Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!m.cs.uiuc.edu!kenny
From: kenny@m.cs.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: sizeof a struc field
Message-ID: <4700046@m.cs.uiuc.edu>
Date: 25 Oct 89 17:25:00 GMT
References: <7710@microsoft.UUCP>
Lines: 39
Nf-ID: #R:microsoft.UUCP:7710:m.cs.uiuc.edu:4700046:000:1855
Nf-From: m.cs.uiuc.edu!kenny    Oct 25 12:25:00 1989


Hey, wait a minute here.

Is it just the constant NULL that can't be `dereferenced' in a sizeof
expression, or is it any possibly invalid pointer?  The discussion
seems to indicate that the common idiomatic macro definition

#define NEW(p) ((p) = malloc_and_check ((size_t) (sizeof *(p))))

(assuming that malloc-and-check never returns NULL, but throws a
signal, longjmps, or something), is illegal under the proposed
standard, because it dereferences the uninitialized pointer p.  I find
it hard to believe that this behavior was the intent of the Committee.

If this interpretation is correct, I can still live with it by making
a NEW macro for every type (typeof wasn't accepted, so I can't
determine the size of the object without knowing the name of the type,
unless I can write sizeof *p), or by passing the type name as a macro
parameter, but it's pretty bogus.  I hope at least that the Standard
doesn't require that I go out of my way to make this NOT work.  I
can't imagine any way that a sane compiler writer could have this
fail, since you never even need to load the pointer p -- all the
discussion of possible faults upon loading a pointer into a register
doesn't apply, since this expression never examines the value of the
pointer at all.

I would submit that `dereferencing' a pointer should mean reading from
or writing into the location to which it points.  Just putting a * or
-> on it doesn't do this in C.

Kevin

| /         o            Kevin Kenny   KB9DLU                    (217) 333-5821
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