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1<html>
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3<head>
4<title>Vorbisfile - function - ov_open</title>
5<link rel=stylesheet href="style.css" type="text/css">
6</head>
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8<body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff">
9<table border=0 width=100%>
10<tr>
11<td><p class=tiny>Vorbisfile documentation</p></td>
12<td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td>
13</tr>
14</table>
15
16<h1>ov_open</h1>
17
18<p><i>declared in "vorbis/vorbisfile.h";</i></p>
19
20<p>ov_open is one of three initialization functions used to initialize
21an OggVorbis_File structure and prepare a bitstream for playback.
22
23<p><em><b> WARNING for Windows developers: </b> Do not use ov_open() in
24Windows applications; Windows linking places restrictions on
25passing <tt>FILE *</tt> handles successfully, and ov_open() runs
26afoul of these restrictions <a href="#winfoot">[a]</a>.  See the <a
27href="ov_open_callbacks.html">ov_open_callbacks() page </a> for
28details on using <a
29href="ov_open_callbacks.html">ov_open_callbacks()</a> instead. </em>
30
31<p>The first argument must be a file pointer to an already opened file
32or pipe (it need not be seekable--though this obviously restricts what
33can be done with the bitstream). <tt>vf</tt> should be a pointer to the
34OggVorbis_File structure -- this is used for ALL the externally visible libvorbisfile
35functions. Once this has been called, the same <a href="OggVorbis_File.html">OggVorbis_File</a>
36struct should be passed to all the libvorbisfile functions.<p>
37
38The <tt>vf</tt> structure initialized using ov_fopen() must eventually
39be cleaned using <a href="ov_clear.html">ov_clear()</a>.  Once a
40<tt>FILE *</tt> handle is passed to ov_open() successfully, the
41application MUST NOT <tt>fclose()</tt> or in any other way manipulate
42that file handle.  Vorbisfile will close the file in <a
43href="ov_clear.html">ov_clear()</a>.  If the application must be able
44to close the <tt>FILE *</tt> handle itself, see <a
45href="ov_open_callbacks.html">ov_open_callbacks()</a> with the use of
46<tt>OV_CALLBACKS_NOCLOSE</tt>.
47
48<p>It is often useful to call <tt>ov_open()</tt> simply to determine
49whether a given file is a Vorbis bitstream. If the <tt>ov_open()</tt>
50call fails, then the file is not recognizable as Vorbis.  If the call
51succeeds but the initialized <tt>vf</tt> structure will not be used,
52the application is responsible for calling <a
53href="ov_clear.html">ov_clear()</a> to clear the decoder's buffers and
54close the file.<p>
55
56If [and only if] an <tt>ov_open()</tt> call fails, the application
57must explicitly <tt>fclose()</tt> the <tt>FILE *</tt> pointer itself.
58
59
60<br><br>
61<table border=0 color=black cellspacing=0 cellpadding=7>
62<tr bgcolor=#cccccc>
63        <td>
64<pre><b>
65int ov_open(FILE *f,<a href="OggVorbis_File.html">OggVorbis_File</a> *vf,char *initial,long ibytes);
66</b></pre>
67        </td>
68</tr>
69</table>
70
71<h3>Parameters</h3>
72<dl>
73<dt><i>f</i></dt>
74<dd>File pointer to an already opened file
75or pipe (it need not be seekable--though this obviously restricts what
76can be done with the bitstream).</dd>
77<dt><i>vf</i></dt>
78<dd>A pointer to the OggVorbis_File structure--this is used for ALL the externally visible libvorbisfile
79functions. Once this has been called, the same <tt>OggVorbis_File</tt>
80struct should be passed to all the libvorbisfile functions.</dd>
81<dt><i>initial</i></dt>
82<dd>Typically set to NULL.  This parameter is useful if some data has already been
83read from the file and the stream is not seekable. It is used in conjunction with <tt>ibytes</tt>.  In this case, <tt>initial</tt>
84should be a pointer to a buffer containing the data read.</dd>
85<dt><i>ibytes</i></dt>
86<dd>Typically set to 0.  This parameter is useful if some data has already been
87read from the file and the stream is not seekable. In this case, <tt>ibytes</tt>
