更改存储在C中变量中的内存地址

Question

I have the following C program and I would like to change the variable secret by changing the memory address. Can you please show me with an example, what I should put for the following two input to accomplish this. Any help would be greatly appriciated

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

unsigned secret = 0xdeadbeef;

int main(int argc, char **argv){
    unsigned *ptr;
    unsigned value;


    printf("Welcome! I will grant you one arbitrary write!\n");
    printf("Where do you want to write to? ");
    scanf("%p", &ptr);
    printf("Okay! What do you want to write there? ");
    scanf("%p", (void **)&value);

    printf("Writing %p to %p...\n", (void *)value, (void *)ptr);
    *ptr = value;
    printf("Value written!\n");

    if (secret == 0x1337beef){
        printf("Woah! You changed my secret!\n");
        exit(0);
    }

    printf("My secret is still safe! Sorry.\n");
}

If you can, please show me with an example

Answer 1

Make a variation that prints the address of secret . Compile and run the variant program.

Edit: The details of addresses and allocations are implementation-specific. So there's no way to discover this number just from the C standard. The value returned by printf("%p",... (and affected by scanf("%p",... ) is dependant upon the specific OS, and the particular setup of the compiler. For example, I have two versions of gcc on my Windows machine, one under Cygwin and one under MinGW. And it would be very surprising if printf("%p",some_static_variable) printed the same value in both environments.

Answer 2

Just add a printf("Address of secret is: %p", &secret); above anything else, then when you run the program, give the first input the actual address it printed out (&secret), and for the second input give the program "1337beef". For example, on my machine, it stated that the variable secret is located at 0x804a02c, so for the first input I gave it "804a02c" and for the second "1337beef" so it enters the if statement. Have fun.