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Message-ID: <602EF4562299D711978800065BEECE9A0D2D30CD@cbrml50.ag.gov.au>
From: "Bitmead, Chris" <Chris DOT Bitmead AT ag DOT gov DOT au>
To: "'cygwin AT cygwin DOT com'" <cygwin AT cygwin DOT com>
Subject: Cygwin.dll crash, alloca and custom stack
Date: Fri, 12 Aug 2005 17:07:19 +1000
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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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