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<h4 class="subsection">13.5.1 Linked Channels</h4>

<p><a name="index-linked-channels-1246"></a>
Channels that come from a single opening share the same file position;
we call them <dfn>linked</dfn> channels.  Linked channels result when you
make a stream from a descriptor using <code>fdopen</code>, when you get a
descriptor from a stream with <code>fileno</code>, when you copy a descriptor
with <code>dup</code> or <code>dup2</code>, and when descriptors are inherited
during <code>fork</code>.  For files that don't support random access, such as
terminals and pipes, <em>all</em> channels are effectively linked.  On
random-access files, all append-type output streams are effectively
linked to each other.

   <p><a name="index-cleaning-up-a-stream-1247"></a>If you have been using a stream for I/O (or have just opened the stream),
and you want to do I/O using
another channel (either a stream or a descriptor) that is linked to it,
you must first <dfn>clean up</dfn> the stream that you have been using. 
See <a href="Cleaning-Streams.html#Cleaning-Streams">Cleaning Streams</a>.

   <p>Terminating a process, or executing a new program in the process,
destroys all the streams in the process.  If descriptors linked to these
streams persist in other processes, their file positions become
undefined as a result.  To prevent this, you must clean up the streams
before destroying them.

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