linux - Can you use a PortAudio callback to write back to a stream's input buffer? -

at highlevel, goal take microphone input 1 stream, processing on it, , copy microphone input stream. latter being default device, other applications (which reasons out of hands, can't specify other device) can record default device , still processed input.

here's snippet of code:

int stream1_callback(const void *card_capture, void *card_playback, unsigned long framecount, const pastreamcallbacktimeinfo *timeinfo, pastreamcallbackflags statusflags, void *userdata) {     main_t *data = (main_t *) userdata;      // unused(card_capture);     unused(card_playback);     unused(framecount);     unused(timeinfo);     unused(statusflags);     // unused(userdata);      deinterleave_i16_i16(card_capture, data->mic_unprocessed, num_mics, blocksize_48khz);     printf("de-interleaved!\n");      process_microphones(data->mic_unprocessed, data->mic_processed);     printf("processed!\n");      return 0; }  int stream2_callback(const void *inputbuffer, void *outputbuffer, unsigned long framecount, const pastreamcallbacktimeinfo *timeinfo, pastreamcallbackflags statusflags, void *userdata) {     main_t *data = (main_t *) userdata;      // unused(inputbuffer);     unused(outputbuffer);     unused(framecount);     unused(timeinfo);     unused(statusflags);     // unused(userdata);      interleave_i16_i16(data->mic_processed, (int16 *)inputbuffer, 1, blocksize_16khz);     printf("interleaved!\n");      return 0; }  int main() {     // ...      /* -- setup stream1 -- */     err = pa_openstream(&stream1, &stream1inputparams, null, 48000, blocksize_48khz, panoflag, stream1_callback, &main_data);     if (err != panoerror) {         goto error;     }      /* -- setup stream2 -- */     err = pa_opendefaultstream(&stream2, 1, 0, paint16, 16000, blocksize_16khz, stream2_callback, &main_data);     if (err != panoerror) {         goto error;     }      //... } 

so i'm wondering if callback's input buffer writable. or if there other (better) way can write input of device.

in general, capture devices cannot written to; go directly hardware.

to able inject own samples data capture device, driver must written allow that. case loopback driver, snd-aloop.