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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.lang.c
Subject: Re: Re: how has C bitten you?  (Really,
Message-ID: <5938@utzoo.UUCP>
Date: Sat, 7-Sep-85 19:36:12 EDT
Article-I.D.: utzoo.5938
Posted: Sat Sep  7 19:36:12 1985
Date-Received: Sat, 7-Sep-85 19:36:12 EDT
References: <259@watmum.UUCP>, <30000012@ISM780.UUCP>
Organization: U of Toronto Zoology
Lines: 33

> Exactly, but also consider what K&R says in section 7.14:
> 
>     The compilers currently allow a pointer to be assigned to an integer, an
>     integer to a pointer, and a pointer to a pointer of another type.  The
>     assignment is a pure copy operation, with no conversion.

Note that they do not say that this is a legitimate feature of the language!
All they say is that the current compilers will let you get away with it.
This is no longer generally true, by the way.  K&R is quite old.

> Also, in section 14.4:
> 
>     A pointer may be converted to any of the integral types large enough to
>     hold it. [...]  The mapping function is also machine dependent, but is
>     intended to be unsurprising to those who know the addressing structure
>     of the machine.
>
> Although this does not seal it up completely, it seems that K&R had it in
> mind that putting pointers into integers (and taking them back again) would
> have no overhead....

True, but there is a subtle point here:  they say you can convert pointers
to (sufficiently large) integers, they may say that you can convert the
result back, but they don't say what the integer will look like.  A NULL
pointer will not necessarily show up as an integer zero.  The equality
between NULL pointers and 0 works only when 0 is a literal constant, in
which case it is (potentially) treated specially by the compiler when
encountered in a "pointer" context.  The conversion of literal 0 to the
NULL pointer is *not* an instance of the general "putting pointers into
integers (and taking them back again)" conversion.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry