Re: How to extract bytes from a long?

From: Joe Wright (joewwright_at_earthlink.net)
Date: 10/16/03 

Date: Thu, 16 Oct 2003 13:57:37 GMT

cody wrote:
>
> > > #include <stdio.h>
> > > union _x
> > > {
> > > long lng;
> > > char byt[sizeof long];
> > > }
> > >
> > > better?
> >
> > Marginally. It still doesn't address the issue raised
> > by both David Rubin and Martin Ambuhl, and it still uses
> > an identifier reserved for the implementation,
>
> What reserved identifier? Do you mean the underscore before x?
>
> > and it still
> > risks trouble with trap representations or if the system
> > uses signed magnitude or ones' complement arithmetic ...
>
> He just wanted to extract the bytes of a long. Sometimes you do not care how
> the
> bytes are represented are in that long, just getting them is enough.
>
> You should not assign new values to byt, that could cause trap
> representations as you said, but reading is allowed and safe.
>
> > ... but other than that, Mrs. Lincoln, how did you
> > like the play?
>
> What do you mean with that?
>

Cody,

They might be picking on you now. I'll move tentatively to your side.

This snippet fixes your remaining syntax error (you'll see it easily)
but provides fodder for our detractors.

I contend, as you seem to, that accessing u.lng as components of u.byt[]
is what unions are all about. We could be wrong.

Accessing memory through (unsigned char *) seems to be guaranteed to
work. I demonstrate both methods below. I don't understand why the first
is wrong while the second is right. Duck! :-)

#include <stdio.h>

typedef unsigned long ulong;
typedef unsigned char uchar;

union {
   ulong lng;
   uchar byt[ sizeof (ulong) ];
} u;

int main(void) {
   int i, j = sizeof (ulong);
   uchar *ucr = (uchar *)&u.lng;
   u.lng = 0x12345678;
   for (i = 0; i < j; ++i)
      printf(" 0x%02X", u.byt[i]);
   puts("");
   for (i = 0; i < j; ++i)
      printf(" 0x%02X", ucr[i]);
   return 0;
} 

-- 
Joe Wright                                 http://www.jw-wright.com
"Everything should be made as simple as possible, but not simpler."
                    --- Albert Einstein ---