memory in C

Question

#include <stdio.h>

int main() {
int a=3;
int b [] = {1,2};
int c = 4;
b[3]=10;
printf ("a = %d",a);
return 0;
}

output is a = 10;

lifo rule, how compiller allocate memory?

Answer 1

It subtracts the stack pointer by a certain value to store local variables.
In this case the compiler allocates the variables at an offset of
a at 0x2C
b+0 at 0x20 and b+1 at 0x24
c at 0x28

By doing b+3 (b[3]) you get 0x20 + 4 * 3 = 0x2C which matches the variable a, so it overwrites a instead.

With optimizations enabled the compiler, in this case, loads 0x3 directly into the edx register of the cpu so there this doesn't happen.