Pointers — The Stupid and The Informative

Pointers can do arithmetic, but if you have int a[3] = {1, 2, 3}, then when you access the second element with a[1], C will automatically convert that into *(a + 1). You might think that +1 is just plus one like in maths, but in reality, since an int is four bytes, it really means “jump” by 1 * sizeof(int) bytes.

Stupid What Ifs:

• If p points to 0x10 and q points to 0x11, q - p < NULL might result in true, but what we know is that the value would be useless as it’d point to a garbage location, and because pointers are unsigned, maybe it will overflow back and become 0xMAX - 0x-1. (And it’s also very stupid.
• Because in normal circumstances, -1 on type pointer doesn’t actually deduct one, if you were delusional, it wouldn’t be easy to treat an (say) int* as a normal number.

• Trying to bypass the const limitation via code like the below is undefined behavior, i.e. anything can happen:

    #include <stdio.h>
  
    int main(void)
    {
        const int a = 1;
        int * b = (int *) & a;
        *b = 5;
        printf("a: %d, b: %d\n", a, *b);
        return 0;
    }
  

void *

C’s version of “generics” (variables of a generic type, aka, no specific type), but for pointers (the non-pointer equivalent doesn’t exist).

E.g. the free() function is a function that accepts the pointer to the memory you want to free as void*, since it frees anything of any type.

#C #C/conceptual #memory