Question

With a 32-bit OS, we know that the pointer size is 4 bytes, so sizeof(char*) is 4 and sizeof(int*) is 4, etc. We also know that when you increment a char*, the byte address (offset) changes by sizeof(char); when you increment an int*, the byte address changes by sizeof(int).

My question is:

• How does the OS know how much to increment the byte address for sizeof(YourType)?

Answer 1

The compiler only knows how to increment a pointer of type YourType * if it knows the size of YourType, which is the case if and only if the complete definition of YourType is known to the compiler at this point.

For example, if we have:

struct YourType *a;
struct YourOtherType *b;

struct YourType {
    int x;
    char y;
};

Then you are allowed to do this:

a++;

but you are not allowed to do this:

b++;

..since struct YourType is a complete type, but struct YourOtherType is an incomplete type.

The error given by gcc for the line b++; is:

error: arithmetic on pointer to an incomplete type