UNIX for Dummies Questions & Answers

Is it possible to write to file descriptor 0 and read from 1 or 2? How could this be implemented?

I fail to see what you gain by doing this as a general idea.

In theory close the descriptors, then open them again directed somewhere else.

As a special case:
When you write a daemon all three of these descriptors are closed and then often reopened and directed to /dev/null. This means because the process runs "in the dark" it cannot directly write or read any of the standard descriptors. This prevents signals like SIGTTOU being sent to the process if it inadvertantly tries to write to stdout while running in detached mode.

Thank you for getting back to me. I understand that stdin is normally used for incoming data (from the keyboard) and that we usually write to stdout and/or stderr. I was just wondering how I could write something to stdin and read from stdout and/or stderr. I figure that it involves the file descriptors and using pipes but I'm really not sure how that works.

You can write to stdin all you want, it just won't work. If you want it to do anything meaningful you have to reopen stdin as something you can write to.