Re: modifying local const variable run time

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: modifying local const variable run time
Date	2017-02-22 16:06 +0000
Organization	A noiseless patient Spider
Message-ID	<87shn66wgx.fsf@bsb.me.uk> (permalink)
References	<c5e88b2a-20a5-4083-a40a-a17d57aa42f7@googlegroups.com> <20170222160611.91c1f3085d6c9e3571a68156@gmail.com> <3cba5f3c-735a-45b7-87f2-d03dd8a18720@googlegroups.com> <o8kaog$9cc$2@dont-email.me> <abdf9730-1fd0-4943-80fc-95c1644ec7fb@googlegroups.com>

Show all headers | View raw

rahul.dev.agg@gmail.com writes:

> On Wednesday, February 22, 2017 at 9:01:28 PM UTC+5:30, David Brown wrote:
>> On 22/02/17 16:26, rahul.dev.agg@gmail.com wrote:
>> > On Wednesday, February 22, 2017 at 8:38:13 PM UTC+5:30, GOTHIER Nathan wrote:
>> >> On Wed, 22 Feb 2017 06:39:51 -0800 (PST)
>> >> rahul.dev.agg@gmail.com wrote:
>> >>
>> >>> int func(void)
>> >>> {
>> >>>    const int i = getc();
>> >>> }
>> >>>
>> >>> Is this correct ? Can we modify const local variable run time ?
>> >>
>> >> It should be correct if you don't try to assign a value to your constant
>> >> integer. Constant objects should only be initialized.
>> > 
>> > But, how this would be implemented ?
>> > I read that const variable can be stored in read only memory.
>> > In that case, it's value should be known at compile time.
>> > Or is there I am missing something ?
>> 
>> A compiler /may/ put a const object in read-only memory, if it knows the
>> initialisation value when the program is compiled.  It can't do that for
>> const objects that are initialised at run time.  Think of your code here
>> as a saying "I want an "int" called "i", initialised to the result of a
>> call to getc(), and it will not be changed after initialisation".
>
> ok. Great !!!  Thanks.
>
> Just curious how this will be implemented ?

The only thing a compiler must do is to report certain cases where i is
used inappropriately.  The compiler has to remember that i is const
(just like it must remember that i is an int) so that if, in the same
scope, it sees i++ or i = 99 is can report it.  It does not even have to
stop compiling the code.  It can generate code to do the operation as if
i were not const.  It can refuse to generate a runable binary or it can
do pretty much anything at all since the code now has undefined
behaviour.

C has no way to stop the programmer from modifying an object.  Using
const just provides some help in preventing unintended modifications and
it can make the code easier to understand by removing one less thing to
think about.

-- 
Ben.

Thread

modifying local const variable run time rahul.dev.agg@gmail.com - 2017-02-22 06:39 -0800
  Re: modifying local const variable run time GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-22 16:06 +0100
    Re: modifying local const variable run time rahul.dev.agg@gmail.com - 2017-02-22 07:26 -0800
      Re: modifying local const variable run time David Brown <david.brown@hesbynett.no> - 2017-02-22 16:31 +0100
        Re: modifying local const variable run time rahul.dev.agg@gmail.com - 2017-02-22 07:43 -0800
          Re: modifying local const variable run time Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-22 16:06 +0000
      Re: modifying local const variable run time James Kuyper <jameskuyper@verizon.net> - 2017-02-22 11:13 -0500
      Re: modifying local const variable run time supercat@casperkitty.com - 2017-02-22 08:34 -0800
        Re: modifying local const variable run time David Brown <david.brown@hesbynett.no> - 2017-02-22 23:03 +0100
      Re: modifying local const variable run time GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-22 17:34 +0100
        Re: modifying local const variable run time Robert Wessel <robertwessel2@yahoo.com> - 2017-02-22 10:52 -0600
          Re: modifying local const variable run time GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-22 17:52 +0100
            Re: modifying local const variable run time Keith Thompson <kst-u@mib.org> - 2017-02-22 09:33 -0800
            Re: modifying local const variable run time James Kuyper <jameskuyper@verizon.net> - 2017-02-22 15:50 -0500
              Re: modifying local const variable run time supercat@casperkitty.com - 2017-02-22 13:13 -0800
                Re: modifying local const variable run time Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-22 21:55 +0000
                Re: modifying local const variable run time supercat@casperkitty.com - 2017-02-22 14:43 -0800
        Re: modifying local const variable run time Keith Thompson <kst-u@mib.org> - 2017-02-22 09:23 -0800
          Re: modifying local const variable run time GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-22 18:32 +0100
            Re: modifying local const variable run time Keith Thompson <kst-u@mib.org> - 2017-02-22 09:50 -0800
              Re: modifying local const variable run time David Brown <david.brown@hesbynett.no> - 2017-02-22 23:07 +0100
                Re: modifying local const variable run time Keith Thompson <kst-u@mib.org> - 2017-02-22 15:20 -0800
                Re: modifying local const variable run time David Brown <david.brown@hesbynett.no> - 2017-02-23 09:50 +0100
  Re: modifying local const variable run time James Kuyper <jameskuyper@verizon.net> - 2017-02-22 10:16 -0500
  Re: modifying local const variable run time Keith Thompson <kst-u@mib.org> - 2017-02-22 09:17 -0800

csiph-web