1<html> 2 3<head> 4<title>Vorbisfile - function - ov_open</title> 5<link rel=stylesheet href="style.css" type="text/css"> 6</head> 7 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 © 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> 184