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Channels that come from a single opening share the same file position;
we call them linked channels. Linked channels result when you
make a stream from a descriptor using
fdopen, when you get a
descriptor from a stream with
fileno, when you copy a descriptor
dup2, and when descriptors are inherited
fork. For files that don't support random access, such as
terminals and pipes, all channels are effectively linked. On
random-access files, all append-type output streams are effectively
linked to each other.
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 clean up the stream that you have been using. See section 13.5.3 Cleaning Streams.
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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