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Re: Do not cast pointers to functions to pointers to primitive types

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pacman@kosh.dhis.org (Alan Curry) writes:
> In article <lnpq62vkuk.fsf@nuthaus.mib.org>,
> Keith Thompson  <kst-u@mib.org> wrote:
>>James Kuyper <jameskuyper@verizon.net> writes:
>>> On 09/03/2012 04:21 PM, Keith Thompson wrote:
>>>> James Kuyper <jameskuyper@verizon.net> writes:
>>>> [...]
>>>>> Example: the dlsym() function is specified by the X-open standard to
>>>>> take a void* argument which can be either a pointer to an object or a
>>>>> pointer to a function. The Rationale for that function notes;
>>>>>
>>>> [...]
>>>>>> Note that compilers conforming to the ISO C standard are required to
>>>>>> generate a warning if a conversion from a void * pointer to a
>>>>>> function pointer is attempted.
>>>> [...]
>>>> 
>>>> I don't believe that's correct.  The behavior of converting a void*
>>>> pointer to a function pointer is not defined, but as far as I can tell
>>>> it doesn't violate any constraint.
>>[...]
>>>
>>> It's possible that they're referring to implicit conversion, rather than
>>> one using a cast.
>>
>>It's possible, but if so they phrased it poorly.
>
> Any idiot should be able to read that and know without being told that using
> a cast will get rid of the warning. Casts do that. It's obvious.

Were you aware that we're talking about conversion from void* to
a function pointer, a conversion whose behavior is not defined by
the standard, whether it's done by a cast or not?  This is quite
distinct from other pointer-to-pointer conversions, which in most
cases merely yield an implementation-defined result.

I posted a code snippet upthread.  Here's a small complete program:

    int main(void) {
        int obj;
        void *vp = &obj;
        void (*fp)(void) = (void (*)(void))vp; /* line 4 */
        return 0;
    }

When I compile this with gcc 4.7, with "-pedantic" and any of
"-std={c90,c99,c11}", I get the following warning:

    c.c:4:24: warning: ISO C forbids conversion of object pointer to function pointer type [-pedantic]

I get the same warning with or without the (void (*)(void)) cast.
(Without the cast, it's definitely a constraint violation.)

You might want to reconsider your "Any idiot" statement.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

Do not cast pointers to functions to pointers to primitive types ajaybgr <ajay.bgr@gmail.com> - 2012-09-03 01:50 -0700
  Re: Do not cast pointers to functions to pointers to primitive types Ian Collins <ian-news@hotmail.com> - 2012-09-03 20:59 +1200
  Re: Do not cast pointers to functions to pointers to primitive types Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-09-03 02:13 -0700
  Re: Do not cast pointers to functions to pointers to primitive types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-03 11:24 +0100
  Re: Do not cast pointers to functions to pointers to primitive types James Kuyper <jameskuyper@verizon.net> - 2012-09-03 08:31 -0400
    Re: Do not cast pointers to functions to pointers to primitive types Keith Thompson <kst-u@mib.org> - 2012-09-03 13:21 -0700
      Re: Do not cast pointers to functions to pointers to primitive types James Kuyper <jameskuyper@verizon.net> - 2012-09-03 16:57 -0400
        Re: Do not cast pointers to functions to pointers to primitive types Keith Thompson <kst-u@mib.org> - 2012-09-03 14:48 -0700
          Re: Do not cast pointers to functions to pointers to primitive types pacman@kosh.dhis.org (Alan Curry) - 2012-09-03 21:57 +0000
            Re: Do not cast pointers to functions to pointers to primitive types Keith Thompson <kst-u@mib.org> - 2012-09-03 18:11 -0700
              Re: Do not cast pointers to functions to pointers to primitive types pacman@kosh.dhis.org (Alan Curry) - 2012-09-04 02:03 +0000
                Re: Do not cast pointers to functions to pointers to primitive types Keith Thompson <kst-u@mib.org> - 2012-09-03 19:20 -0700
                Re: Do not cast pointers to functions to pointers to primitive types pacman@kosh.dhis.org (Alan Curry) - 2012-09-04 20:27 +0000
                Re: Do not cast pointers to functions to pointers to primitive types James Kuyper <jameskuyper@verizon.net> - 2012-09-04 17:10 -0400
                Re: Do not cast pointers to functions to pointers to primitive types Barry Schwarz <schwarzb@dqel.com> - 2012-09-04 14:14 -0700
          Re: Do not cast pointers to functions to pointers to primitive types pete <pfiland@mindspring.com> - 2012-09-03 22:51 -0400
      Re: Do not cast pointers to functions to pointers to primitive types Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2012-09-04 00:02 +0200
        Re: Do not cast pointers to functions to pointers to primitive types Keith Thompson <kst-u@mib.org> - 2012-09-03 18:19 -0700
  Re: Do not cast pointers to functions to pointers to primitive types Kaz Kylheku <kaz@kylheku.com> - 2012-09-04 01:18 +0000
    Re: Do not cast pointers to functions to pointers to primitive types "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2012-09-04 11:59 -0400
      Re: Do not cast pointers to functions to pointers to primitive types Philip Lantz <prl@canterey.us> - 2012-09-06 21:34 -0700

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