Chaining Example Code

The following is a run-through of the chaining example program supplied with vorbisfile - chaining_example.c. This program demonstrates how to work with a chained bitstream.

First, relevant headers, including vorbis-specific "codec.h" and "vorbisfile.h" have to be included.

#include "vorbis/codec.h" #include "vorbis/vorbisfile.h" #include "../lib/misc.h"

Inside main(), we declare our primary OggVorbis_File structure. We also declare a other helpful variables to track our progress within the file.

int main(){ OggVorbis_File ov; int i;

This example takes its input on stdin which is in 'text' mode by default under Windows; this will corrupt the input data unless set to binary mode. This applies only to Windows.

#ifdef _WIN32 /* We need to set stdin to binary mode under Windows */ _setmode( _fileno( stdin ), _O_BINARY ); #endif

We call ov_open_callbacks() to initialize the OggVorbis_File structure. ov_open_callbacks() also checks to ensure that we're reading Vorbis format and not something else. The OV_CALLBACKS_NOCLOSE callbacks instruct libvorbisfile not to close stdin later during cleanup.

if(ov_open_callbacks(stdin,&ov,NULL,-1,OV_CALLBACKS_NOCLOSE)<0){ printf("Could not open input as an OggVorbis file.\n\n"); exit(1); }

First we check to make sure the stream is seekable using ov_seekable.

Then we're going to find the number of logical bitstreams in the physical bitstream using ov_streams.

We use ov_time_total to determine the total length of the physical bitstream. We specify that we want the entire bitstream by using the argument -1.

if(ov_seekable(&ov)){ printf("Input bitstream contained %ld logical bitstream section(s).\n", ov_streams(&ov)); printf("Total bitstream playing time: %ld seconds\n\n", (long)ov_time_total(&ov,-1)); }else{ printf("Standard input was not seekable.\n" "First logical bitstream information:\n\n"); }

Now we're going to iterate through each logical bitstream and print information about that bitstream.

We use ov_info to pull out the vorbis_info struct for each logical bitstream. This struct contains bitstream-specific info.

ov_serialnumber retrieves the unique serial number for the logical bistream. ov_raw_total gives the total compressed bytes for the logical bitstream, and ov_time_total gives the total time in the logical bitstream.

for(i=0;i<ov_streams(&ov);i++){ vorbis_info *vi=ov_info(&ov,i); printf("\tlogical bitstream section %d information:\n",i+1); printf("\t\t%ldHz %d channels bitrate %ldkbps serial number=%ld\n", vi->rate,vi->channels,ov_bitrate(&ov,i)/1000, ov_serialnumber(&ov,i)); printf("\t\tcompressed length: %ld bytes ",(long)(ov_raw_total(&ov,i))); printf(" play time: %lds\n",(long)ov_time_total(&ov,i)); }

When we're done with the entire physical bitstream, we need to call ov_clear() to release the bitstream.

ov_clear(&ov); return 0; }

The full source for chaining_example.c can be found with the vorbis distribution in chaining_example.c.