how to write an address (pointer) in a text file that can be read and dereferenced in C

Question

I would like to write an address in a text file, which can be read by fscanf and dereferenced by C after reading the pointer. How do I write the address in the file?

edit: i don't mean to be rude, but i know that this is exactly what i need, so if anyone can please just list the answer, and not ethical reasons for why i shouldn't be doing this, that would be great!

further edit: ah, i was misclear about what I want to do. In emacs, I want to (with my fingers!!) write in an address that a C program could read in using fscanf and use as a pointer. How do I physically (with my fingers!!) write an address in emacs. For example, if I wanted C to read in 0x11111111, I am trying to write 0x11111111 in emacs, but it's not becoming the right address in C when i read it in.

Answer 1

You can't do it. There is no guarantee the pointer will be valid when it is read in in again - if it is read by another process it almost certainly won't be. And if it is read by the same process, why write it to a file in the first place?

Answer 2

fprintf(file, "%p", pointer);

should do the job; but it is questionable if the address is of any use, if you read the file...

Perhaps for some hardware register / IO-port or shared memory this could be interesting, but in the general case: do not use it. Segfault, sense-less data or data-corruption will be the result...

[added]

fscanf(... "%x" ...)

should read a hexadecimal coded integer, a.k.a. address.

Answer 3

Something like this maybe?

#include<stdio.h>

int main()
{
    FILE *out = fopen("test.out","w");
    int i = 6;
    int *pi = &i;

    fprintf(out, "%d", pi);
    fclose(out);

    printf("pi points to an int that has the value of %d\n", *pi);

    FILE *in = fopen("test.out","r");
    fscanf(in, "%d", &pi);
    fclose(in);

    printf("After reading the value of pi from the file, it points to an int that has the value of %d\n", *pi); 
}

If you only want to read the value, then this is all you should care about:

int *pi;
fscanf(in, "%d", &pi);
// use *pi to dereference normally

Edit to your further edit: try using "%x" to read in a hex number.

Answer 4

I don't see the problem, here is the snippet using a string instead of a file:

#include <stdio.h>

int main() {
    void *p1,*p2;
    p1 = (void*)0x12FE0FE0A;
    char str[] = "12FE0FE0A";

    sscanf(str, "%p", &p2);
    printf("%p %p\n",p1,p2);
}

the pointers are identical.

BTW, if you write "0x12345678", the format string should be "0x%p" to process correctly the "0x" prefix.

Answer 5

You can assign an integer value to a pointer type so you use a type cast, e.g.

unsigned char *vram = (unsigned char *)0xA0000000;

The caveat being that the result is implementation-defined (not undefined, mind you), and might not even be usable.