Question

Let’s say we have an array defined as a global variable.

int array[] = {16, 25, 36, 49, 64};

If this is compiled as a shared library, compiler will produce a binary with a symbol “array” pointing to the location in memory of the array.

Is it possible to add a global variable that will represent a memory location that is inside of the array.

int elem;

Can it be some how made that elem represents the same location as array[2]? Is that even possible with just C?

EDIT:

Can it be done without involving pointers? I am interested into making elem being the location in memory at witch array[2] resides. With int* elem = &array[2] memory is set aside for a pointer and elem becomes a symbol for that pointer, and then the memory adress of array[2] is put in there. I would like that elem becomes a symbol for a location of array[2], so that assert(elem == array[2]) would pass always. Like an identity in math (≡).

Does anybody know is the thing I am interested in possible in plain C, or only in assembly.

Answer 1

This is impossible in plain C without using a pointer.

You can, however, use some linker trickery to achieve this (beware that this is not portable and very, very hacky): Get your default linker script from ld -verbose and edit it to include something like elem = (array) + 4 * 2;, then compile with -Wl,-Tyour_script.ld. elem should now occupy the same memory address as array + 8 Bytes (which is array[2] assuming sizeof(int) == 4):

 $ cat a.c
 #include <stdio.h>

 int foo[4] = {1,2,3,4};

 int bar;

 int main() {
    printf("%p %p\n", &foo[2], &bar);
    return 0;
 }
 $ gcc -Wl,-Ta.ld a.c
 $ ./a.out
 0x6008e8 0x6008e8
 $