ov_open

declared in "vorbis/vorbisfile.h";

This is the main function used to open and initialize an OggVorbis_File structure. It sets up all the related decoding structure.

The first argument must be a file pointer to an already opened file or pipe (it need not be seekable--though this obviously restricts what can be done with the bitstream). vf should be 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.

Also, you should be aware that ov_open(), once successful, takes complete possession of the file resource. After you have opened a file using ov_open(), you MUST close it using ov_clear(), not fclose() or any other function.

It is often useful to call ov_open() simply to determine whether a given file is a vorbis bitstream. If the ov_open() call fails, then the file is not recognizable as such. When you use ov_open() for this, you should fclose() the file pointer if, and only if, the ov_open() call fails. If it succeeds, you must call ov_clear() to clear the decoder's buffers and close the file for you.

(Note that ov_test() provides a less expensive way to test a file for Vorbisness.)

int ov_open(FILE *f,OggVorbis_File *vf,char *initial,long ibytes);

Parameters

f

File pointer to an already opened file or pipe (it need not be seekable--though this obviously restricts what can be done with the bitstream).

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 file 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 file and the stream is not seekable. In this case, ibytes should contain the length (in bytes) of the buffer. Used together with initial

Return Values

0 indicates success

less than zero for failure:

• OV_EREAD - A read from media returned an error.
• OV_ENOTVORBIS - Bitstream is not Vorbis data.
• OV_EVERSION - Vorbis version mismatch.
• OV_EBADHEADER - Invalid Vorbis bitstream header.
• OV_EFAULT - Internal logic fault; indicates a bug or heap/stack corruption.

Notes

If your decoder is threaded, it is recommended that you NOT call ov_open() in the main control thread--instead, call ov_open() IN your decode/playback thread. This is important because ov_open() 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.

WARNING for windows developers: this function cannot be used on win32 if your application dynamically links to libvorbisfile (see this microsoft page for details of why). Instead, you must use ov_open_callbacks(). A simple set of callbacks that will work is:

  static int _fseek64_wrap(FILE *f,ogg_int64_t off,int whence){
    if(f==NULL)return(-1);
    return fseek(f,off,whence);
  }

  ov_callbacks callbacks = {
    (size_t (*)(void *, size_t, size_t, void *))  fread,
    (int (*)(void *, ogg_int64_t, int))           _fseek64_wrap,
    (int (*)(void *))                             fclose,
    (long (*)(void *))                            ftell
  };