Re: weird printf() behavior

[Matthias Urlichs]

Hi, Kai-Min Sung wrote:

> Hi,
>       I'm not sure if this is a valid bug or just a known quirk, but I
> wrote the following test code and saw some rather strange output:
> 
Floats are promoted to double when passed as arguments.

>       printf("%x, %f\n", f, f);
> lever:~/tmp>./foo
> a0000000, -0.000000
> 
So what you're really doing here is print a dword-swapped double.  ;-)

To demonstrate what happens behind the scenes:

@linux tmp $ ./foo
1.100000, 3ff19999a0000000, 3f8ccccd, 1.100000
@linux tmp $ cat foo.c
#include <stdio.h>

int main()
{
        float f = 1.1;
        double ff = f;
        
        /* Yes, the first two format arguments are swapped intentionally. */
        printf("%f, %llx, %lx, %f\n", *(long long *)&ff, f, *(long *)&f, f);

        return 0;
}


-- 
Matthias Urlichs   |   {M:U} IT Design @ m-u-it.de   |  address@hidden
Disclaimer: The quote was selected randomly. Really. | http://smurf.noris.de
-- 
She's so tough she won't take 'yes' for an answer.