Date: Thu, 24 Oct 2002 12:54:47 +0100 From: "Peter Edwards" <pmedwards@eircom.net> To: current@freebsd.org Subject: Floating point problems Message-ID: <20021024115918.057A443E65@mx1.FreeBSD.org>
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Hi,
There was some discussion about issues with interactions between the floating
point context and signal handling in a thread a week or so ago, and a suggestion
that someone try and get a simple test that would fail. I was surprised how
easy it was: The following program just spins calculating the value of 6.0 /
3.0, and traps SIGINT.
If you run it on -current (as of a few hours ago), 99% of the time, hitting
ctl-C will cause the program to exit with an error. A 4.5 kernel never causes
any problems.
I'm pretty sure this is what's causing the stalls and crashes in X. I've taken
stack traces of crashes, and from "spinning" processes, and I can spot NaNs on
the stack that shouldn't be there, etc.
I just thought it might be useful to send this, to firmly take the blame
away from the X server, and give someone a way of reproducing the bug.
fptest.c:
#include <math.h>
#include <stdio.h>
#include <signal.h>
#include <assert.h>
int count = 0;
void sigint(int sig)
{
write(2, "INT\n", 4);
count++;
}
int
main(int argc, char **argv)
{
double x = 6.0;
double y = 3.0;
double d;
double err;
struct sigaction sa;
sa.sa_handler = sigint;
sa.sa_flags = 0;
sigemptyset(&sa.sa_mask);
sigaction(SIGINT, &sa, 0);
while (count < 30) {
d = x / y;
err = 2.0 - d;
if (err != 0.0) {
fprintf(stderr, "err %f!\n", err);
exit(-1);
}
}
}
--
Peter Edwards.
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