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Re: “ChattyG takes a college freshman C/C++ programming exam”

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From Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups comp.lang.c++
Subject Re: “ChattyG takes a college freshman C/C++ programming exam”
Date Tue, 10 Oct 2023 18:46:04 -0700
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On 10/8/23 18:31, Keith Thompson wrote:
>> Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> writes:
>>> Anton Shepelev wrote:
>>>> You are converting void to function pointer fptr.  This is not supported
>>>> in C.  You might have taken advantage of void*, which is silently
>>>> convertible to any pointer, but then again it does not work with
>>>> function pointer, and C has not analog of void* for functions pointers,
>>>> so an explicit conversion is seem inevitable.
>>>
>>> It mayn't be supported in standard C, but it's a common extension—in fact, 
>>> POSIX _requires_ it in order for the dlsym subroutine to work.
>> 
>> The POSIX requirement is limited to values returned by dlsym().
>> 
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html
>
> I've heard that before, and it might be true - but I don't see words to
> that effect on that page. It says:
>
> "Note that conversion from a void * pointer to a function pointer as in:
>
> fptr = (int (*)(int))dlsym(handle, "my_function");
>
> is not defined by the ISO C standard. This standard requires this
> conversion to work correctly on conforming implementations."
>
> As I read it, that requires that the conversion work in general - it
> says nothing about it working only for values returned by dlsym.

I'd say it's ambiguous.  One could argue that "this conversion" refers
to converting the result of a call to dlsym().

And if I were trying to create a POSIX-conforming implementation on a
system where function pointers cannot in general be converted to void*
and back again, I'd argue exactly that.  (Imagine, for example, a system
where function pointers are 128 bits and object pointers, including
void*, are 64 bits, but the implementation can arrange for functions
whose addresses might be returned by dlsym() to have all zeros in their
high-order 64 bits.)

I *think* the wording was changed at some point, and that an earlier
version of POSIX required all function pointers to be convertible to
void*, but I can't find a reference.  (opengroup.org has links to older
versions of POSIX from 2008, 2013, and 2016, but when I search for
"dlsym" in any of them it takes me to the current version.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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“ChattyG takes a college freshman C/C++ programming exam” Lynn McGuire <lynnmcguire5@gmail.com> - 2023-10-05 17:40 -0500
  Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-05 15:57 -0700
  Re: “ChattyG takes a college freshman C/C++ programming exam” Christian Gollwitzer <auriocus@gmx.de> - 2023-10-06 07:36 +0200
    Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-06 11:59 -0700
  Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-07 23:54 +0300
    Re: “ChattyG takes a college freshman C/C++ programming exam” jak <nospam@please.ty> - 2023-10-08 09:42 +0200
      Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-08 04:27 -0400
      Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 12:55 +0300
        Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 13:06 +0300
        Re: “ChattyG takes a college freshman C/C++ programming exam” candycanearter07 <no@thanks.net> - 2023-10-08 06:22 -0500
          Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 15:03 +0300
            Re: “ChattyG takes a college freshman C/C++ programming exam” Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-10-15 15:51 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” Richard Damon <richard@damon-family.org> - 2023-10-15 17:53 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-16 00:07 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-10-16 22:01 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” Richard Damon <richard@damon-family.org> - 2023-10-16 22:41 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-17 12:22 -0400
          Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-08 12:31 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” Richard Damon <news.x.richarddamon@xoxy.net> - 2023-10-08 21:17 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 01:13 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-10 12:18 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” MarioCCCP <NoliMihiFrangereMentulam@libero.it> - 2023-10-13 11:22 +0200
              Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-13 10:04 -0700
        Re: “ChattyG takes a college freshman C/C++ programming exam” Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-08 13:54 +0100
        Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-08 11:25 -0400
          Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 20:07 +0300
        Re: “ChattyG takes a college freshman C/C++ programming exam” Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-10-08 16:37 +0000
          Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 20:05 +0300
            Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 01:13 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-09 13:30 -0700
                Re: Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 20:33 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-10 12:04 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-10-09 12:17 +0000
              Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-09 23:03 +0300
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 20:30 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-09 23:45 +0300
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 21:21 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 21:26 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 22:41 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-09 21:47 +0100
          Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-08 15:31 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 01:12 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-10 18:46 -0700
                Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-11 01:14 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-11 13:47 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-11 07:56 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-09 23:06 +0300
              Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-09 17:15 -0700
      Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-08 11:08 -0400
  Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-11 21:06 -0700
    Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-11 21:10 -0700
    Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 10:56 +0300
    Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 10:57 +0300
      Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-12 12:59 -0700
  Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 10:53 +0300
    Re: “ChattyG takes a college freshman C/C++ programming exam” Phil Carmody <pc+usenet@asdf.org> - 2023-10-23 15:39 +0300
      Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-23 16:45 +0300

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