DESCRIPTION

This tutorial descipes some basics with working with libroar. We will create a simple application that can play a file and one that can play some sines. A lot of other examples can be found in RoarAudio's sources in the roarclients directory.

This tutorial will cover some basics of the so called VS API. The VS API is a abstract layer ontop of the normal API. It is designed to be simple yet powerful. The VS API is all you need for most applications. If you need more control over what you do you must use the normal. If you need only a little of those extra power you can mix VS API and normal API.

PLAYING A FILE

Playing back a file is a easy task with libroar. The VS API has some special support to play back files in a very simple way. This is shown here.

First of all we need to include the needed header files:
 #include <roaraudio.h>  /* libroar */

This main header already includes all we need to use the VS API.

Now we can start our main(). We need to declare a object for the VS API as it is object oriented. This object is used to interact with the server and send all audio data to it:
 roar_vs_t * vss;
 int err; /* see later */

Next we need to open the connection to the server. The most simple function to do this is roar_vs_new_from_file(3) if we are going to play a file. It takes the following arguments:

server address

This is the address of the server. In general case This should be set to NULL.

program name

This is the name of our program. This should be set to some name the user will recognize like "some App", "some Game". It should not contain the filename of the process like "/usr/bin/someapp.bin".

file name

This is the name of the file we want to play. In fact this is a URL. VS API uses so called DSTR API to open files. DSTR API supports local files as well as for example HTTP. Examples include: "somefile.ogg", "file:///data/bla.wav", "http://radiostation.org:8000/bla.ogg".

error var

This is a pointer to a int used to store the error value in case of error. This can be set to NULL but should not. The function roar_vs_strerr(3) can be used to get a lion readable string of the error.

Our call to roar_vs_new_from_file(3) will look like this:
 vss = roar_vs_new_from_file(NULL, "some App", "somefile.ogg", &err);
 if ( vss == NULL ) {
  roar_vio_printf(roar_stderr, "Error: Can not connect to server: %s, roar_vs_strerr(err));
  return 1;
 }

Next we need to continuously feed in the data so the server can do the playback. Again most simple way is to use roar_vs_run(3).

 if ( roar_vs_run(vss, &err) == -1 ) {
  roar_vio_printf(roar_stderr, "Error: can not loop: %s, roar_vs_strerr(err));
 }

This will block until all of the file is played.

After it returned must close the VS object. This should be done directly after roar_vs_run(3) returned. This is done this way:
 if ( roar_vs_close(vss, ROAR_VS_FALSE, &err) == -1 ) {
  roar_vio_printf(roar_stderr, "Error: Can not close connection to server: %s, roar_vs_strerr(err));
  return 1;
 }

After adding some standard main() construct we should have something like this:
 //vsfile.c:

 #include <roaraudio.h>

 int main (void) {
  roar_vs_t * vss;
  int err; /* see later */

  vss = roar_vs_new_from_file(NULL, "some App", "somefile.ogg", &err);
  if ( vss == NULL ) {
   roar_vio_printf(roar_stderr, "Error: Can not connect to server: %s, roar_vs_strerr(err));
   return 1;
  }

  if ( roar_vs_run(vss, &err) == -1 ) {
   roar_vio_printf(roar_stderr, "Error: can not loop: %s, roar_vs_strerr(err));
  }

  if ( roar_vs_close(vss, ROAR_VS_FALSE, &err) == -1 ) {
   roar_vio_printf(roar_stderr, "Error: Can not close connection to server: %s, roar_vs_strerr(err));
   return 1;
  }

  return 0;
 }

 //ll

To compile and link we can use a command like this one:
 cc -o vsfile vsfile.c `roar-config --libs --cflags`

The tool roar-config(1) will keep care for us about all flags needed for libroar.

PLAYING A SINE

Now we want to write a application playing a sine for some secs. We start the same way by including the correct header files:
 #include <math.h>       /* sin() */
 #include <roaraudio.h>  /* libroar */

After that we need some basic variables with data about the audio we want to play back:
 int rate     = ROAR_RATE_DEFAULT;
 int bits     = 16;
 int channels =  1; /* mono */

Next we need to set the 'codec'. The codec is how the data is encoded. We want PCM as signed ints in the native byte order of our machine.
 int codec    = ROAR_CODEC_DEFAULT;

Now we need to store the frequency of our sine:
 float freq = 523.2;            /* middle C */
 float step = M_PI*2*freq/rate; /* how much time per sample we have to encode ... */

In addition we need some variables to store the current time and the length of time sine:
 float t      = 0; /* current time */
 float length = 5; /* 5 sec */

Next we need the buffer to hold the data as well as a varible used to go thru the buffer on generation of data.
 int16_t out[1024];
 int i;

last we need the VS object again as well as our error var:
 roar_vs_t * vss;
 int err;

This time we open the connection to the server using roar_vs_new_playback(3). It is similar to roar_vs_new_from_file(3) but takes some other options:

server address

Same as above.

program name

same as above.

sample rate

The number of audio frames per sec.

channels

The number of samples (one per channel) per audio frame.

codec

The codec to be used. This is one of ROAR_CODEC_*. In our case we use ROAR_CODEC_DEFAULT which is signed PCM in CPU native format.

bits

The number of bits per sample.

error var

same as above.

The call looks like this:
 vss = roar_vs_new_playback(NULL, "vssin", rate, channels, codec, bits, &err);
 if ( vss == NULL ) {
  roar_vio_printf(roar_stderr, "Error: Can not connect to server: %s, roar_vs_strerr(err));
  return 1;
 }

Now we want to loop for length seconds:
 while (t < 2*M_PI*freq*length) {
 }

In this loop we need to calculate our samples:
  for (i = 0; i < (sizeof(out)/sizeof(*out)); i++) {
   out[i] = 32767.f*sin(t);
   t += step;
  }

The sine is multiplyed by 32767 as our amplitude range for 16 bit signed int is from -32768 to +32767.

After we have our current data in out we want to write them to the server:
  if ( roar_vs_write(vss, out, sizeof(out), &err) == -1 ) {
   roar_vio_printf(roar_stderr, "Error: Can not write audio data to server: %s, roar_vs_strerr(err));
   break;
  }

NOTE: In a real application you may want to check the return value for short writes: Those are writes shorter than the requested amount of data to be written. If you got any short writes you should try to rewrite the rest of your buffer later. This is not a error case.

After we are finished with our main loop we have to close the connection to the server. This is done by roar_vs_close(3) as we already done in the file playback example:
 if ( roar_vs_close(vss, ROAR_VS_FALSE, &err) == -1 ) {
  roar_vio_printf(roar_stderr, "Error: Can not close connection to server: %s, roar_vs_strerr(err));
  return 1;
 }

After adding some standard main() construct we should have something like this:
 //vssin.c:

 #include <roaraudio.h>
 #include <math.h>

 int main (void) {
  roar_vs_t * vss;
  int rate     = ROAR_RATE_DEFAULT;
  int bits     = 16;
  int channels =  1; /* mono */
  int codec    = ROAR_CODEC_DEFAULT;
  float freq = 523.2;            /* middle C */
  float step = M_PI*2*freq/rate; /* how much time per sample we have to encode ... */
  float t      = 0; /* current time */
  float length = 5; /* 5 sec */
  int16_t out[1024];
  size_t i;
  int err;

  vss = roar_vs_new_playback(NULL, "vssin", rate, channels, codec, bits, &err);
  if ( vss == NULL ) {
   roar_vio_printf(roar_stderr, "Error: Can not connect to server: %s, roar_vs_strerr(err));
   return 1;
  }

  while (t < 2*M_PI*freq*length) {
   for (i = 0; i < (sizeof(out)/sizeof(*out)); i++) {
    out[i] = 32768.f*sin(t);
    t += step;
   }

   if ( roar_vs_write(vss, out, sizeof(out), &err) == -1 ) {
    roar_vio_printf(roar_stderr, "Error: Can not write audio data to server: %s, roar_vs_strerr(err));
    break;
   }
  }

  if ( roar_vs_close(vss, ROAR_VS_FALSE, &err) == -1 ) {
   roar_vio_printf(roar_stderr, "Error: Can not close connection to server: %s, roar_vs_strerr(err));
   return 1;
  }

  return 0;
 }

 //ll

To compile and link we can use a command like this one:
 cc -o vssin vssin.c -lm `roar-config --libs --cflags`

We need to use -lm to link the math library for sin(). The tool roar-config(1) will keep care for us about all flags needed for libroar.

Now we should have a working binary vssin playing a sin() for 5 sec.

Happy hacking!