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 , and when you copy a descriptor with dup or dup2 . 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, 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 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.