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| <h4 class="subsection">13.5.2 Independent Channels</h4> |
| |
| <p><a name="index-independent-channels-1248"></a> |
| When you open channels (streams or descriptors) separately on a seekable |
| file, each channel has its own file position. These are called |
| <dfn>independent channels</dfn>. |
| |
| <p>The system handles each channel independently. Most of the time, this |
| is quite predictable and natural (especially for input): each channel |
| can read or write sequentially at its own place in the file. However, |
| if some of the channels are streams, you must take these precautions: |
| |
| <ul> |
| <li>You should clean an output stream after use, before doing anything else |
| that might read or write from the same part of the file. |
| |
| <li>You should clean an input stream before reading data that may have been |
| modified using an independent channel. Otherwise, you might read |
| obsolete data that had been in the stream's buffer. |
| </ul> |
| |
| <p>If you do output to one channel at the end of the file, this will |
| certainly leave the other independent channels positioned somewhere |
| before the new end. You cannot reliably set their file positions to the |
| new end of file before writing, because the file can always be extended |
| by another process between when you set the file position and when you |
| write the data. Instead, use an append-type descriptor or stream; they |
| always output at the current end of the file. In order to make the |
| end-of-file position accurate, you must clean the output channel you |
| were using, if it is a stream. |
| |
| <p>It's impossible for two channels to have separate file pointers for a |
| file that doesn't support random access. Thus, channels for reading or |
| writing such files are always linked, never independent. Append-type |
| channels are also always linked. For these channels, follow the rules |
| for linked channels; see <a href="Linked-Channels.html#Linked-Channels">Linked Channels</a>. |
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