Windows segfault-catching only works for the main thread

[simonmar]

On Windows, the RTS tries to catch segmentation faults and divide-by-zero exceptions using structured exception handling in rts/Main.c. Unfortunately this only works for the main thread, so if the exception occurs in another thread it won't be caught. GHCi runs all its computations in a separate thread, hence derefnull(ghci) and divbyzero(ghci) are failing.

[igloo]

See also the mingw GCC 4.4.0 announcement info on exceptions:  http://sourceforge.net/project/shownotes.php?release_id=691876

[igloo]

This program, with the seh_excn.* files in the GHC tree:

#include "seh_excn.h"

void g(int *p) {
    printf("1 Points to %d\n", *p);
    printf("2 Not %d\n", 5 / 0);
    printf("3 Points to %d\n", *p);
}

void h() {
    printf("1 Points to %d\n", 6);
    printf("2 Not %d\n", 5 / 0);
    printf("3 Points to %d\n", 7);
}

void f(int *p) {
    BEGIN_CATCH
    g(p);
    // h();
    END_CATCH
}

int main(void) {
    int p;
    p = 28;
    printf("Start\n");
    f(&p);
    printf("End\n");
}

doesn't work properly:

$ ./seh
Start
1 Points to 28

(note that we don't get a "divide by zero" message) whereas if you change it to call h rather than g then it does:

$ ./seh
Start
1 Points to 6
divide by zero

This is a problem because I think we want to do the exception handling in workerStart, where we need to pass the task argument on.

I've looked at using the simpler-looking AddVectoredExceptionHandler instead (which would mean we have to drop Win 2k support, I think), but I'm getting

C:\cygwin\tmp/ccYnOVfO.o:seh2.c:(.text+0xe3): undefined reference to `AddVectoredExceptionHandler'
C:\cygwin\tmp/ccYnOVfO.o:seh2.c:(.text+0xf6): undefined reference to `RemoveVectoredExceptionHandler'
collect2: ld returned 1 exit status