Warning: pointer of type 'void *' used in subtraction

Question

Although it runs correctly, the following results in the aforementioned compiler warning:

return ((item - (my->items))/(my->itemSize));

'item' is a 'void *'; 'my->items' is a 'void *'; 'my->itemSize' is an 'int'

Casting 'item' and 'my->items' as an 'int *' caused the program to run improperly. What is the best way to remove the warning?

Answer 1

Additions and subtractions with pointers work with the size of the pointed type:

int* foo = 0x1000;
foo++;
// foo is now 0x1004 because sizeof(int) is 4

Semantically speaking, the size of void should be zero, since it doesn't represent anything. For this reason, pointer arithmetic on void pointers should be illegal.

However, for several reasons, sizeof(void) returns 1, and arithmetic works as if it was a char pointer. Since it's semantically incorrect, you do, however, get a warning.

To suppress the warning, use char pointers.