"swapfd" (more unix ideas)

[Jeff Anton]

In article <288 at azure.UUCP> stevesu at azure.UUCP (Steve Summit) writes:
>> 1) have a swapfd system call
>>         Usage:  swapfd(fd1,proc,fd2)
>> or possibly:    swapfd(proc1,fd1,proc2,fd2)
>> 
>> what this does, is makes procs (usually another process) fd2, to become
>> my fd1, and my fd1 to become (point to a file structure) of procs fd2.
>
>I don't know if it's what the original submittor had in mind, but
>the reason I've thought a call like this would be neat is because
>you could use it to give a shell the ability to manipulate i/o
>"after the fact," just like job control lets you manipulate
>backgroundness/foregroundness "after the fact."  Is that command
>taking too long?  Type control-Z and background it.  Don't want
>the output coming out asysynchronously?  Well, I haven't thought
>of a syntax, but the idea is that you want to reconnect the
>standard output of the process to a file.  It gets hard if the
>process in question has played with its file descriptors, like
>repoening file descriptor 1 as something else, or dup'ing file
>descriptor 1 to some other file descriptor and then using it
>there.  It gets messy.
It gets very messy very quickly.  csh dups it's input and output
descriptors to around 17 from 0,1,2.  Simply assumeing 0,1,2 are
stdin, stdout, and stderr is not the way to go.  Also, if a process
forks it's children will also need to be changed.  I've worked
on what I called 'deferred redirection' where you would be allowed
to redirect a process group after it's started execution.  I believe
the most sensable solution is to allow the following call:
changefds(pgrp, fd1, fd2)
int	pgrp, fd1, fd2;

Change all occerences of file decriptors in process group pgrp
that are decendants of fd1 to refer to the file fd2.
Decendants of a file descriptor are dup's and a child's inherited
descriptor.

This can be implemented in the kernel easily by moveing the
descriptors from the user structure to the process table.
This call would allow a shell to change a commands input or output
to a file instead of the default.  However, the changes to csh
were too tricky for a simple hack job.
-- 
C knows no bounds.
					Jeff Anton
					U.C.Berkeley
					ucbvax!anton
					anton at berkeley.ARPA