warning: pointer of type ‘void *’ used in arithmet

Question

I am writing and reading registers from a memory map, like this:

//READ
return *((volatile uint32_t *) ( map + offset ));

//WRITE
*((volatile uint32_t *) ( map + offset )) = value;

However the compiler gives me warnings like this:

warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]

How can I change my code to remove the warnings? I am using C++ and Linux.

Answer 1

If the use of arithmetic on void pointers is really what you want as it is made possible by GCC (see Arithmetic on void- and Function-Pointers) you can use -Wno-pointer-arith to suppress the warning.

Answer 2

Type void is incomplete type. Its size is unknown. So the pointer arithmetic with pointers to void has no sense. You have to cast the pointer to type void to a pointer of some other type for example to pointer to char. Also take into account that you may not assign an object declared with qualifier volatile.

Answer 3

Since void* is a pointer to an unknown type you can't do pointer arithmetic on it, as the compiler wouldn't know how big the thing pointed to is.

Your best bet is to cast map to a type that is a byte wide and then do the arithmetic. You can use uint8_t for this:

//READ
return *((volatile uint32_t *) ( ((uint8_t*)map) + offset ));

//WRITE
*((volatile uint32_t *) ( ((uint8_t*)map)+ offset )) = value;