Date: Tue, 13 Dec 2011 00:07:54 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: John Levine <johnl@iecc.com> Cc: freebsd-questions@freebsd.org Subject: Re: What's wrong with this code? Message-ID: <20111213060754.GG53453@dan.emsphone.com> In-Reply-To: <20111213051220.45894.qmail@joyce.lan> References: <20111213051220.45894.qmail@joyce.lan>
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In the last episode (Dec 13), John Levine said:
> This tiny routine is in a .so loadable module I use. (It's part of the
> mailfront SMTP daemon.)
>
> static const char* date_string(void)
> {
> static char datebuf[64];
> time_t now = time(0);
> struct tm* tm = gmtime(&now);
> strftime(datebuf, sizeof datebuf - 1, "%d %b %Y %H:%M:%S -0000", tm);
> return datebuf;
> }
>
> I was getting bogus dates. Running it under GDB, time() is returning 1,
> -and setting errno to 22, which is EINVAL. Changing the call to
> time to time(NULL) or time(&now) made no difference.
The manpage says that time() can fail for any of the reasons described in
gettimeofday(2), but time() actually calls clock_gettime(CLOCK_SECOND),
which technically could return EINVAL if the first argument isn't a valid
clock_id. CLOCK_SECOND is valid, though, so in practice it should never
fail with EINVAL. You could try adding a printf to
sys/kern/kern_time.c:kern_clock_gettime() to see if it's really failing
there.
> I changed it to a call to gettimeofday(), which works fine. But what
> could the problem have been? When I splice this routine into a tiny test
> program that calls it and prints out the result, it works fine.
>
> The obvious problem, since it's in a .so, is that it's linking to
> something other than the system library time() function, but I did an nm
> on the .so, and it said this, which sure looks like the system time()
> function to me:
>
> U time@@FBSD_1.0
>
> