Confused about Passing user data to PortAudio Callbacks - c

Question

This is my first post here and I'm fairly new to programming and especially with C. A couple weeks ago I started working through the Audio Programming Book(MIT press) and have been expand on some examples to try to understand things further.
I think my question lies with how I'm trying to pass data (retrieved from the user in an initialization function) to a PortAudio callback. I feel like what I've done isn't that different from the examples (both from the book and PortAudio's examples like paex_sine.c), but for some reason I can't my code to work and I've been banging my head against a wall trying to understand why. I've tried searching pretty extensively for solutions or example code to study, but I kind of don't know what I don't know, so that hasn't returned much.
How do I get user data into the callback?
Am I just not understanding how pointers and structs work and trying to force them to do things they don't want to?
Or, am I just overlooking something really obvious?
The following code either gives a really high pitched output, short high pitched blips, or no (audible) output:
#include <stdio.h>
#include <math.h>
#include "portaudio.h"
#define FRAME_BLOCK_LEN 64
#define SAMPLING_RATE 44100
#define TWO_PI (3.14159265f * 2.0f)
PaStream *audioStream;
double si = 0;
typedef struct
{
float frequency;
float phase;
}
paTestData;
int audio_callback (const void *inputBuffer, void *outputBuffer,
unsigned long framesPerBuffer,
const PaStreamCallbackTimeInfo* timeinfo,
PaStreamCallbackFlags statusFlags,
void *userData )
{
paTestData *data = (paTestData*)userData;
float *out = (float*)outputBuffer;
unsigned long i;
// data->frequency = 400;
for(i = 0; i < framesPerBuffer; i++){
si = TWO_PI * data->frequency / SAMPLING_RATE; // calculate sampling-incr
*out++ = sin(data->phase);
*out++ = sin(data->phase);
data->phase += si; // add sampling-incr to phase
}
return paContinue;
}
void init_stuff()
{
float frequency;
int i;
PaStreamParameters outputParameters;
paTestData data;
printf("type the modulator frequency in Hz: ");
scanf("%f", &data.frequency); // get modulator frequency
printf("you chose data.frequency %.2f\n",data.frequency);
data.phase = 0.0;
printf("initializing Portaudio. Please wait...\n");
Pa_Initialize(); // initialize Portaudio
outputParameters.device = Pa_GetDefaultOutputDevice(); /* default output device */
outputParameters.channelCount = 2; /* stereo output */
outputParameters.sampleFormat = paFloat32; /* 32 bit floating point output */
outputParameters.suggestedLatency = Pa_GetDeviceInfo( outputParameters.device )->defaultLowOutputLatency;
outputParameters.hostApiSpecificStreamInfo = NULL;
Pa_OpenStream( // open paStream object
&audioStream, // portaudio stream object
NULL, // input params
&outputParameters, // output params
SAMPLING_RATE, // SampleRate
FRAME_BLOCK_LEN, // frames per buffer
paNoFlag, // set no Flag
audio_callback, // callbak function address
&data ); // user data
Pa_StartStream(audioStream); // start the callback mechanism
printf("running... press space bar and enter to exit\n");
}
void terminate_stuff()
{
Pa_StopStream(audioStream); // stop callback mechanism
Pa_CloseStream(audioStream); // destroy audio stream object
Pa_Terminate(); // terminate portaudio
}
int main(void)
{
init_stuff();
while(getchar() != ' ') Pa_Sleep(100);
terminate_stuff();
return 0;
}
Uncommenting data->frequency = 400; at least plays a 400hz sine wave, but that ignores any user input done in init_stuff()
If I put a printf("%f\n",data->frequency); inside the callback, it prints 0.000000 or something like -146730090609497866240.000000.
It's pretty unpredictable, and this really makes me think it's pointer related.
My goal for this code is to eventually incorporate envelope generators to change the pitch and possibly incorporate wavetable oscillators so I'm not calculating sin(x) for every iteration.
I can get envelopes and wavetables to work while using a blocking API like portsf that's used in the book, but trying to adapt any of that code from earlier chapters to use PortAudio callbacks is turning my brain to mush.
Thanks so much!

Answer 1

The problem you're having with your callback data is that it goes out of scope and memory is deallocated as soon as init_stuff finishes execution.
You should allocate memory for your callback data using malloc or new and passing the pointer to it for the callback.
For example:
void init_stuff()
{
float frequency;
int i;
PaStreamParameters outputParameters;
paTestData *data = (paTestData *) malloc(sizeof(paTestData));
printf("type the modulator frequency in Hz: ");
scanf("%f", &(data->frequency)); // get modulator frequency
printf("you chose data.frequency %.2f\n",data->frequency);
data->phase = 0.0;
...
Pa_OpenStream( // open paStream object
&audioStream, // portaudio stream object
NULL, // input params
&outputParameters, // output params
SAMPLING_RATE, // SampleRate
FRAME_BLOCK_LEN, // frames per buffer
paNoFlag, // set no Flag
audio_callback, // callbak function address
data );
...

Answer 2

I wasn't able to get the original code working using malloc but based on both suggestions, I realized another workable solution. Because running init_stuff() caused my data to get deallocated, I'm for now just making all my assignments and calls to Pa_OpenStream() from main.
Works beautifully and I can now send whatever data I want to the callback. Thanks for the help!