How do I detect child input?

[Jim Hayes @wayward]

Hi all,

  I'm working on a source-level debugger which runs on a Sun3 using SunOS 4.0.
We spawn the target program (the program being debugged) using vfork(2) and keep
control over it using ptrace(2). We want to have complete control over the
child's terminal input and output so we can direct it to an i/o window we've
set up; in order to do this we replace the child's stdin, stdout, and stderr
descriptors with one end of a pipe, the other end of which the debugger watches.
This works fine for the child's output: we check the pipe periodically using
fstat(2), and when the child writes something onto the pipe we read it and
display it to the window. But I sure can't figure out how to detect when the
child is waiting for input. Right now we make the assumption that anything the
user types while the child is running is intended for the child, and we send it
down the pipe. This works ok, but it's not "right" and has several problems
associated with it. Any suggestions? We're working under the restriction that
we're not allowed to muck with the target's code (at least not much). I've beat
my head against this one for quite a while now, so I'd greatly appreciate any
help folks can offer.

--Jim

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"There's no problem so awful that you can't    |   Jim Hayes
 add some guilt to it and make it even worse!" |   ...!ucsd!telesoft!jjh