Question

Okay I know that main()‘s automatic local variables are stored in the stack and also any function automatic local variables too, but when I have tried the following code on gcc version 4.6.3:

#include <stdio.h>

    int main(int argc, char *argv[]) {

        int var1;
        int var2;
        int var3;
        int var4;
        printf("%p\n%p\n%p\n%p\n",&var1,&var2,&var3,&var4);
        }

the results are :

0xbfca41e0
0xbfca41e4
0xbfca41e8
0xbfca41ec

according to the results var4 on the top of the stack and var1 on the bottom of the stack and the stack pointer now pointing on the address below var1 address….but why var4 on the
top of the stack and var1 on the bottom…its declared after var1 so I think logically that var1 should be on the top of the stack and any variable declared after var1 should be below
it in memory…so in my example like this:

>>var1  at 0xbfca41ec
>>var2  at 0xbfca41e8
>>var3  at 0xbfca41e4
>>var4  at 0xbfca41e0
>>and stack pointer pointing here  
..
..

---

EDIT 1:

After reading the comment by @AusCBloke I’ve tried the following code :

#include <stdio.h>


 void fun(){
 int var1;
 int var2;
 printf("inside the function\n");
 printf("%p\n%p\n",&var1,&var2);

 }


 int main(int argc, char *argv[]) {


int var1;
int var2;
int var3;
int var4;



printf("inside the main\n");
printf("%p\n%p\n%p\n%p\n",&var1,&var2,&var3,&var4);
fun();
return 0;
}

And the results :

inside the main
0xbfe82d60
0xbfe82d64
0xbfe82d68
0xbfe82d6c
inside the function
0xbfe82d28
0xbfe82d2c

so the variables inside fun() stack frame are below the variables inside main() stack frame and that’s true according to the nature of the stack ,..but inside the same stack frame its not necessary to be ordered from top to the bottom.

thanks @AusCBloke….. your comment helped me a lot

Answer 1

There is no requirement for these variables to be allocated in the order in which they were declared. They can be moved around by the compiler, or even optimized out entirely. If you need the relative addresses to stay the same, use a struct.