bug in float on Pentium

Not sure but I think I found a NEW bug.

I know that there have been some issues with pentiums and floating point arrithmatic, but this takes the cake...

Linux Lserver.org 2.2.18 #43 SMP Fri Mar 9 14:19:41 EST 2001 i586 unknown

egcs-2.91.66

RH 6.2.x / 7.0

try this program

#include <stdio.h>

int main() {

    char tmpx[100];

char tmpy[100];

double x = 5483.99;
float y = 5483.99;

    sprintf (tmpx, "%f",x );
    sprintf (tmpy, "%f",y );

printf ("%s\n%s\n", tmpx, tmpy);
return 0;
}

I am getting the following as output

joeja@Lserver$ ./testf
5483.990000
5483.990234

what is with the .990234?? it should be .990000

any ideas on this??

--
Joe Acosta ........
home: [email protected]

Hardly. Floats are inaccurate.

...

Your second number is a 32-bit float - which has roughly 7 digits of
precision. Just like your program output clearly shows.

(I do however fail to see the relevance of this to linux-kernel)

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Well, duh. Floats are less accurate than doubles, so what? Read your C textbook again.

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It's not that you found a new bug or that floats are inaccurate (they are just less exact than doubles)... For example, if you make your program print some more digits, you'll get:

5483.98999999999978172127157449722290039062500000000000 5483.99023437500000000000000000000000000000000000000000

#include <stdio.h>

int main() {

    unsigned int *X;
    unsigned int *Y;

double x = 5483.99;
float y = 5483.99;

    X = (unsigned int *)&x;
    Y = (unsigned int *)&y;

printf ("%60.50lf\n%60.50f\n", x, y);

printf("%lf %x%x %f %x\n", x, X[1], X[0], y, *Y); return 0;
}

which you can verify by hand:

5483.990000 40b56bfd70a3d70a
0 10000001011 1.0101 0110 1011 1111 1101 0111 0000 1010 0011 1101 0111 0000 1010
+ 2^12 * 1.3388647460937499467092948179925 =
5483.989999999999781721271574497222900390625

5483.990234 45ab5fec
0 10001011 1.010 1011 0101 1111 1110 1100
+ 2^12 * 1.338864803314208984375 = 5489.990234375

check out:
http://www.psc.edu/general/software/packages/ieee/ieee.html

l8r

m

Matt Billenstein
mbillens (at) one (dot) net
http://w3.one.net/~mbillens/

----- Original Message -----
From: "Joe" <[email protected]>
To: <[email protected]>
Sent: Friday, April 13, 2001 7:23 PM
Subject: bug in float on Pentium

| Not sure but I think I found a NEW bug.
|
| I know that there have been some issues with pentiums and floating point
| arrithmatic, but this takes the cake...
|
| Linux Lserver.org 2.2.18 #43 SMP Fri Mar 9 14:19:41 EST 2001 i586
| unknown
|
| >kgcc --version
| egcs-2.91.66
|
| RH 6.2.x / 7.0
|
| try this program
|
| #include <stdio.h>
|
| int main() {
|
|     char tmpx[100];
|  char tmpy[100];
|
|  double x = 5483.99;
|  float y = 5483.99;
|
|     sprintf (tmpx, "%f",x );
|     sprintf (tmpy, "%f",y );
|
|  printf ("%s\n%s\n", tmpx, tmpy);
|  return 0;
| }
|
|
| I am getting the following as output
|
| joeja@Lserver$ ./testf
| 5483.990000
| 5483.990234
|
|
| what is with the .990234??  it should be .990000
|
| any ideas on this??
|
| --
| Joe Acosta ........
| home: [email protected]
|
|
|
| -
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