what can be used in a signal handler

Nick Maclaren nmm1 at cus.cam.ac.uk
Thu Jan 18 14:41:49 CET 2007

In article <3zKrh.30692$sE7.26486 at newsfe21.lga>,
hg <hg at nospam.org> writes:
|> I posted an equivalent question earlier ... but am still not sure:
|> I currently (under Linux) have a program that uses Queue.put (raw_input(''))
|> in a signal handler and Queue.get() in the main/only thread.
|> It works but I want to know if it is legal / what type of synchro API I have
|> the right to use in a signal handler.

Strictly, no and none.

C and POSIX's specifications of signal handling is a complete mess.
ALL handling of ALL real signals is undefined behaviour, though few
programmers realise that and POSIX depends on it.  As that would make
all Unices (and Microsoft systems) unusable, you can reasonably assume
that there is some signal handling that will work.

But what?  Which is where you came in.

The situation is that most things that seem to work do actually work,
in some environments and under some circumstances.  You can never be
sure when they will stop working, however, and all bets are off when
you port signal handling code to a new system.  And, really but REALLY,
do not mix it with very high levels of optimisation or rely on it
being in any way thread safe.

I can't tell you whether the code of Queue is intended to work if an
active Queue.get is interrupted by a signal which then does a Queue.put,
but that is the sort of circumstance where things get very hairy.  Even
if Python has code to enable that case, it won't be 100% reliable on all

Sorry.  But that is the situation :-(

Nick Maclaren.

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