Save audio stream in file using C libwebsockets - c

Question

I have a C server that uses libwebsockets and I want to save a received audio stream in a file on disk.
Here is my code snippet:
#define FILENAME "/home/ubuntu/Desktop/file.wav"
FILE *received_file;
struct lws_context *Audiocontext;
static int callback_audio(
struct lws *wsi,
enum lws_callback_reasons reason,
void *user, void *in, size_t len)
{
switch (reason) {
case LWS_CALLBACK_ESTABLISHED:
{
printf("client is connected\n");
received_file = fopen(FILENAME, "w+");
if (received_file == NULL)
{
printf("Failed to open file\n");
exit(EXIT_FAILURE);
}
}
break;
case LWS_CALLBACK_RECEIVE: {
if(strcmp((char*)in,"EOS")==0)
{
printf("End of stream!\n");
fclose(received_file);
}
else
{
fwrite(in, 1, len, received_file);
}
}
}
}
I got the message "client is connected" and also the file, but the content isn't ok, I cannot play it. I think there is a problem regarding the way I save the stream on file using fwrite().
The client is sending audio chunks encoded as wav, 16Khz, mono. Here is a snippet from client (it's a javascript client, the full code is here:http://kaljurand.github.io/dictate.js/).
if (recorder) {
recorder.stop();
config.onEvent(MSG_STOP, 'Stopped recording');
// Push the remaining audio to the server
recorder.export16kMono(function(blob) {
socketSend(blob);
socketSend(TAG_END_OF_SENTENCE);
recorder.clear();
}, 'export16kMono');
config.onEndOfSpeech();
} else {
config.onError(ERR_AUDIO, "Recorder undefined");
}
The client is working well, I use it for exactly the same task, but using a Java server. I would appreciate if someone could indicate me how to save these audio chunks in a valid file.

Answer 1

I think that you do not write header in your wav file. To do so,
you can write some function to do so see specification here
or you can use a dedicaded library, like libsndfile, which is not so complicated to use:
// instead of fopen, write something like
SF_INFO info;
SNDFILE * sf;
info.samplerate = sample_rate;
info.channels = 1;
info.sections = 1;
info.seekable = 0;
info.frames = 0;
info.format = SF_FORMAT_WAV | SF_FORMAT_PCM_16;
sf = sf_open(filename, SFM_WRITE, &info);
// instead of fwrite, something like
sf_write_short(sf, in, len / sizeof(short));
// and instead of fclose
sf_close(sf);

Answer 2

I didn't use libsndfile, but I change something on the client side. This is the current hexdump output:
00000000 52 49 46 46 20 60 00 00 57 41 56 45 66 6d 74 20 |RIFF `..WAVEfmt |
00000010 10 00 00 00 01 00 02 00 44 ac 00 00 10 b1 02 00 |........D.......|
00000020 04 00 10 00 64 61 74 61 00 60 00 00 00 00 00 00 |....data.`......|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00006020 00 00 00 00 00 00 00 00 00 00 00 00 52 49 46 46 |............RIFF|
00006030 20 60 00 00 57 41 56 45 66 6d 74 20 10 00 00 00 | `..WAVEfmt ....|
00006040 01 00 02 00 44 ac 00 00 10 b1 02 00 04 00 10 00 |....D...........|
00006050 64 61 74 61 00 60 00 00 00 00 00 00 00 00 00 00 |data.`..........|
00006060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
0000c050 00 00 00 00 00 00 00 00 52 49 46 46 20 60 00 00 |........RIFF `..|
0000c060 57 41 56 45 66 6d 74 20 10 00 00 00 01 00 02 00 |WAVEfmt ........|
0000c070 44 ac 00 00 10 b1 02 00 04 00 10 00 64 61 74 61 |D...........data|
0000c080 00 60 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |.`..............|
0000c090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
It seems to be wav encoded, but i don't have a content.