Path: utzoo!attcan!uunet!mcsun!ukc!edcastle!elee24
From: elee24@castle.ed.ac.uk (H Bruce)
Newsgroups: comp.lang.c
Subject: Type conversion within arithmetic expressions. What does ANSI say ?
Message-ID: <6584@castle.ed.ac.uk>
Date: 4 Oct 90 11:16:15 GMT
Organization: Dept. Electrical Engineering, Edinburgh University, Scotland, UK.
Lines: 21

How should the following code fragment compile ?

unsigned char x,y;
unsigned short int z;
z=x*y;

According to the ANSI standard, if the result of x*y exceeds 255, 
will all 16 bits be copied to z  or only the lower 8 ?
(i.e if x=2 and y=200 does z=400 or 400%256 ?)

I have a C compiler for the 8051 which does not copy the upper byte across
even if z=x*y is replaced with z=(unsigned short int)x*y.
I want to confirm that this is a bug.
Microsoft C does convert the result into 16 bits without a type cast.

I can't find reference to this in any "ANSI" C books that we have.


Thanks,

Henry Bruce.