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Date: Sat, 17 Oct 2009 06:41:28 +0100
From: Dave Korn <dave DOT korn DOT cygwin AT googlemail DOT com>
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Subject: Re: fork failure?
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Charles Wilson wrote:

> I have a hunch that an STC (okay, less-hellaciously-complicated test
> case) could be developed, using just GNU pth and avoiding all the
> libassuan/gnupg gobbledygook.

  Oh yuck.  So there's this alternative user-land pthreads library that runs a
scheduler within a single real machine thread, using some hairy sjlj hackery
to perform context switches?  That's kinda asking for trouble, isn't it?

  Anyway, look here: pth_mctx.c line ~ 514

> /*
>  * VARIANT 5: WIN32 SPECIFIC JMP_BUF FIDDLING
>  *
>  * Oh hell, Win32 has setjmp(3), but no sigstack(2) or sigaltstack(2).
>  * So we have to fiddle around with the jmp_buf here too...
>  */
> 
> #elif PTH_MCTX_MTH(sjlj) && PTH_MCTX_DSP(sjljw32)
> intern int
> pth_mctx_set(pth_mctx_t *mctx, void (*func)(void),
>                      char *sk_addr_lo, char *sk_addr_hi)
> {
>     pth_mctx_save(mctx);
> #if i386
>     mctx->jb[7] = (int)sk_addr_hi;
>     mctx->jb[8] = (int)func;
> #else
> #error "Unsupported Win32 architecture"
> #endif
>     sigemptyset(&mctx->sigs);
>     mctx->error = 0;
>     return TRUE;
> }

  Umm, yes.  Poking around directly inside a sigjmp_buf.  Wonder if the layout
is actually what that code expects it to be or not?  That's where I'd start
looking next, anyway, if I was wondering why maybe things were randomly
jumping to unexpected places ...

    cheers,
      DaveK



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