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From: newman@mit-trillian.MIT.EDU (Ron Newman)
Newsgroups: comp.lang.c
Subject: Determing alignment of (char *) pointer
Message-ID: <1510@mit-trillian.MIT.EDU>
Date: Thu, 4-Dec-86 21:05:39 EST
Article-I.D.: mit-tril.1510
Posted: Thu Dec  4 21:05:39 1986
Date-Received: Fri, 5-Dec-86 04:23:05 EST
Reply-To: newman@athena.mit.edu (Ron Newman)
Organization: MIT Project Athena
Lines: 24

I am using a (char *) pointer to store a sequence of differently-typed
and differently-sized objects.  I want to make sure that each object
will be properly aligned before storing it.

In particular, I need to determine whether the pointer is 32-bit
aligned before attempting to store a long by casting it to a (long *).
Which is a better, more portable way of determining whether a pointer
is aligned?

   char *p;

   /* method 1 */

   if ((long)p & 3) ...


   /* method 2 */

   if ((long)(p - (char *) 0) & 3) ...


or if neither of these is best, what is better?

/Ron Newman