[Python-Dev] Strategy for converting the decimal module to C (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Wed Jul 26 09:39:40 CEST 2006

Nick Maclaren wrote:

The compiled code has made a data structure temporarily inconsistent because the operation is safe (say, list insertion), and then gets an asynchronous interrupt (e.g. SIGINT). The SIGINT handler does some operation (e.g. I/O) that implicitly uses floating-point, which then interrupts.

Well, of course anything can be made to happen asynchronously by calling it from something asynchronous, such as a SIGINT handler. That doesn't change the fact that the floating point operation itself is deterministic, including whether it causes an exception.

Well-written programs don't do any more in a signal handler than is absolutely necessary, for reasons which apply equally well whether floating point is involved or not. I'd say the mistake was made right at the beginning by assuming that the data structure in question was safe while allowing a SIGINT to occur to a handler that's not careful enough about what it does.

BTW, it seems to me you could get exactly the same problem if FP exceptions were handled entirely in user mode, as you suggest. Not that I don't agree that would be a good idea -- I do -- but it wouldn't prevent this particular kind of mistake.

And all of this is getting rather far away from where we started, which was simply instrumenting a piece of code to count floating point exceptions. Such a program isn't going to be doing I/O in SIGINT handlers or installing FP exception handlers that mess with unrelated critical data structures.

-- Greg

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