Re: Strange bit shifting result with gcc - am I missing something obvious?
- From: "Bartc" <bc@xxxxxxxxxx>
- Date: Mon, 31 Mar 2008 09:47:38 GMT
"Gordon Burditt" <gordonb.obxrf@xxxxxxxxxxx> wrote in message
news:13v0jkqh24l8r46@xxxxxxxxxxxxxxxxxxxxx
main()
{
int bits = 32;
printf("%d\n",(int)1 << (int)32);
printf("%d\n",(int)1 << bits);
}
The first printf gives a result of 0, the second gives 1. I checked
with sizeof() and ints are definately 32 bits in size.
I'm sure I'm missing something obvious but can someone tell me what?
Thanks
B2003
When I compile tyour code with lcc-win I get:
Warning tshift.c: 2 no type specified. Defaulting to int
The prototype for main is:
int main(void)
Warning tshift.c: 5 missing prototype for printf
You did not include <stdio.h>
Warning tshift.c: 5 shift by 32 is undefined
The number of bits shifted is greater than sizeof(int)*CHAR_BIT
This is undefined!
Isn't the above error message misleading (assuming you're running
in a typical 32-bit setup)? The number of bits shifted is *NOT*
greater than sizeof(int)*CHAR_BIT, and the result is still undefined.
The actual warning (not error) is:
Warning tshift.c: 5 shift by 32 is undefined
Which is correct.
I was interested however in investigating why 1<<32 gave 16777216 and found
that slightly odd code. You could argue it doesn't matter, but just blindly
shifting the value by whatever was in a register seemed wrong. And jacob
fixed this although he wasn't asked to (and will doubtless now break some
programs :-).
--
Bart
.
- References:
- Prev by Date: Re: A plea for sanity!
- Next by Date: Re: strcat strncat and strlen
- Previous by thread: Re: Strange bit shifting result with gcc - am I missing something obvious?
- Next by thread: Re: Strange bit shifting result with gcc - am I missing something obvious?
- Index(es):
Relevant Pages
|
|
