Re: long long problem

From: Keith Thompson (kst-u_at_mib.org)
Date: 03/04/05 

Date: Fri, 04 Mar 2005 21:32:10 GMT

karl_m@acm.org writes:
> I'm having trouble with unsigned long long declarations on my SCO unix
> 7.1.1 compiler. Can you test the following code? I'm expecting both
> printf functions to report 1. Thanks, karl m

But you don't tell us how it behaves on your system.

> #include <stdio.h>
>
> typedef struct {
> unsigned long long cnt:16;
> unsigned long long right:48;
> } Page;

C only requires support for bitfields of types signed int, plain int,
and unsigned int (and _Bool in C99). Many compilers support other
types of bitfields as an extension.

> typedef struct {
> Page *frame;
> } Bt1;
>
> typedef struct {
> Page frame[1];
> } Bt2;
>
> char buff[4096];
>
> int main (int argc, char **argv)
> {
> Bt1 bt1[1];
> Bt2 bt2[1];
>
> bt1->frame = (Page*)buff;

That's interesting. I was about to complain that bt1 is an array, not
a struct pointer, but of course the array name decays to a pointer in
most contexts, including the left operand of a "->". It's legal and
it works, but I've never seen that idiom before.

I would have declared:

    Bt1 bt1;
    Bt2 bt2;

and used bt1.frame rather than bt1->frame.

As for assigning frame to point to buff, there's no guarantee that
buff is properly aligned for an object of type Page (though it's
likely that you'll be "lucky" and it will be). If you want to
allocate space for a Page, either declare an object of type Page or
use malloc().

> bt1->frame->cnt = 1;
> bt1->frame->right = 30;
> printf ("cnt1 = %d\n", bt1->frame->cnt);

You're using a "%d" format for an unsigned long long value. Try
"%lld". (I suspect that's the cause of your problem, but you didn't
tell us what problem, if any, you're actually having.)

> bt2->frame->cnt = 1;
> bt2->frame->right = 30;
> printf ("cnt2 = %d\n", bt2->frame->cnt);

And again.

> }

You should also return 0 from the main program. (It's not required in
C99, but it's good style anyway.)

BTW, when I ran the program, I got:

cnt1 = 1
cnt2 = 1 

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
We must do something.  This is something.  Therefore, we must do this.

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