1 <html> 2 3 <head> 4 <title>Vorbisfile - Callbacks and non-stdio I/O</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>Callbacks and non-stdio I/O</h1> 17 18 Although stdio is convenient and nearly universally implemented as per 19 ANSI C, it is not suited to all or even most potential uses of Vorbis. 20 For additional flexibility, embedded applications may provide their 21 own I/O functions for use with Vorbisfile when stdio is unavailable or not 22 suitable. One common example is decoding a Vorbis stream from a 23 memory buffer.<p> 24 25 Use custom I/O functions by populating an <a 26 href="ov_callbacks.html">ov_callbacks</a> structure and calling <a 27 href="ov_open_callbacks.html">ov_open_callbacks()</a> or <a 28 href="ov_test_callbacks.html">ov_test_callbacks()</a> rather than the 29 typical <a href="ov_open.html">ov_open()</a> or <a 30 href="ov_test.html">ov_test()</a>. Past the open call, use of 31 libvorbisfile is identical to using it with stdio. 32 33 <h2>Read function</h2> 34 35 The read-like function provided in the <tt>read_func</tt> field is 36 used to fetch the requested amount of data. It expects the fetch 37 operation to function similar to file-access, that is, a multiple read 38 operations will retrieve contiguous sequential pieces of data, 39 advancing a position cursor after each read.<p> 40 41 The following behaviors are also expected:<p> 42 <ul> 43 <li>a return of '0' indicates end-of-data (if the by-thread errno is unset) 44 <li>short reads mean nothing special (short reads are not treated as error conditions) 45 <li>a return of zero with the by-thread errno set to nonzero indicates a read error 46 </ul> 47 <p> 48 49 <h2>Seek function</h2> 50 51 The seek-like function provided in the <tt>seek_func</tt> field is 52 used to request non-sequential data access by libvorbisfile, moving 53 the access cursor to the requested position. The seek function is 54 optional; if callbacks are only to handle non-seeking (streaming) data 55 or the application wishes to force streaming behavior, 56 <tt>seek_func</tt> and <tt>tell_func</tt> should be set to NULL. If 57 the seek function is non-NULL, libvorbisfile mandates the following 58 behavior: 59 60 <ul> 61 <li>The seek function must always return -1 (failure) if the given 62 data abstraction is not seekable. It may choose to always return -1 63 if the application desires libvorbisfile to treat the Vorbis data 64 strictly as a stream (which makes for a less expensive open 65 operation).<p> 66 67 <li>If the seek function initially indicates seekability, it must 68 always succeed upon being given a valid seek request.<p> 69 70 <li>The seek function must implement all of SEEK_SET, SEEK_CUR and 71 SEEK_END. The implementation of SEEK_END should set the access cursor 72 one past the last byte of accessible data, as would stdio 73 <tt>fseek()</tt><p> 74 </ul> 75 76 <h2>Close function</h2> 77 78 The close function should deallocate any access state used by the 79 passed in instance of the data access abstraction and invalidate the 80 instance handle. The close function is assumed to succeed; its return 81 code is not checked.<p> 82 83 The <tt>close_func</tt> may be set to NULL to indicate that libvorbis 84 should not attempt to close the file/data handle in <a 85 href="ov_clear.html">ov_clear</a> but allow the application to handle 86 file/data access cleanup itself. For example, by passing the normal 87 stdio calls as callback functions, but passing a <tt>close_func</tt> 88 that is NULL or does nothing (as in the case of OV_CALLBACKS_NOCLOSE), an 89 application may call <a href="ov_clear.html">ov_clear()</a> and then 90 later <tt>fclose()</tt> the file originally passed to libvorbisfile. 91 92 <h2>Tell function</h2> 93 94 The tell function is intended to mimic the 95 behavior of <tt>ftell()</tt> and must return the byte position of the 96 next data byte that would be read. If the data access cursor is at 97 the end of the 'file' (pointing to one past the last byte of data, as 98 it would be after calling <tt>fseek(file,SEEK_END,0)</tt>), the tell 99 function must return the data position (and thus the total file size), 100 not an error.<p> 101 102 The tell function need not be provided if the data IO abstraction is 103 not seekable, or the application wishes to force streaming 104 behavior. In this case, the <tt>tell_func</tt> and <tt>seek_func</tt> 105 fields should be set to NULL.<p> 106 107 <br><br> 108 <hr noshade> 109 <table border=0 width=100%> 110 <tr valign=top> 111 <td><p class=tiny>copyright © 2007 Xiph.org</p></td> 112 <td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td> 113 </tr><tr> 114 <td><p class=tiny>Vorbisfile documentation</p></td> 115 <td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td> 116 </tr> 117 </table> 118 119 </body> 120 121 </html> 122
