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Re: Cygwin.dll crash, alloca and custom stack
Bitmead, Chris <Chris.Bitmead <at> ag.gov.au> writes:
>
>
> I'm writing an implemention of a scheme interpreter in C, and as people
> familiar with that language know, it requires some stack manipulation. Now
> to test this out I wrote a little program....
>
> #include <stdio.h>
>
> int main() {
> char * st1;
> st1 = (void *)malloc(5000) + 5000;
> asm("mov %0, %%esp" : : "r" (st1));
> fprintf(stderr, "hello\n");
> }
>
> This program crashes silently and prints nothing under cygwin. However this
> kind of thing works fine under Linux.
>
> Now if I run it under gdb it says 0x610dfbb3 in cygwin1!_alloca () from
> /usr/bin/cygwin1.dll
>
> Which seems to indicate that the problem has something to do with alloca.
>
> Q1. Does cygwin and gcc have a real stack based alloca, or is it a fake
> simulation? Intuitively I assume that this kind of error would be caused by
> the latter.
>
> Q2. Why does this problem occur? My vague guess is that something in the
> bootstrap code for cygwin calls alloca, and because of the memory layout of
> stack and heap, creating a heap based stack makes it free it prematurely.
>
> Q3. Shouldn't this be fixed in cygwin? While this is an unusual case, it
> seems reasonable to me that a program should be able to create a new stack
> if it wants to.
>
> Q4. Is there any work around? I'm guessing that if cygwin uses a fake
> alloca, then a better implementation would fix it, but is there any way to
> substitute another one without actually rebuilding cygwin.dll (which I am
> loath to do).
>
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Do you mean the following:
=====================================================================
[ito2@intra C++]$ cat stack.c
#include <stdio.h>
int main()
{
char * st1;
st1 = (void *)malloc(5000) + 5000;
asm("mov %0, %%esp" : : "r" (st1));
fprintf(stderr, "hello\n");
}
[ito2@intra C++]$ rm stack
[ito2@intra C++]$ cat stack.c
#include <stdio.h>
int main()
{
char * st1;
st1 = (void *)malloc(5000) + 5000;
asm("mov %0, %%esp" : : "r" (st1));
fprintf(stderr, "hello\n");
}
[ito2@intra C++]$ make stack
cc stack.c -o stack
[ito2@intra C++]$ ./stack
Segmentation fault
=====================================================================
I just can see that your code crashes there in both cygwin and linux.
Jason
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