Can I do arithmetic on void * pointers in C?

Question

is this valid

void *p = &X; /* some thing */
p += 12;

and if so what does p now point to? I have (third party) code that does this (and compiles cleanly) and my guess is that the void * was treated as a char *. My trusty K&R is silent(ish) on the topic

EDIT: My little test app runs fine on gcc 4.1.1 and treats void * as char *. But g++ barfs

I know how to do it properly. I need to know if I have to clean this code base to find all the places its done.

BTW gcc -pedantic throws up a warning

Summary:

The C spec is ambiguous. It says that in terms of representation and use as function parameters void* =char*. But it is silent regarding pointer arithmetic.

• gcc (4) permits it and treats it as char *
• g++ refuses it
• gcc -pedantic warns about it
• vs2010 both c and c++ refuses it

Answer 1

No this is not legal. A void* cannot be arbitrarily incremented. It needs to be cast to a specific type first.

If you want to increment it by a specific number of bytes then this is the solution I use.

p = ((char*)p) + 12;

The char type is convenient because it has a defined size of 1 byte.

EDIT

It's interesting that it runs on gcc with a warning. I tested on Visual Studio 2010 and verified it does not compile. My limited understanding of the standard would say that gcc in the error here. Can you add the following compilation flags

-Wall -ansi -pedantic