how do I know when a process wants data

[Perry Hutchison]

In article <511 at ccadfa.adfa.oz.au> cjsv at cs.adfa.oz.au (Christopher J S Vance)
writes:

+ I want to run a process (call it dexec) which fork's and exec's an
+ ordinary Unix process (call it X) using a file descriptor shared with
+ dexec.  I want dexec to be notified that X is blocked waiting for input
+ from the shared fd ...

Would it work to make like ps(1) and read /dev/kmem?  I suppose that's polling
of a sort, but short of kernel hacking I don't know what else would work in
the general case.  If you know enough about X, you could do like dbx(1) and
set breakpoints in X whenever it's about to read.