Get name of variable from its address?

Question

I'm wondering if it's possible to get access to a variable through its memory adress in C. For example, I create a variable "aa", then I print its memory adress through printf and &aa ... Assume that its adress is 12345. Is there a way to get the variable by only using 12345 and not pointers, like that :

printf("%d", 12345);

By this way, I would like that the instruction print "aa".

Sorry if it's not clear, I will show you with the code:

int main()
{
    int aa=1; //assume its adress is 12345
    printf("%d", 12345); // I want to use 12345 as parameter to print the variable
}

Thank you very much.

Answer 1

Assuming the address of your variable is definitely 12345:

printf("Value at the address %d:%d", 12345, *(int*)12345);

In a more general way:

int aa = 10; // Your variable 'aa'
int* aa_ptr = &aa; // 'aa_ptr' contains the address of the variable aa.

printf("The variable 'aa' is at the address %p and contains the value: %d", aa_ptr, *aa_ptr); // '*aa_ptr' would show the value of the variable 'aa'

The address of a memory is called 'pointer' in C. Have a look to the concept of pointer in C for more explanation: first tutorial in google

Note: For your information variable are often at least 8bit aligned (and more often 32bit aligned) - the adress should be a even number. So it is unlikely you will see a variable address at an odd address like 12345, 12344 or 12346 are more likely to be valid addresses. Accessing a non-aligned address could sometime crash your program.

Answer 2

That is what pointers are made for:

int aa = 12      ;   // defines an integer variable

int *pt = &aa;       // declare a pointer to the above variable

printf("Variable aa is at address %p\n", pt);
printf("Its value is %d\n", *pt);

In real mode programming when hardware registers are mapped at well know locations, it is even legal to store an absolute address in a pointer:

char * screenmem = 0xB800; // absolute address of the screen text buffer on a PC in REAL MODE
screenmem[0] = 'A';        // writes a A in upper left corner

But apart from:

• taking the address of an existing object
• using a well known address

dereferencing a pointer pointing to an arbitrary address (int * pt = 12345; *pt = 12;) invokes Undefined Behaviour.

Answer 3

Well, for sake of theory, yes you can do it. The question you should be asking yourself is why or when do you need it?

One way of what you expect,can be done as:

int var = 0;
int * pVar = NULL;

pVar  = &var;

printf ("The address is %p\n", (void *)&var);
printf ("The same address (via pointer variable) is %p\n", (void *)pVar );

printf ("The value at the address is %d\n", *(&var));
printf ("The value at the address (via pointer) is %d\n", *pVar );

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