| 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> |
|---|
