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(libc.info)Stream/Descriptor Precautions

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Dangers of Mixing Streams and Descriptors
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   You can have multiple file descriptors and streams (let's call both
streams and descriptors "channels" for short) connected to the same
file, but you must take care to avoid confusion between channels.  There
are two cases to consider: "linked" channels that share a single file
position value, and "independent" channels that have their own file
positions.

   It's best to use just one channel in your program for actual data
transfer to any given file, except when all the access is for input.
For example, if you open a pipe (something you can only do at the file
descriptor level), either do all I/O with the descriptor, or construct a
stream from the descriptor with `fdopen' and then do all I/O with the
stream.

Linked Channels

Dealing with channels sharing a file position.

Independent Channels

Dealing with separately opened, unlinked channels.

Cleaning Streams

Cleaning a stream makes it safe to use

another channel.