Can't assign values to variable and pointer

You leave the pointer with uninitialized value. So when you dereference it (*ptr), you access arbitrary place in memory, resulting in a segmentation fault.

Point ptr at something by assigning to ptr itself (not *ptr) an address of a variable (like &i) or some freshly allocated memory (like malloc(sizeof(int))).

Here is the answer for C:

int main(void) {
    int i;
    int * ptr = (int *)malloc(sizeof(int));

    i = 2;
    *ptr = 5;     
    printfn("%d",*ptr); 
    free(ptr);
}

Alternatively you could for the i and *ptr assignment lines use something like:

int main(void) {
    int i;
    int * ptr;

    i = 2;
    ptr = &i;     
    printfn("%d",*ptr); // should print 2
}

Notice also that the free came out!!!