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1 <html>
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2
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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 1.3.2 - 20101101</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 © 2000-2010 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 1.3.2 - 20101101</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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