Question

I want to read the stack pointer register value without writing inline assembly.The reason I want to do this is because I want to assign the stack pointer register value to an element of an array and I find it cumbersome to access an array using inline assembly. So I would want to do something like that.

register "rsp" long rsp_alias; <--- How do I achieve something like that in gcc?
long current_rsp_value[NUM_OF_THREADS];

current_rsp_value[tid] = rsp_alias;

Is there anything like that possible with gcc?

Answer 1

There’s a shortcut:

register long rsp asm ("rsp");

Demo:

#include<stdio.h>

void foo(void)
{
    register long rsp asm ("rsp");
    printf("RSP: %lx\n", rsp);
}

int main()
{
    register long rsp asm ("rsp");
    printf("RSP: %lx\n", rsp);
    foo();
    return 0;
}

Gives:

 $ gdb ./a.out 
GNU gdb (Gentoo 7.2 p1) 7.2
...
Reading symbols from /home/user/tmp/a.out...done.
(gdb) break foo
Breakpoint 1 at 0x400538: file t.c, line 7.
(gdb) r
Starting program: /home/user/tmp/a.out 
RSP: 7fffffffdb90

Breakpoint 1, foo () at t.c:7
7       printf("RSP: %lx\n", rsp);
(gdb) info registers
....
rsp            0x7fffffffdb80   0x7fffffffdb80
....
(gdb) n
RSP: 7fffffffdb80
8   }

Taken from the Variables in Specified Registers documentation.