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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: Mon, 08 Jun 2015 14:52:45 -0700
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glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> wrote:
> (snip) 
>> Because this strictly conforming program:
>  
>> #include <stdio.h>
>> #include <stddef.h>
>> int main(void) {
>> #ifdef size_t
>>    puts("size_t is a macro");
>> #else
>>    puts("size_t is not a macro");
>> #endif
>>    {
>>        char *size_t = "Because, among other things, if size_t were "
>>                       "a macro, this would be a syntax error";
>>    }
>>    return 0;
>> }
>  
>> must print "size_t is not a macro".
>
> Maybe I am missing something (likely), but why can't size_t be
> a macro that expands to a typedef name?  That is, with the appropriate
> macro expansion it wouldn't be a syntax error, but still would print
> "size_t is a macro".

Interesting question.  If an implementation is permitted to, for
example, have the following in <stddef.h>

    typedef unsigned long size_t;
    #define size_t size_t

then the above program is *not* strictly conforming, but I don't think
the macro definition would affect anything that doesn't explicitly test
whether size_t is defined as a macro.

I have some fairly weak arguments that it's not permitted.

N1570 7.1.2p1 says:

    The header declares a set of related functions, plus any
    necessary types and additional macros needed to facilitate
    their use.

The macro is not *needed* to facilitate the use of the type.

7.1.2p5 says:

    Any definition of an object-like macro described in this clause
    shall expand to code that is fully protected by parentheses
    where necessary, so that it groups in an arbitrary expression
    as if it were a single identifier.

This cannot apply to a macro that expands to a typedef name.

7.1.4p1 gives explicit permission to define functions as macros.  No
such permission is given for type names.

In general, I'd like to say that implementations aren't permitted to
define identifiers in ways not explicitly permitted by the standard,
but I'm not sure I can justify it rigorously.

-- 
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"