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<head>
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<title>Vorbisfile - function - ov_open</title>
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<link rel=stylesheet href="style.css" type="text/css">
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</head>
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<body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff">
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Vorbisfile documentation
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vorbisfile version 1.3.2 - 20101101
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ov_open
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declared in "vorbis/vorbisfile.h";
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ov_open is one of three initialization functions used to initialize
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an OggVorbis_File structure and prepare a bitstream for playback.
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WARNING for Windows developers: Do not use ov_open() in
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Windows applications; Windows linking places restrictions on
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passing <tt>FILE *</tt> handles successfully, and ov_open() runs
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afoul of these restrictions [a]. See the
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href="ov_open_callbacks.html">ov_open_callbacks() page for
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details on using
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href="ov_open_callbacks.html">ov_open_callbacks() instead.
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The first argument must be a file pointer to an already opened file
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or pipe (it need not be seekable--though this obviously restricts what
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can be done with the bitstream). <tt>vf</tt> should be a pointer to the
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OggVorbis_File structure -- this is used for ALL the externally visible libvorbisfile
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functions. Once this has been called, the same OggVorbis_File
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struct should be passed to all the libvorbisfile functions.
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The <tt>vf</tt> structure initialized using ov_fopen() must eventually
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be cleaned using ov_clear(). Once a
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<tt>FILE *</tt> handle is passed to ov_open() successfully, the
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application MUST NOT <tt>fclose()</tt> or in any other way manipulate
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that file handle. Vorbisfile will close the file in
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href="ov_clear.html">ov_clear(). If the application must be able
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to close the <tt>FILE *</tt> handle itself, see
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href="ov_open_callbacks.html">ov_open_callbacks() with the use of
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<tt>OV_CALLBACKS_NOCLOSE</tt>.
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It is often useful to call <tt>ov_open()</tt> simply to determine
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whether a given file is a Vorbis bitstream. If the <tt>ov_open()</tt>
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call fails, then the file is not recognizable as Vorbis. If the call
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succeeds but the initialized <tt>vf</tt> structure will not be used,
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the application is responsible for calling
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href="ov_clear.html">ov_clear() to clear the decoder's buffers and
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close the file.
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If [and only if] an <tt>ov_open()</tt> call fails, the application
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must explicitly <tt>fclose()</tt> the <tt>FILE *</tt> pointer itself.
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int ov_open(FILE *f,OggVorbis_File *vf,char *initial,long ibytes);
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Parameters
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f
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File pointer to an already opened file
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or pipe (it need not be seekable--though this obviously restricts what
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can be done with the bitstream).
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vf
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A pointer to the OggVorbis_File structure--this is used for ALL the externally visible libvorbisfile
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functions. Once this has been called, the same <tt>OggVorbis_File</tt>
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struct should be passed to all the libvorbisfile functions.
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initial
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Typically set to NULL. This parameter is useful if some data has already been
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read from the file and the stream is not seekable. It is used in conjunction with <tt>ibytes</tt>. In this case, <tt>initial</tt>
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should be a pointer to a buffer containing the data read.
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ibytes
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Typically set to 0. This parameter is useful if some data has already been
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read from the file and the stream is not seekable. In this case, <tt>ibytes</tt>
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should contain the length (in bytes) of the buffer. Used together with <tt>initial</tt>
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Return Values
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0 indicates success
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less than zero for failure:
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OV_EREAD - A read from media returned an error.
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OV_ENOTVORBIS - Bitstream is not Vorbis data.
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OV_EVERSION - Vorbis version mismatch.
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OV_EBADHEADER - Invalid Vorbis bitstream header.
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OV_EFAULT - Internal logic fault; indicates a bug or heap/stack corruption.
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Notes
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[a] Windows and ov_open()
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Under Windows, stdio file access is implemented in each of many
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variants of crt.o, several of which are typically installed on any one
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Windows machine. If libvorbisfile and the application using
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libvorbisfile are not linked against the exact same
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version/variant/build of crt.o (and they usually won't be, especially
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using a prebuilt libvorbis DLL), <tt>FILE *</tt> handles cannot be
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opened in the application and then passed to vorbisfile to be used
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by stdio calls from vorbisfile's different version of CRT. For this
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reason, using ov_open() under Windows
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without careful, expert linking will typically cause a protection
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fault. Windows programmers should use
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href="ov_fopen.html">ov_fopen() (which will only use libvorbis's
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crt.o) or ov_open_callbacks()
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(which will only use the application's crt.o) instead.
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This warning only applies to Windows and only applies to
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href="ov_open.html">ov_open(). It is perfectly safe to use
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href="ov_open.html">ov_open() on all other platforms.
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For more information, see the following microsoft pages on
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href="http://msdn2.microsoft.com/en-us/library/abx4dbyh(VS.80).aspx">C
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runtime library linking and a specific description of
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href="http://msdn2.microsoft.com/en-us/library/ms235460(VS.80).aspx">restrictions
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on passing CRT objects across DLL boundaries.
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[b] Threaded decode
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If your decoder is threaded, it is recommended that you NOT call
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<tt>ov_open()</tt>
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in the main control thread--instead, call <tt>ov_open()</tt> in your decode/playback
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thread. This is important because <tt>ov_open()</tt> may be a fairly time-consuming
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call, given that the full structure of the file is determined at this point,
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which may require reading large parts of the file under certain circumstances
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(determining all the logical bitstreams in one physical bitstream, for
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example). See Thread Safety for other information on using libvorbisfile with threads.
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[c] Mixed media streams
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As of Vorbisfile release 1.2.0, Vorbisfile is able to access the
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Vorbis content in mixed-media Ogg streams, not just Vorbis-only
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streams. For example, Vorbisfile may be used to open and access the
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audio from an Ogg stream consisting of Theora video and Vorbis audio.
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Vorbisfile 1.2.0 decodes the first logical audio stream of each
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physical stream section.
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[d] Faster testing for Vorbis files
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ov_test() and
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href="ov_test_callbacks.html">ov_test_callbacks() provide less
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computationally expensive ways to test a file for Vorbisness, but
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require more setup code.
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copyright © 2000-2010 Xiph.Org
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Ogg Vorbis
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Vorbisfile documentation
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vorbisfile version 1.3.2 - 20101101
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</body>
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</html>
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