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Path: utzoo!mnetor!uunet!nbires!isis!scicom!milehi!Ray_Gardner
From: Ray_Gardner@milehi (Ray Gardner)
Newsgroups: comp.lang.c
Subject: pointer arithmetic question
Message-ID: <0.21787448@milehi>
Date: Sat, 17-Oct-87 22:40:42 EDT
Article-I.D.: milehi.0.21787448
Posted: Sat Oct 17 22:40:42 1987
Date-Received: Sun, 18-Oct-87 23:38:29 EDT
Organization: FidoNet node 104/56 - Mile Hi Tech, Littleton CO
Lines: 36

I have coded something like this:
struct s {
   char junk1[72];
   long fpos;         /* file pos. of buffer contents */
   int junk2;
   char junk3;
   char buf[4097];
   } *p;              /* pointer to buffer structure */
char *p1;             /* pointer to somewhere in p->buf */
 
  (p1 - p->buf + p->fpos) + 101    <--- is this well-defined?
 
This worked as I expected on a DOS compiler and a UNIX compiler, but 
XENIX on a TI 1100 (an 80286 machine) gave me something different. 
Roughly, the code generated was
   mov   bx,_p          ; pointer to structure

   mov   ax,[bx+72]     ; lsw of p->fpos
   add   ax,_p1         ; add pointer to loc. in p->buf

   sub   ax,_p          ; subtract pointer to structure
   add   ax,22          ; add 22 = 101 - (offset of buf)
   cwd                  ; convert word to double
 
Is the compiler free to make this interpretation?  I cured the problem 
with a cast:
   ((long)(p1 - p->buf) + p->fpos) + 101


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