition: src/pipewire/keys.h:264
pw_log_warn
#define pw_log_warn(...)
Definition: src/pipewire/log.h:87
spa_buffer::datas
struct spa_data * datas
array of data members
Definition: buffer/buffer.h:97
pw_buffer::buffer
struct spa_buffer * buffer
the spa buffer
Definition: stream.h:176
spa_data::data
void * data
optional data pointer
Definition: buffer/buffer.h:88
pipewire.h
pw_stream_queue_buffer
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:2034
PW_STREAM_FLAG_MAP_BUFFERS
@ PW_STREAM_FLAG_MAP_BUFFERS
mmap the buffers except DmaBuf
Definition: stream.h:255
PW_KEY_MEDIA_TYPE
#define PW_KEY_MEDIA_TYPE
Media.
Definition: src/pipewire/keys.h:260
spa_pod_builder
Definition: builder.h:63
pw_stream_events
Events for a stream.
Definition: stream.h:211
pw_main_loop_destroy
void pw_main_loop_destroy(struct pw_main_loop *loop)
Destroy a loop.
Definition: main-loop.c:96