x86 stack change code

stack.c

// Program to demonstrate how to change to a new stack section on x86

#include <stdlib.h>
#include <stdio.h>

#define ALTSTACK
void* altStack = 0;

void core()
{
    char dummy;
    
    printf("in core! [dummy = %p]\n", &dummy);
}

void test()
{
#ifdef ALTSTACK
    // Setup the alternate stack and call core
    void* oldStack = 0;
    
    __asm__("movl %%esp, %[oldstack]\n\
             movl %[altstack], %%esp"
        : [oldstack] "=m" (oldStack)
        : [altstack] "m" (altStack));
#endif
        
    core();

#ifdef ALTSTACK
    __asm__("movl %[oldstack], %%esp"
        : /* no outputs */
        : [oldstack] "m" (oldStack));        
#endif
}

int main(int argc, char** argv)
{
    altStack = malloc(1024000);
    printf("altStack = %p\n", altStack);
    test();
    free(altStack);
}