Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!mentor.cc.purdue.edu!acu
From: acu@mentor.cc.purdue.edu (Floyd McWilliams)
Newsgroups: comp.lang.c
Subject: Re: The Dangers of sizeof
Summary: sizeof is unary op.
Keywords: sizeof
Message-ID: <2280@mentor.cc.purdue.edu>
Date: 10 Apr 89 14:57:52 GMT
References: <9969@smoke.BRL.MIL> <3.UUL1.3#5109@pantor.UUCP> <105@servio.UUCP>
Reply-To: acu@mentor.cc.purdue.edu (Floyd McWilliams)
Organization: PHAP
Lines: 51

In article <105@servio.UUCP> penneyj@servio.UUCP (D. Jason Penney) writes:

>sizeof (aType) is actually the same as

>sizeof (aType)(1)

>(i.e, casting the empty expression).  People who uniformly treat sizeof
>as if it were a function run a grave risk.  Trick question:  what is the
>value of:

>sizeof (char) - 1

>Answer:  1 -- the (char) is a cast, which is of higher precedence than
>sizeof, hence it parses as,

>sizeof ((char)(- 1))

	Oh really?  Then take a look at this:

--
Script started on Sun Apr  9 13:41:38 1989
tcsh% more foo.c
#include        <stdio.h>

main()

{
  printf("sizeof(char) - 1 == %d.\n",sizeof(char) - 1);
}
tcsh% cc foo.c
tcsh% a.out
sizeof(char) - 1 == 0.
tcsh% 

script done on Sun Apr  9 13:42:09 1989
--

>I recommend ALWAYS using the operator form of sizeof and parenthesizing
>the entire sizeof expression for safety.  Note:  you MUST parenthesize
>the operand of sizeof if it is a type:

>(sizeof (char)) - 1

	"sizeof" may not be a function, but it does have a high precedence
(it's a unary operator).  If what you posted were true, then everyone who
tried a "sizeof(foo) + CONST" under pre-ANSI standard C would have been
screwed; C didn't have a unary plus.

--
"Life's for my own, to live my own way."
Floyd McWilliams			mentor.cc.purdue.edu!acu