Re: [PATCH] block-sha1/sha1.c: silence compiler complaints by casting void* to uintptr_t

From: Brandon Casey <casey@nrlssc.navy.mil>
Date: Aug 15, 2009, 00:52 UTC
Message-ID: <qZXfoQ5Gp5jKnRfKM6ee07YVX-hyS2HsJmQwAYdPVHw92cOuUcufJA@cipher.nrlssc.navy.mil>
In-Reply-To: <alpine.DEB.1.00.0908150207270.8306@pacific.mpi-cbg.de>

Johannes Schindelin wrote:

Show 9 quoted lines

> Hi,
> 
> On Fri, 14 Aug 2009, Brandon Casey wrote:
> 
>> Some compilers produce errors when arithmetic is attempted on pointers to
>> void.  So cast to uintptr_t when performing arithmetic on void pointers.
> 
> I am confused.  Is sizeof(*(uintptr_t)NULL) not larger than 1, and as a 
> consequence ((uintptr_t)p)+1 not different from ((void *)p)+1?

If you try this:

   printf("NULL + 1: %u\n", (void*)NULL + 1);

the MIPSpro compiler complains like:

   The expression must be a pointer to a complete object type.

        printf("NULL + 1: %u\n", (void*)NULL + 1);
                                 ^

   1 error detected in the compilation of "test.c".

Compilers other than gcc at least issue a warning, if they do not also fail.

-brandon