| [16] | 1 | <html> |
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| 3 | <head> |
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| 4 | <title>Vorbisfile - Callbacks and non-stdio I/O</title> |
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| 5 | <link rel=stylesheet href="style.css" type="text/css"> |
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| 6 | </head> |
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| 7 | |
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| 8 | <body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff"> |
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| 9 | <table border=0 width=100%> |
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| 10 | <tr> |
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| 11 | <td><p class=tiny>Vorbisfile documentation</p></td> |
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| 12 | <td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td> |
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| 13 | </tr> |
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| 14 | </table> |
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| 15 | |
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| 16 | <h1>Callbacks and non-stdio I/O</h1> |
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| 17 | |
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| 18 | Although stdio is convenient and nearly universally implemented as per |
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| 19 | ANSI C, it is not suited to all or even most potential uses of Vorbis. |
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| 20 | For additional flexibility, embedded applications may provide their |
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| 21 | own I/O functions for use with Vorbisfile when stdio is unavailable or not |
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| 22 | suitable. One common example is decoding a Vorbis stream from a |
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| 23 | memory buffer.<p> |
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| 24 | |
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| 25 | Use custom I/O functions by populating an <a |
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| 26 | href="ov_callbacks.html">ov_callbacks</a> structure and calling <a |
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| 27 | href="ov_open_callbacks.html">ov_open_callbacks()</a> or <a |
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| 28 | href="ov_test_callbacks.html">ov_test_callbacks()</a> rather than the |
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| 29 | typical <a href="ov_open.html">ov_open()</a> or <a |
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| 30 | href="ov_test.html">ov_test()</a>. Past the open call, use of |
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| 31 | libvorbisfile is identical to using it with stdio. |
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| 32 | |
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| 33 | <h2>Read function</h2> |
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| 34 | |
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| 35 | The read-like function provided in the <tt>read_func</tt> field is |
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| 36 | used to fetch the requested amount of data. It expects the fetch |
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| 37 | operation to function similar to file-access, that is, a multiple read |
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| 38 | operations will retrieve contiguous sequential pieces of data, |
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| 39 | advancing a position cursor after each read.<p> |
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| 40 | |
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| 41 | The following behaviors are also expected:<p> |
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| 42 | <ul> |
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| 43 | <li>a return of '0' indicates end-of-data (if the by-thread errno is unset) |
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| 44 | <li>short reads mean nothing special (short reads are not treated as error conditions) |
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| 45 | <li>a return of zero with the by-thread errno set to nonzero indicates a read error |
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| 46 | </ul> |
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| 47 | <p> |
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| 48 | |
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| 49 | <h2>Seek function</h2> |
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| 50 | |
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| 51 | The seek-like function provided in the <tt>seek_func</tt> field is |
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| 52 | used to request non-sequential data access by libvorbisfile, moving |
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| 53 | the access cursor to the requested position. The seek function is |
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| 54 | optional; if callbacks are only to handle non-seeking (streaming) data |
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| 55 | or the application wishes to force streaming behavior, |
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| 56 | <tt>seek_func</tt> and <tt>tell_func</tt> should be set to NULL. If |
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| 57 | the seek function is non-NULL, libvorbisfile mandates the following |
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| 58 | behavior: |
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| 59 | |
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| 60 | <ul> |
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| 61 | <li>The seek function must always return -1 (failure) if the given |
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| 62 | data abstraction is not seekable. It may choose to always return -1 |
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| 63 | if the application desires libvorbisfile to treat the Vorbis data |
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| 64 | strictly as a stream (which makes for a less expensive open |
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| 65 | operation).<p> |
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| 66 | |
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| 67 | <li>If the seek function initially indicates seekability, it must |
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| 68 | always succeed upon being given a valid seek request.<p> |
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| 69 | |
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| 70 | <li>The seek function must implement all of SEEK_SET, SEEK_CUR and |
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| 71 | SEEK_END. The implementation of SEEK_END should set the access cursor |
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| 72 | one past the last byte of accessible data, as would stdio |
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| 73 | <tt>fseek()</tt><p> |
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| 74 | </ul> |
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| 75 | |
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| 76 | <h2>Close function</h2> |
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| 77 | |
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| 78 | The close function should deallocate any access state used by the |
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| 79 | passed in instance of the data access abstraction and invalidate the |
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| 80 | instance handle. The close function is assumed to succeed; its return |
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| 81 | code is not checked.<p> |
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| 82 | |
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| 83 | The <tt>close_func</tt> may be set to NULL to indicate that libvorbis |
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| 84 | should not attempt to close the file/data handle in <a |
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| 85 | href="ov_clear.html">ov_clear</a> but allow the application to handle |
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| 86 | file/data access cleanup itself. For example, by passing the normal |
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| 87 | stdio calls as callback functions, but passing a <tt>close_func</tt> |
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| 88 | that is NULL or does nothing (as in the case of OV_CALLBACKS_NOCLOSE), an |
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| 89 | application may call <a href="ov_clear.html">ov_clear()</a> and then |
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| 90 | later <tt>fclose()</tt> the file originally passed to libvorbisfile. |
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| 91 | |
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| 92 | <h2>Tell function</h2> |
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| 93 | |
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| 94 | The tell function is intended to mimic the |
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| 95 | behavior of <tt>ftell()</tt> and must return the byte position of the |
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| 96 | next data byte that would be read. If the data access cursor is at |
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| 97 | the end of the 'file' (pointing to one past the last byte of data, as |
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| 98 | it would be after calling <tt>fseek(file,SEEK_END,0)</tt>), the tell |
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| 99 | function must return the data position (and thus the total file size), |
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| 100 | not an error.<p> |
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| 101 | |
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| 102 | The tell function need not be provided if the data IO abstraction is |
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| 103 | not seekable, or the application wishes to force streaming |
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| 104 | behavior. In this case, the <tt>tell_func</tt> and <tt>seek_func</tt> |
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| 105 | fields should be set to NULL.<p> |
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| 106 | |
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| 107 | <br><br> |
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| 108 | <hr noshade> |
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| 109 | <table border=0 width=100%> |
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| 110 | <tr valign=top> |
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| 111 | <td><p class=tiny>copyright © 2007 Xiph.org</p></td> |
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| 112 | <td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td> |
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| 113 | </tr><tr> |
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| 114 | <td><p class=tiny>Vorbisfile documentation</p></td> |
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| 115 | <td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td> |
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| 116 | </tr> |
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| 117 | </table> |
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| 118 | |
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| 119 | </body> |
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| 120 | |
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| 121 | </html> |
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