Vorbisfile documentation

vorbisfile version 1.2.0 - 20070723

ov_open_callbacks

declared in "vorbis/vorbisfile.h";

This is an alternative function used to open and initialize an OggVorbis_File structure when using a data source other than a file, when its necessary to modify default file access behavior, or to initialize a Vorbis decode from a FILE * pointer under Windows where ov_open() cannot be used. It allows the application to specify custom file manipulation routines and sets up all the related decoding structures.

Once ov_open_callbacks() has been called, the same OggVorbis_File struct should be passed to all the libvorbisfile functions. Unlike ov_fopen() and ov_open(), ov_open_callbacks() may be used to instruct vorbisfile to either automatically close or not to close the file/data access handle in ov_clear(). Automatic closure is disabled by passing NULL as the close callback, or using one of the predefined callback sets that specify a NULL close callback. The application is responsible for closing a file when a call to ov_open_callbacks() is unsuccessful.

See also Callbacks and Non-stdio I/O for information on designing and specifying custom callback functions.




int ov_open_callbacks(void *datasource, OggVorbis_File *vf, char *initial, long ibytes, ov_callbacks callbacks);

Parameters

datasource
Pointer to a data structure allocated by the calling application, containing any state needed by the callbacks provided.
vf
A pointer to the OggVorbis_File structure--this is used for ALL the externally visible libvorbisfile functions. Once this has been called, the same OggVorbis_File struct should be passed to all the libvorbisfile functions.
initial
Typically set to NULL. This parameter is useful if some data has already been read from the stream and the stream is not seekable. It is used in conjunction with ibytes. In this case, initial should be a pointer to a buffer containing the data read.
ibytes
Typically set to 0. This parameter is useful if some data has already been read from the stream and the stream is not seekable. In this case, ibytes should contain the length (in bytes) of the buffer. Used together with initial.
callbacks
A completed ov_callbacks struct which indicates desired custom file manipulation routines. vorbisfile.h defines several preprovided callback sets; see ov_callbacks for details.

Return Values

  • 0 for success
  • less than zero for failure:

    • Notes

    [a] Windows and use as an ov_open() substitute

    Windows applications should not use ov_open() due to the likelihood of CRT linking mismatches and runtime protection faults [ov_open:a]. ov_open_callbacks() is a safe substitute; specifically: 

    ov_open_callbacks(f, vf, initial, ibytes, OV_CALLBACKS_DEFAULT);
    ... provides exactly the same functionality as ov_open() but will always work correctly under Windows, regardless of linking setup details.

    [b] Threaded decode

    If your decoder is threaded, it is recommended that you NOT call ov_open_callbacks() in the main control thread--instead, call ov_open_callbacks() in your decode/playback thread. This is important because ov_open_callbacks() may be a fairly time-consuming call, given that the full structure of the file is determined at this point, which may require reading large parts of the file under certain circumstances (determining all the logical bitstreams in one physical bitstream, for example). See Thread Safety for other information on using libvorbisfile with threads.

    [c] Mixed media streams

    A href="ov_comment.html">ov_comment(). 

    
  {
    char **ptr=ov_comment(&vf,-1)->user_comments;
    vorbis_info *vi=ov_info(&vf,-1);
    while(*ptr){
      fprintf(stderr,"%s\n",*ptr);
      ++ptr;
    }
    fprintf(stderr,"\nBitstream is %d channel, %ldHz\n",vi->channels,vi->rate);
    fprintf(stderr,"Encoded by: %s\n\n",ov_comment(&vf,-1)->vendor);
  }

    Here's the read loop: 

    

  while(!eof){
    long ret=ov_read(&vf,pcmout,sizeof(pcmout),0,2,1,¤t_section);
    switch(ret){
    case 0:
      /* EOF */
      eof=1;
      break;
    case -1:
      break;
    default:
      fwrite(pcmout,1,ret,stdout);
      break;
    }
  }

    The code is reading blocks of data using