vorbis/doc/vorbisfile/ov_open.html

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<title>Vorbisfile - function - ov_open</title>
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<td><p class=tiny>Vorbisfile documentation</p></td>
<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td>
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<h1>ov_open</h1>

<p><i>declared in "vorbis/vorbisfile.h";</i></p>

<p>ov_open is one of three initialization functions used to initialize
an OggVorbis_File structure and prepare a bitstream for playback.

<p><em><b> WARNING for Windows developers: </b> Do not use ov_open() in
Windows applications; Windows linking places restrictions on
passing <tt>FILE *</tt> handles successfully, and ov_open() runs
afoul of these restrictions <a href="#winfoot">[a]</a>.  See the <a
href="ov_open_callbacks.html">ov_open_callbacks() page </a> for
details on using <a
href="ov_open_callbacks.html">ov_open_callbacks()</a> instead. </em>

<p>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). <tt>vf</tt> 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 <a href="OggVorbis_File.html">OggVorbis_File</a>
struct should be passed to all the libvorbisfile functions.<p>

The <tt>vf</tt> structure initialized using ov_fopen() must eventually
be cleaned using <a href="ov_clear.html">ov_clear()</a>.  Once a
<tt>FILE *</tt> handle is passed to ov_open() successfully, the
application MUST NOT <tt>fclose()</tt> or in any other way manipulate
that file handle.  Vorbisfile will close the file in <a
href="ov_clear.html">ov_clear()</a>.  If the application must be able
to close the <tt>FILE *</tt> handle itself, see <a
href="ov_open_callbacks.html">ov_open_callbacks()</a> with the use of
<tt>OV_CALLBACKS_NOCLOSE</tt>.

<p>It is often useful to call <tt>ov_open()</tt> simply to determine
whether a given file is a Vorbis bitstream. If the <tt>ov_open()</tt>
call fails, then the file is not recognizable as Vorbis.  If the call
succeeds but the initialized <tt>vf</tt> structure will not be used,
the application is responsible for calling <a
href="ov_clear.html">ov_clear()</a> to clear the decoder's buffers and
close the file.<p>

If [and only if] an <tt>ov_open()</tt> call fails, the application
must explicitly <tt>fclose()</tt> the <tt>FILE *</tt> pointer itself.


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<pre><b>
int ov_open(FILE *f,<a href="OggVorbis_File.html">OggVorbis_File</a> *vf,char *initial,long ibytes);
</b></pre>
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<h3>Parameters</h3>
<dl>
<dt><i>f</i></dt>
<dd>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).</dd>
<dt><i>vf</i></dt>
<dd>A pointer to the OggVorbis_File structure--this is used for ALL the externally visible libvorbisfile
functions. Once this has been called, the same <tt>OggVorbis_File</tt>
struct should be passed to all the libvorbisfile functions.</dd>
<dt><i>initial</i></dt>
<dd>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 <tt>ibytes</tt>.  In this case, <tt>initial</tt>
should be a pointer to a buffer containing the data read.</dd>
<dt><i>ibytes</i></dt>
<dd>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, <tt>ibytes</tt>
should contain the length (in bytes) of the buffer.  Used together with <tt>initial</tt></dd>
</dl>


<h3>Return Values</h3>
<blockquote>
<li>0 indicates success</li>

<li>less than zero for failure:</li>
<ul>
<li>OV_EREAD - A read from media returned an error.</li>
<li>OV_ENOTVORBIS - Bitstream is not Vorbis data.</li>
<li>OV_EVERSION - Vorbis version mismatch.</li>
<li>OV_EBADHEADER - Invalid Vorbis bitstream header.</li>
<li>OV_EFAULT - Internal logic fault; indicates a bug or heap/stack corruption.</li>
</ul>
</blockquote>
<p>

<a name="notes"></a>
<h3>Notes</h3>
<dl>

<a name="winfoot"></a>
<dt><b>[a] Windows and ov_open()</b><p>

<dd>Under Windows, stdio file access is implemented in each of many
variants of crt.o, several of which are typically installed on any one
Windows machine.  If libvorbisfile and the application using
libvorbisfile are not linked against the exact same
version/variant/build of crt.o (and they usually won't be, especially
using a prebuilt libvorbis DLL), <tt>FILE *</tt> handles cannot be
opened in the application and then passed to vorbisfile to be used
by stdio calls from vorbisfile's different version of CRT.  For this
reason, using <a href="ov_open.html">ov_open()</a> under Windows
without careful, expert linking will typically cause a protection
fault.  Windows programmers should use <a
href="ov_fopen.html">ov_fopen()</a> (which will only use libvorbis's
crt.o) or <a href="ov_open_callbacks.html">ov_open_callbacks()</a>
(which will only use the application's crt.o) instead.<p>

This warning only applies to Windows and only applies to <a
href="ov_open.html">ov_open()</a>.  It is perfectly safe to use <a
href="ov_open.html">ov_open()</a> on all other platforms.<p>

For more information, see the following microsoft pages on <a
href="http://msdn2.microsoft.com/en-us/library/abx4dbyh(VS.80).aspx">C
runtime library linking</a> and a specific description of <a
href="http://msdn2.microsoft.com/en-us/library/ms235460(VS.80).aspx">restrictions
on passing CRT objects across DLL boundaries</a>.

<p>

<dt><b>[b] Threaded decode</b><p>
<dd>If your decoder is threaded, it is recommended that you NOT call
<tt>ov_open()</tt>
in the main control thread--instead, call <tt>ov_open()</tt> in your decode/playback
thread. This is important because <tt>ov_open()</tt> 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 <a href="threads.html">Thread Safety</a> for other information on using libvorbisfile with threads.
<p>

<dt><b>[c] Mixed media streams</b><p>
<dd>
As of Vorbisfile release 1.2.0, Vorbisfile is able to access the
Vorbis content in mixed-media Ogg streams, not just Vorbis-only
streams.  For example, Vorbisfile may be used to open and access the
audio from an Ogg stream consisting of Theora video and Vorbis audio.
Vorbisfile 1.2.0 decodes the first logical audio stream of each
physical stream section.<p>

<dt><b>[d] Faster testing for Vorbis files</b><p>
<dd><a href="ov_test.html">ov_test()</a> and <a
href="ov_test_callbacks.html">ov_test_callbacks()</a> provide less
computationally expensive ways to test a file for Vorbisness, but
require more setup code.<p>

</dl>

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<td><p class=tiny>copyright &copy; 2000-2010 Xiph.Org</p></td>
<td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td>
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<td><p class=tiny>Vorbisfile documentation</p></td>
<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td>
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