Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #21665

Re: Q: Local variables initialization shortcut.

Show key headers only | View raw

Jens Thoms Toerring wrote:
> 
> Jean-Christophe <5.d@free.fr> wrote:
> > On 2 juin, 12:31, j...@toerring.de :
 
> > > '&z - &a' isn't legal - you can only subtract addresses
> > > of locations within a single object (e.g. an array),
> 
> > I've done it many times and it always worked fine :
> > &a is the address of 'a' so it's its memory location.
> 
> Well, you were lucky. The standard doesn't define what
> the result of subtracting pointers not within the same
> object is (or, in the case of arrays, one beyond the
> end of the array), so you are relying on whatever your
> compiler makes out of it. Some other compiler might
> give you a different result.
> 
> > Except for this actual example (when variables are stored
> > in stack w/ their adresses allocated by the compiler)
> 
> There also is no requirement for a stack in the C stan-
> dard. Most (or probably all) compilers will employ
> something stack-like for local variables, but that is
> a decision the compiler writer makes, not something
> prescribed by the standard.
> 
> > if the location of 'a' thru 'z' are well stored
> > (i.e, like in an array) then the following code works fine :
> > unsigned int size = (unsigned int)( &z - &a );
> 
> Yes, it may seem to work on your system with the specific
> compiler you are using. If you want your program to work
> only on that system and with that compiler then this is
> probably all fine, just dont expect it to work anymore
> under different circumstances.
> 
> > > everything else is undefined behaviour, And even if
> > > disregarding this and assuming that 'a' is at the lowest
> > > and 'z' at the highest address, '&z - &a' will be the
> > > number of variable between 'a' and 'z' (25) and not the
> > > number of bytes used in between, so you would have to
> > > multiply by the size of a double.
> 
> > Hum, no : if &z is the highest address
> > and &a is the lowest address then (&z - &a)
> > is the number of bytes (less one sizeof) and not the
> > number of variables (unless 'a' and 'z' are chars)
> 
> That may well be the case. Since you are relying on un-
> defined behaviour everything you get is "right". Just for
> the fun of it: on my system (x86-64 Linux and with gcc)
> the output of this program
> 
> #include <stdio.h>
> int main( void )
> {
>     double a, b;
>     printf( "%d\n", ( int ) ( &a - &b ) );
>     return 0;
> }
> 
> is -1. So obviously at least some of the deductions you
> were led to make by the behaviour on your system aren't
> valid from what I see on my machine. And that's quite
> ok since the behaviour of that program isn't defined,
> so whatever the output is is correct. (And I have de-
> finitely worked with machines were variables defined
> later had lower addresses).
> 
> For even more giggles take this program:
> 
> #include <stdio.h>
> int main( void )
> {
>     double a, b, c;
>     printf( "%d\n", ( int ) ( &a - &c ) );
>     return 0;
> }
> 
> The output on my machine is still -1...
> 
> But I am a bit puzzled - I would expect the difference
> between two pointers of the same type to be number of
> variables of that type in between, not the number of
> bytes. Otherwise the behaviour (while undefined) would
> be even more weird on your machine since e.g. for
> arrays you can rely on the difference between the
> addresses of two of its elements to be the number of
> elements in between. But then, again, the moment you
> do something undefined everything can happen.

On the machine that I am using, 
the following program outputs a pair of 1's.

#include <stdio.h>

int 
main( void )
{
    {
        double a, b;
        printf( "%d\n", ( int ) ( &a - &b ) );
    }
    {
        double b, a;
        printf( "%d\n", ( int ) ( &a - &b ) );
    }
    return 0;
}

