| The Android Open Source Project | 28350ca | 2008-10-21 07:00:00 -0700 | [diff] [blame] | 1 | <html> |
| 2 | |
| 3 | <head> |
| 4 | <title>Tremor - 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>Tremor documentation</p></td> |
| 12 | <td align=right><p class=tiny>Tremor version 1.0 - 20020403</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 Tremor 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 | libvorbisidec 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 libvorbisidec, moving |
| 53 | the access cursor to the requested position.<p> |
| 54 | |
| 55 | libvorbisidec expects the following behavior: |
| 56 | <ul> |
| 57 | <li>The seek function must always return -1 (failure) if the given |
| 58 | data abstraction is not seekable. It may choose to always return -1 |
| 59 | if the application desires libvorbisidec to treat the Vorbis data |
| 60 | strictly as a stream (which makes for a less expensive open |
| 61 | operation).<p> |
| 62 | |
| 63 | <li>If the seek function initially indicates seekability, it must |
| 64 | always succeed upon being given a valid seek request.<p> |
| 65 | |
| 66 | <li>The seek function must implement all of SEEK_SET, SEEK_CUR and |
| 67 | SEEK_END. The implementation of SEEK_END should set the access cursor |
| 68 | one past the last byte of accessible data, as would stdio |
| 69 | <tt>fseek()</tt><p> |
| 70 | </ul> |
| 71 | |
| 72 | <h2>Close function</h2> |
| 73 | |
| 74 | The close function should deallocate any access state used by the |
| 75 | passed in instance of the data access abstraction and invalidate the |
| 76 | instance handle. The close function is assumed to succeed.<p> |
| 77 | |
| 78 | One common use of callbacks and the close function is to change the |
| 79 | behavior of libvorbisidec with respect to file closure for applications |
| 80 | that <em>must</em> <tt>fclose</tt> data files themselves. By passing |
| 81 | the normal stdio calls as callback functions, but passing a |
| 82 | <tt>close_func</tt> that does nothing, an application may call <a |
| 83 | href="ov_clear.html">ov_clear()</a> and then <tt>fclose()</tt> the |
| 84 | file originally passed to libvorbisidec. |
| 85 | |
| 86 | <h2>Tell function</h2> |
| 87 | |
| 88 | The tell function is intended to mimic the |
| 89 | behavior of <tt>ftell()</tt> and must return the byte position of the |
| 90 | next data byte that would be read. If the data access cursor is at |
| 91 | the end of the 'file' (pointing to one past the last byte of data, as |
| 92 | it would be after calling <tt>fseek(file,SEEK_END,0)</tt>), the tell |
| 93 | function must return the data position (and thus the total file size), |
| 94 | not an error.<p> |
| 95 | |
| 96 | The tell function need not be provided if the data IO abstraction is |
| 97 | not seekable.<p. |
| 98 | |
| 99 | <br><br> |
| 100 | <hr noshade> |
| 101 | <table border=0 width=100%> |
| 102 | <tr valign=top> |
| 103 | <td><p class=tiny>copyright © 2002 Xiph.org</p></td> |
| 104 | <td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td> |
| 105 | </tr><tr> |
| 106 | <td><p class=tiny>Tremor documentation</p></td> |
| 107 | <td align=right><p class=tiny>Tremor version 1.0 - 20020403</p></td> |
| 108 | </tr> |
| 109 | </table> |
| 110 | |
| 111 | </body> |
| 112 | |
| 113 | </html> |