Re: Could you please discuss whether casting a size_t variable to unsigned int variable is the correct way to handle...

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Could you please discuss whether casting a size_t variable to unsigned int variable is the correct way to handle...
Date	2015-05-10 11:26 -0700
Organization	None to speak of
Message-ID	<876180gufr.fsf@kst-u.example.com> (permalink)
References	<a148e69a-0699-419d-ac3e-2fff86c8a158@googlegroups.com> <87ioc3j29h.fsf@kst-u.example.com> <3cd2ac79-aeaa-4b7a-91d7-6fc63f592f72@googlegroups.com> <87a8xfi01c.fsf@kst-u.example.com> <kfnvbg0e76h.fsf@x-alumni2.alumni.caltech.edu>

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Tim Rentsch <txr@alumni.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> writes:
>> G G <gdotone@gmail.com> writes:
>> [...]
>>> I didn't of think of it.  I was just puzzled because I thought
>>> size_t was really and unsigned int.
>>
>> size_t is an implementation-defined unsigned integer type.  It's
>> defined via a typedef, which creates an alias for an existing
>> type, not a new type.  [snip]
>
> What makes you say size_t must be defined using 'typedef'?  I
> agree that this will very likely be true, but I haven't found
> anything in the Standard that implies that it _must_ be true.  Is
> there some reason an implementation could not, for example, have
> something like
>
>     _Declare size_t;
>
> in those header files required to provide size_t?  AFAICT the
> type size_t may be distinct from any integer type that is
> nameable without using 'size_t'.

You're right, size_t doesn't *necessarily* have to be a typedef.

The standard (7.19) says that <stddef.h>

    declares the following types.
    ...
    The types are
    ...
        size_t
    which is the unsigned integer type of the result of the sizeof
    operator;

The only way to define size_t in standard C is to use a typedef
-- but standard headers aren't restricted to using standard C.
Using an extension would be permissible.

I think it still has to be equivalent to some named predefined type.
size_t is an "unsigned integer type", which means (6.2.5) that it's
either one of the six "standard unsigned integer types" (it can't
be _Bool), or an "extended unsigned integer types".  A footnote
suggests, but doesn't quite say, that extended integer types have
names that are implementation-defined keywords, which "shall have the
form of an identifier reserved for any use as described in 7.1.3".
The name "size_t" doesn't qualify.  (Certainly "size_t" can't be
treated as a keyword.)

If an implementation used an extension to define size_t without using
"typedef", I'd have difficulty arguing that it's non-conforming -- but
it still quacks like a typedef.

I suspect the authors of the standard assumed size_t is a typedef and
didn't bother saying so explicitly.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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