Can an address be assigned to a variable in C?

Question

Is it possible to assign a variable the address you want, in the memory?

I tried to do so but I am getting an error as "Lvalue required as left operand of assignment".

int main() {
  int i = 10;
  &i = 7200;
  printf("i=%d address=%u", i, &i);
}

What is wrong with my approach? Is there any way in C in which we can assign an address we want, to a variable?

Solution 1

Not directly. You can do this though : int* i = 7200; .. and then use i (ie. *i = 10) but you will most likely get a crash. This is only meaningful when doing low level development - device drivers, etc... with known memory addreses.

Solution 2

C language provides you with no means for "attaching" a name to a specific memory address. I.e. you cannot tell the language that a specific variable name is supposed to refer to a lvalue located at a specific address. So, the answer to your question, as stated, is "no". End of story.

Moreover, formally speaking, there's no alternative portable way to work with specific numerical addresses in C. The language itself defines no features that would help you do that.

However, a specific implementation might provide you with means to access specific addresses. In a typical implementation, converting an integral value Ato a pointer type creates a pointer that points to address A. By dereferencing such pointer you can access that memory location.

Solution 3

Assuming you are on an x86-type processer on a modern operating system, it is not possible to write to aribtray memory locations; the CPU works in concert with the OS to protect memory so that one process cannot accidentally (or intentionally) overwrite another processes' memory. Allowing this would be a security risk (see: buffer overflow). If you try to anyway, you get the 'Segmentation fault' error as the OS/CPU prevents you from doing this.

For technical details on this, you want to start with 1, 2, and 3.

Instead, you ask the OS to give you a memory location you can write to, using malloc. In this case, the OS kernel (which is generally the only process that is allowed to write to arbitrary memory locations) finds a free area of memory and allocates it to your process. The allocation process also marks that area of memory as belonging to your process, so that you can read it and write it.

However, a different OS/processor architecture/configuration could allow you to write to an arbitrary location. In that case, this code would work:

#include <stdio.h>
void main() {
  int *ptr;
  ptr = (int*)7000;
  *ptr = 10;
  printf("Value: %i", *ptr);
}

Solution 4

Not portably. But some compilers (usually for the embedded world) have extensions to do it.

For example on IAR compiler (here for MSP430), you can do this:

static const char version[] @ 0x1000 = "v1.0";

This will put object version at memory address 0x1000.

View more solutions