-- 
pete

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 02:49 -0700
  Re: Q: Local variables initialization shortcut. China Blue Max <chine.bleu@yahoo.com> - 2012-06-02 03:03 -0700
    Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 03:17 -0700
  Re: Q: Local variables initialization shortcut. jacob navia <jacob@spamsink.net> - 2012-06-02 12:06 +0200
    Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 03:25 -0700
      Re: Q: Local variables initialization shortcut. James Kuyper <jameskuyper@verizon.net> - 2012-06-02 10:59 -0400
        Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 13:02 -0700
          Re: Q: Local variables initialization shortcut. Keith Thompson <kst-u@mib.org> - 2012-06-02 14:51 -0700
  Re: Q: Local variables initialization shortcut. Ian Collins <ian-news@hotmail.com> - 2012-06-02 22:10 +1200
  Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 10:31 +0000
    Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 03:59 -0700
      Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 11:55 +0000
        Re: Q: Local variables initialization shortcut. pete <pfiland@mindspring.com> - 2012-06-02 08:15 -0400
        Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 05:37 -0700
          Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 13:35 +0000
            Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 06:47 -0700
              Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 15:08 +0100
                Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 07:37 -0700
                Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 16:17 +0100
                Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 19:02 +0100
              Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 17:39 +0000
                Re: Q: Local variables initialization shortcut. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 15:27 -0400
                Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 12:58 -0700
                Re: Q: Local variables initialization shortcut. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 16:15 -0400
                Re: Q: Local variables initialization shortcut. David Brown <david.brown@hesbynett.no> - 2012-06-03 15:27 -0500
                Re: Q: Local variables initialization shortcut. Noob <root@localhost> - 2012-06-05 09:12 +0200
                Re: Q: Local variables initialization shortcut. David Brown <david@westcontrol.removethisbit.com> - 2012-06-05 09:21 +0200
                Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 12:53 -0700
                Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 21:16 +0100
                Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 21:18 +0000
                Re: Q: Local variables initialization shortcut. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 17:38 -0400
                Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 22:15 +0000
                Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-03 00:51 -0700
                Re: Q: Local variables initialization shortcut. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 11:46 -0700
                Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-03 00:13 -0700
                Re: Q: Local variables initialization shortcut. Keith Thompson <kst-u@mib.org> - 2012-06-02 14:50 -0700
                Re: Q: Local variables initialization shortcut. David Brown <david.brown@hesbynett.no> - 2012-06-03 15:34 -0500
          Re: Q: Local variables initialization shortcut. David Brown <david.brown@hesbynett.no> - 2012-06-03 15:25 -0500
        Re: Q: Local variables initialization shortcut. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 08:46 -0400
        Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 05:46 -0700
          Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 05:53 -0700
            Re: Q: Local variables initialization shortcut. Ike Naar <ike@sverige.freeshell.org> - 2012-06-02 17:08 +0000
            Re: Q: Local variables initialization shortcut. "io_x" <a@b.c.invalid> - 2012-06-03 10:33 +0200
              Re: Q: Local variables initialization shortcut. "io_x" <a@b.c.invalid> - 2012-06-03 11:32 +0200
      Re: Q: Local variables initialization shortcut. pete <pfiland@mindspring.com> - 2012-06-02 08:03 -0400
        Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 05:18 -0700
    Re: Q: Local variables initialization shortcut. "io_x" <a@b.c.invalid> - 2012-06-03 10:32 +0200
  Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 11:45 +0100
    Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 04:17 -0700
      Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 12:40 +0100
        Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 12:51 +0100
        Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 04:57 -0700
          Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 14:00 +0100
            Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 06:12 -0700
              Re: Q: Local variables initialization shortcut. Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 09:58 -0400
                Re: Q: Local variables initialization shortcut. David Brown <david.brown@hesbynett.no> - 2012-06-03 15:41 -0500
              Re: Q: Local variables initialization shortcut. Robert Wessel <robertwessel2@yahoo.com> - 2012-06-02 11:45 -0500
  Re: Q: Local variables initialization shortcut. Noob <root@localhost> - 2012-06-02 16:05 +0200
    Re: Q: Local variables initialization shortcut. Jean-Christophe <5.d@free.fr> - 2012-06-02 07:48 -0700
      Re: Q: Local variables initialization shortcut. "BartC" <bc@freeuk.com> - 2012-06-02 18:31 +0100
      Re: Q: Local variables initialization shortcut. jt@toerring.de (Jens Thoms Toerring) - 2012-06-02 17:50 +0000

csiph-web