Re: void* arithmnetic

[Arnaldo Carvalho de Melo <acme@redhat.com>]

Em Thu, Nov 29, 2007 at 01:05:31AM +0100, J.A. Magallón escreveu:
> Hi all...
> 
> Since begin of the ages the build of the nvidia driver says things like
> this:
> 
> include/asm/compat.h:210: warning: pointer of type 'void *' used in arithmetic
> 
> There are several of this warnings. The code in question for this example
> is:
> 
> static __inline__ void __user *compat_alloc_user_space(long len)
> {
>     struct pt_regs *regs = task_pt_regs(current);
>     return (void __user *)regs->rsp - len;
> }
> 
> As this is dealing with mem blocks, I suppose it's counting in bytes, so
> we could do something like:
> 
>    return (void __user *)((u8*)regs->rsp - len);
> 
> so the arithmetic knows how to inc/dec for each unity...
> I think the warning is correct and that void* arithmetic is undefined in C,
> isn't it ?

Yes, but not in gcc, the language the kernel is written 8)

It is allowed and the size of a void is 1. -Wpointer-arith disables
this.

[acme@doppio ~]$ cat voidptr.c
#include <stdio.h>

int main(int argc, char *argv[])
{
        void *ptr = argv[argc - 1];

        puts(ptr + 4);
        return 0;
}
[acme@doppio ~]$ gcc -Wall voidptr.c -o voidptr
[acme@doppio ~]$ ./a Magallón
llón
[acme@doppio ~]$ gcc -Wall -Wpointer-arith voidptr.c -o voidptr
voidptr.c: In function ‘main’:
voidptr.c:7: warning: pointer of type ‘void *’ used in arithmetic
[acme@doppio ~]$ ./a Magallón
llón
[acme@doppio ~]$

- Arnaldo