88should contain the length (in bytes) of the buffer.  Used together with <tt>initial</tt></dd>
89</dl>
90
91
92<h3>Return Values</h3>
93<blockquote>
94<li>0 indicates success</li>
95
96<li>less than zero for failure:</li>
97<ul>
98<li>OV_EREAD - A read from media returned an error.</li>
99<li>OV_ENOTVORBIS - Bitstream is not Vorbis data.</li>
100<li>OV_EVERSION - Vorbis version mismatch.</li>
101<li>OV_EBADHEADER - Invalid Vorbis bitstream header.</li>
102<li>OV_EFAULT - Internal logic fault; indicates a bug or heap/stack corruption.</li>
103</ul>
104</blockquote>
105<p>
106
107<a name="notes"></a>
108<h3>Notes</h3>
109<dl>
110
111<a name="winfoot"></a>
112<dt><b>[a] Windows and ov_open()</b><p>
113
114<dd>Under Windows, stdio file access is implemented in each of many
115variants of crt.o, several of which are typically installed on any one
116Windows machine.  If libvorbisfile and the application using
117libvorbisfile are not linked against the exact same
118version/variant/build of crt.o (and they usually won't be, especially
119using a prebuilt libvorbis DLL), <tt>FILE *</tt> handles cannot be
120opened in the application and then passed to vorbisfile to be used
121by stdio calls from vorbisfile's different version of CRT.  For this
122reason, using <a href="ov_open.html">ov_open()</a> under Windows
123without careful, expert linking will typically cause a protection
124fault.  Windows programmers should use <a
125href="ov_fopen.html">ov_fopen()</a> (which will only use libvorbis's
126crt.o) or <a href="ov_open_callbacks.html">ov_open_callbacks()</a>
127(which will only use the application's crt.o) instead.<p>
128
129This warning only applies to Windows and only applies to <a
130href="ov_open.html">ov_open()</a>.  It is perfectly safe to use <a
131href="ov_open.html">ov_open()</a> on all other platforms.<p>
132
133For more information, see the following microsoft pages on <a
134href="http://msdn2.microsoft.com/en-us/library/abx4dbyh(VS.80).aspx">C
135runtime library linking</a> and a specific description of <a
136href="http://msdn2.microsoft.com/en-us/library/ms235460(VS.80).aspx">restrictions
137on passing CRT objects across DLL boundaries</a>.
138
139<p>
140
141<dt><b>[b] Threaded decode</b><p>
142<dd>If your decoder is threaded, it is recommended that you NOT call
143<tt>ov_open()</tt>
144in the main control thread--instead, call <tt>ov_open()</tt> in your decode/playback
145thread. This is important because <tt>ov_open()</tt> may be a fairly time-consuming
146call, given that the full structure of the file is determined at this point,
147which may require reading large parts of the file under certain circumstances
148(determining all the logical bitstreams in one physical bitstream, for
149example).  See <a href="threads.html">Thread Safety</a> for other information on using libvorbisfile with threads.
150<p>
151
152<dt><b>[c] Mixed media streams</b><p>
153<dd>
154As of Vorbisfile release 1.2.0, Vorbisfile is able to access the
155Vorbis content in mixed-media Ogg streams, not just Vorbis-only
156streams.  For example, Vorbisfile may be used to open and access the
157audio from an Ogg stream consisting of Theora video and Vorbis audio.
158Vorbisfile 1.2.0 decodes the first logical audio stream of each
159physical stream section.<p>
160
161<dt><b>[d] Faster testing for Vorbis files</b><p>
162<dd><a href="ov_test.html">ov_test()</a> and <a
163href="ov_test_callbacks.html">ov_test_callbacks()</a> provide less
164computationally expensive ways to test a file for Vorbisness, but
165require more setup code.<p>
166
167</dl>
168
169<br><br>
170<hr noshade>
171<table border=0 width=100%>
172<tr valign=top>
173<td><p class=tiny>copyright &copy; 2007 Xiph.org</p></td>
174<td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td>
175</tr><tr>
176<td><p class=tiny>Vorbisfile documentation</p></td>
177<td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td>
178</tr>
179</table>
180
181</body>
182
183</html>
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