here's a different little hack that's more unix-specific,
which i borrowed from unix libthread (i think that's where i saw it,
although it might be well-known in fcntl land).

a parent process needs to know whether a child has successfully done an exec
of an arbitrary program (so it's hard to have that program tell the parent).
the parent makes a pipe, marks the child's end as close-on-exec, forks, and then
reads its end of the pipe.  if the exec succeeds, the read returns 0; if the
exec fails, the child writes the diagnostic on the pipe, so again the read returns something.