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drand48() bug
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[email protected] writes:
> Most of the unix machines do the same thing. On SGI machines
> that have the hardware cycle counter, its value is used in place of the
> srand48(usec), lrand48() sequence. BSDI the code used srandom and random.
Kun Luo, one of our grad students here recently found a bug in Sun's implemen-
tation of the drand48() function. We reported it to Sun, and they acknowledged
the bug exists - it seemed to be the first time they had heard of it, though.
The bug affects Sun's ANSI C compiler shipped with
SPARCWorks3.0 and consists of the following: If you're compiling using the
- -Xc flag (strict ANSI C, no SUN C compatibility extensions), the function
drand48() is BROKEN. It ALWAYS returns the number 9.000000, no matter what
you seeded it with using srand48().
to reproduce, compile the following program under Solaris 2.x using their
C compiler AND the -Xc flag:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <time.h>
void main(){
double number;
int num = 1000;
srand48((int)time(NULL));
while(num --){
number = drand48();
printf("%lf\n", number);
}
}
By the way, Cyphepunks mail seems to indicate that Netscape uses lrand48()
instead of drand48(). lrand48() is NOT broken. The potential for disaster
is high, though, for anyone blindly trusting vendor-provided pseudo-
random number generators. How many products are out there using drand48()
as part of their random number generation scheme?
- -Arve
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