Re: pointer q

Joe Smith wrote:
"Flash Gordon" <spam@xxxxxxxxxxxxxxxxxx>
Joe Smith wrote:
"RSoIsCaIrLiIoA" <zz@xxxx> wrote in message news:9odg62tc13dv6m60mlsq19h4caf5ff06ij@xxxxxxxxxx

[snip]
#include <stdio.h>

int main ( void )
{long l = 420000;
long *lp = &l;

short *sp;
*sp = (short)*lp; /* ill-advised, but Let's Pretend */

This is very different. If you were trying to fix the code further up so it would compile what you wanted was:
short *sp = (short*)*lp; /* ill-advised, but Let's Pretend */
How thick is my head ^ ?

We all make mistakes.

<snip>

Am I correct to think that a type of masking occurs here? joe
No, not masking. More a case of only reading part of the data with my "fix". Or with your code writing to an uninitialised pointer and causing half of your socks to migrate to Alaska.
Am I not reading, with correction, the number that a smaller type thinks a larger type was? My socks are well accounted for, but Scott Knuds not only flew out of my hard drive, but is eating my lunch.

Your version of the code:
#include <stdio.h>

int main ( void )
{long l = 420000;
long *lp = &l;

short *sp;
*sp = (short)*lp; /* ill-advised, but Let's Pretend */

Writes through an uninitialised pointer which is undefined behaviour.

My partial correction:
#include <stdio.h>

int main ( void )
{long l = 420000;
long *lp = &l;

short *sp = (short)*lp; /* ill-advised, but Let's Pretend */

/* Use of *sp */

Tries to read a short from where a long was stored. On normal 2s complement systems the part of the long that is read will depend on the endianness of the system.

The standard also says:
| An object shall have its stored value accessed only by an lvalue
| expression that has one of the following types:74)
| — a type compatible with the effective type of the object,
| — a qualified version of a type compatible with the effective type of
| the object,
| — a type that is the signed or unsigned type corresponding to the
| effective type of the object,
| — a type that is the signed or unsigned type corresponding to a
| qualified version of the effective type of the object,
| — an aggregate or union type that includes one of the aforementioned
| types among its members (including, recursively, a member of a
| subaggregate or contained union), or
| — a character type.
I.e. don't do what the above code does.

It is also possibly in general (though unlikely in this specific case) for short to have trap representations. If the part of the long that is read as a short happens to be a trap representation for short (e.g. -0 on a 1s complement system where -0 is a trap) then you have again invoked undefined behaviour.

The comments about ridiculous things happening are just to make the point that the C standard does not care what happens, so anything your compiler does, including something completely unexpected

BTW, I would not place too much face in anything posted by RSoIsCaIrLiIoA. A Google search and seeing how often it has posted assembler will show you why many people probably have it kill filed.
He sounds like a dude, so you can be assured that I won't give him face, but until otherwise motivated, I'll give him the benefit of the doubt. Thanks for your reply. Joe

Your choice. Just don't expect any errors in its posts to be corrected.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

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