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From: ken@turtlevax.UUCP (Ken Turkowski)
Newsgroups: net.lang.c
Subject: Re: conversion of short to unsigned it
Message-ID: <706@turtlevax.UUCP>
Date: Sat, 23-Mar-85 16:28:15 EST
Article-I.D.: turtleva.706
Posted: Sat Mar 23 16:28:15 1985
Date-Received: Wed, 27-Mar-85 03:59:21 EST
References: <2125@ncrcae.UUCP> <7088@watdaisy.UUCP> <2128@ncrcae.UUCP>
Reply-To: ken@turtlevax.UUCP (Ken Turkowski)
Organization: CADLINC, Inc. @ Menlo Park, CA
Lines: 45

In article <2128@ncrcae.UUCP> wescott@ncrcae.UUCP (Mike Wescott) writes:
>	unsigned short us;
>	short s;
>	   .
>	   .
>	   .
>	s = -3;
>	us = -3;
>	ui = s;
 ...
>
>	if ((unsigned int)s == us) printf("OOPS\n");
>
>prints OOPS meaning that fffffffd == 0000fffd !!!
>The code generated for the comparison is a
>
>	cmpw	_s, _us
>
>No conversion.  K&R confused me with the "otherwise" phrase
>in the section on arithmetic conversions.  The version of the C standard
>was not much more help but it eventually became clear that the code for
>the comparison is improperly generated.  The short should be sign extend
>and the unsigned short should be padded with 0's on the left and the
>compare should be a cmpl.

I disagree.  The "(unsigned int)" is a cast, saying that s is to be
considered unsigned rather than signed.  It is NOT a conversion.  The
fact that s was declared to be a "short int" is immaterial; it is of
type int rather than float, etc.  This has the same effect as saying
"(unsigned)", without the "int".  Int has no inherent size associated
with it; the size of an int is machine-dependent.  If you want an int
of a specific size, you say "short int" or "long int".

I would suggest that you try the statement

	if ((unsigned long int)s == us) printf("OOPS\n");

instead, and see if you get the same results.  This explicitly
specifies a size conversion.

-- 

Ken Turkowski @ CADLINC, Menlo Park, CA
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