== _fork()_, _wait()_, and threads

Something I've discovered through (painful) experience is that threaded
programs cannot portably _fork()_ in one thread and then _wait()_ for
the forked process in another one.

This will work in some environments but fail explosively in others; the
one I stubbed my toe on was a Linux 2.4 kernel based system (without
[[NPTL ../linux/ThreadMemoryUse]], which is the usual state of affairs
for 2.4-based Linux distributions).

Admittedly, mixing threads and _fork()_ is a bit perverse, but
sometimes it's what you need to do.

(I saw this in a Python program, but Python isn't doing anything
special in the POSIX threads department so I have to expect that
it's completely generic.)