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Aucbvax.5573 fa.unix-wizards utzoo!decvax!ucbvax!unix-wizards Sun Dec 27 01:46:39 1981 multiple standard inputs >From mo@LBL-UNIX Sun Dec 27 01:33:39 1981 I have been thinking for a long time how to add the notion of multiple standard inputs (and outputs?) to programs. Mostly this is for the notion of a "command and control input" for control arguments, with the "real data" coming over another stdin. However, it would be nice if "diff3" or "pr" could take multiple inputs from other programs. One simple scheme I have sketched is as follows. It is simply a programming convention. Since on most systems (v7 and later) it is possible to have file descriptors tidied-up when you exec, I think it would be possible to consider any open descriptor greater than 2 (std-error) to be a standard input. This way, a program would probe its descriptor space by either trying a read (and getting an error), or opening /dev/null to see what the first free descriptor was, with everything greater than 2 and less than the descriptor to /dev/null being considered to be a standard input. Another way, cleaner, but requiring a kernel hack, would be to allow marking descriptors with "colors" and a call to probe the descriptor space. The Berkeley proposal for non-blocking I/O uses a similar approach and might integrate the whole notion in one mechanism. There is already a flag word for each file descriptor; adding "stdin", "stdout", and "stderr" bits wouldn't be too bad. That way a common mechanism does it all.

One unresolved problem is the necessary shell syntax for this. The Bourne shell almost has everything you need, if not packaged too elegantly (for this purpose, only!). There has been work done other places (the Software Tools world) with shells allowing networks of processes and they have been fairly sucessful. (And this was an implementation which required pipes to be faked with temp files, after computing a sufficient partial order!)

Any comments or thoughts on this general issue?? (Fool that I am....)

-Mike

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