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From: franka@mmintl.UUCP
Newsgroups: comp.lang.c
Subject: Re: Pointer Comparison and Portability
Message-ID: <2000@mmintl.UUCP>
Date: Mon, 23-Feb-87 15:09:06 EST
Article-I.D.: mmintl.2000
Posted: Mon Feb 23 15:09:06 1987
Date-Received: Sat, 28-Feb-87 00:25:49 EST
References: <814@cullvax.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT
Lines: 20

In article <814@cullvax.UUCP> drw@cullvax.UUCP writes:
>john@viper.UUCP (John Stanley) writes:
>>   In the example he gave:
>> 	  char p[16], q[16];
>> (Details of 8086 memory addressing omitted.)
>> The byte of memory addressed by q[0] and p[17] would
>> be the exact same byte, but the two pointers 0002:0000 and 0001:0010 would
>> be different.
>
>Yes, but comparing &p[17] and &q[0] is comparing two addresses derived
>from different allocations, and as the standard says, that is
>implementation defined (i.e., may not work).  Note also that &p[17] is
>a correct *pointer*, but attempting to fetch or store anything through
>it is not.  &p[18] is not a correct pointer.

Actually, of course, it is p[16] which is equal to q[0], and &p[17] which is
not a correct pointer.  Isn't zero-based array indexing wonderful? (:-)

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Ashton-Tate          52 Oakland Ave North         E. Hartford, CT 06108