[ntp:questions] Ntpd in uninterruptible sleep?

Dave Hart hart at ntp.org
Sat Nov 12 08:01:14 UTC 2011


On Sat, Nov 12, 2011 at 03:19, Dave Hart <hart at ntp.org> wrote:
> Excellent.  I assume the stack trace is from ntpd 4.2.6p3.  I think
> you've found a bug in your system's libc dtoa() exposed by its
> snprintf(s, " %.2f", ...).  I believe you will not be able to
> reproduce the bug using 4.2.7, as that version of ntpd uses
> C99-snprintf [1] if the system snprintf() is not C99-compliant.
> C99-snprintf's rpl_vsnprintf() does not use dtoa(), it hand-rolls the
> double-to-ascii conversion.  Below is the code in ntpd.  NTP_SHIFT is
> 8.  I claim the ntpd code is correct and your system's dtoa() and
> thereby snprintf() of double (floating point) is subject to infinite
> looping for some values.

I was confused.  Thinking of the antique hardware involved, I was
assuming wrongly that the system-provided snprintf() was not
C99-compliant.  In fact, the antique is running modern NetBSD 5.x,
which undoubtedly does provide a C99 snprintf().

But wait, there's more.  I was also probably wrong in saying that this
is a bug in 32-bit SPARC NetBSD 5.x dtoa().  That code is likely quite
mature and such a bug would have been found before now.  My hunch is a
hardware bug, but I'll pursue that on the port-sparc at netbsd.org list.
I will report back here once it's resolved, if Mr. Carver does not.

Cheers,
Dave Hart


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