Accessing specific memory locations in C

Question

In assembly language we have instructions like:

movl ax, [1000]

This allows us to access specific memory locations.

But in C can we do something similar to this?

I know inline assembly code using asm() will allow you to do this, but I would like to know about some C specific technique to achieve this.

I tried the following code and got segmentation error:

int *ptr=0xFE1DB124;
*ptr;

This again was confusing as the memory location was identified by the code given below:

int var;
printf("\nThe Address is %x",&var);

So the memory location is available, but I am still getting a segmentation fault.

Why?

Answer 1

Common C compilers will allow you to set a pointer from an integer and to access memory with that, and they will give you the expected results. However, this is an extension beyond the C standard, so you should check your compiler documentation to ensure it supports it. This feature is not uncommonly used in kernel code that must access memory at specific addresses. It is generally not useful in user programs.

As comments have mentioned, one problem you may be having is that your operating system loads programs into a randomized location each time a program is loaded. Therefore, the address you discover on one run will not be the address used in another run. Also, changing the source and recompiling may yield different addresses.

To demonstrate that you can use a pointer to access an address specified numerically, you can retrieve the address and use it within a single program execution:

#include <inttypes.h>
#include <stdio.h>
#include <stdint.h>


int main(void)
{
    //  Create an int.
    int x = 0;

    //  Find its address.
    char buf[100];
    sprintf(buf, "%" PRIuPTR, (uintptr_t) &x);
    printf("The address of x is %s.\n", buf);

    //  Read the address.
    uintptr_t u;
    sscanf(buf, "%" SCNuPTR, &u);

    //  Convert the integer value to an address.
    int *p = (int *) u;

    //  Modify the int through the new pointer.
    *p = 123;

    //  Display the int.
    printf("x = %d\n", x);

    return 0;
}

Obviously, this is not useful in a normal program; it is just a demonstration. You would use this sort of behavior only when you have a special need to access certain addresses.