122 lines
4.7 KiB
HTML
122 lines
4.7 KiB
HTML
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<html>
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<head>
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<title>Vorbisfile - Callbacks and non-stdio I/O</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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<table border=0 width=100%>
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<tr>
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<td><p class=tiny>Vorbisfile documentation</p></td>
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<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td>
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</tr>
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</table>
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<h1>Callbacks and non-stdio I/O</h1>
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Although stdio is convenient and nearly universally implemented as per
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ANSI C, it is not suited to all or even most potential uses of Vorbis.
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For additional flexibility, embedded applications may provide their
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own I/O functions for use with Vorbisfile when stdio is unavailable or not
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suitable. One common example is decoding a Vorbis stream from a
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memory buffer.<p>
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Use custom I/O functions by populating an <a
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href="ov_callbacks.html">ov_callbacks</a> structure and calling <a
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href="ov_open_callbacks.html">ov_open_callbacks()</a> or <a
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href="ov_test_callbacks.html">ov_test_callbacks()</a> rather than the
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typical <a href="ov_open.html">ov_open()</a> or <a
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href="ov_test.html">ov_test()</a>. Past the open call, use of
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libvorbisfile is identical to using it with stdio.
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<h2>Read function</h2>
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The read-like function provided in the <tt>read_func</tt> field is
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used to fetch the requested amount of data. It expects the fetch
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operation to function similar to file-access, that is, a multiple read
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operations will retrieve contiguous sequential pieces of data,
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advancing a position cursor after each read.<p>
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The following behaviors are also expected:<p>
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<ul>
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<li>a return of '0' indicates end-of-data (if the by-thread errno is unset)
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<li>short reads mean nothing special (short reads are not treated as error conditions)
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<li>a return of zero with the by-thread errno set to nonzero indicates a read error
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</ul>
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<p>
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<h2>Seek function</h2>
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The seek-like function provided in the <tt>seek_func</tt> field is
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used to request non-sequential data access by libvorbisfile, moving
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the access cursor to the requested position. The seek function is
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optional; if callbacks are only to handle non-seeking (streaming) data
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or the application wishes to force streaming behavior,
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<tt>seek_func</tt> and <tt>tell_func</tt> should be set to NULL. If
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the seek function is non-NULL, libvorbisfile mandates the following
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behavior:
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<ul>
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<li>The seek function must always return -1 (failure) if the given
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data abstraction is not seekable. It may choose to always return -1
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if the application desires libvorbisfile to treat the Vorbis data
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strictly as a stream (which makes for a less expensive open
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operation).<p>
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<li>If the seek function initially indicates seekability, it must
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always succeed upon being given a valid seek request.<p>
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<li>The seek function must implement all of SEEK_SET, SEEK_CUR and
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SEEK_END. The implementation of SEEK_END should set the access cursor
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one past the last byte of accessible data, as would stdio
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<tt>fseek()</tt><p>
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</ul>
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<h2>Close function</h2>
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The close function should deallocate any access state used by the
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passed in instance of the data access abstraction and invalidate the
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instance handle. The close function is assumed to succeed; its return
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code is not checked.<p>
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The <tt>close_func</tt> may be set to NULL to indicate that libvorbis
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should not attempt to close the file/data handle in <a
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href="ov_clear.html">ov_clear</a> but allow the application to handle
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file/data access cleanup itself. For example, by passing the normal
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stdio calls as callback functions, but passing a <tt>close_func</tt>
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that is NULL or does nothing (as in the case of OV_CALLBACKS_NOCLOSE), an
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application may call <a href="ov_clear.html">ov_clear()</a> and then
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later <tt>fclose()</tt> the file originally passed to libvorbisfile.
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<h2>Tell function</h2>
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The tell function is intended to mimic the
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behavior of <tt>ftell()</tt> and must return the byte position of the
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next data byte that would be read. If the data access cursor is at
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the end of the 'file' (pointing to one past the last byte of data, as
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it would be after calling <tt>fseek(file,SEEK_END,0)</tt>), the tell
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function must return the data position (and thus the total file size),
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not an error.<p>
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The tell function need not be provided if the data IO abstraction is
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not seekable, or the application wishes to force streaming
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behavior. In this case, the <tt>tell_func</tt> and <tt>seek_func</tt>
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fields should be set to NULL.<p>
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<br><br>
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<hr noshade>
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<table border=0 width=100%>
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<tr valign=top>
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<td><p class=tiny>copyright © 2000-2010 Xiph.Org</p></td>
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<td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td>
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</tr><tr>
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<td><p class=tiny>Vorbisfile documentation</p></td>
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|
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<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td>
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</tr>
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</table>
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</body>
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</html>
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