Conference turris::decc_bugs

1288.0. "Incomplete sign extension when returning result..." by PEACHS::DALEY ('Course it don't come with @!# wafers!) Wed Apr 23 1997 17:55

Hi,

I *think* this is a bug.  Need confirmation, explanation.


Sign extension doesn't seem to work in the code below, which
should return signed long, but seems to return a signed in
and stuffs it in the first 32bits of a long.

The following code returns a long that's a positive value on
an alpha...mainly because it does sign extension out 32 bits
(as if it were an int), but not 64 bits for the long.

Other vendors machines I've tested with show long as a signed
negative value, but their ints are the same as their longs, so
maybe (I thought) they're just lucky.  They do sign extension
from short int to int, and so to we, but not to our long, which
is 8 bytes, compared to everybody else's 4 bytes.

Comments?  Here's the code, below.

Regards,

John 

---------------------------------------------------------
#include <stdio.h>

long foo();
long l;
int i;

main() {

        l = foo();
        printf("l as long = %ld\n", l);
        printf("l as int  = %d\n", l);
        printf("sizeof long l = %d\n", sizeof(l));
        printf("sizeof int i  = %d\n", sizeof(i));
}

long foo() {

        unsigned int x = 1;
        unsigned int y = 26;

        return(x - y);
        }

>        unsigned int x = 1;
>        unsigned int y = 26;
>
>        return(x - y);

This computes (x-y) using "unsigned int" arithmetic, getting a large
positive value (unsigned arithmetic "wraps" on overflow, as in this case).
Then it converts that large positive value to a long int (getting the
same value, of course) and returns it.  This behavior is required by the
C language.

You probably want to write
	return ( (long int)x - (long int)y );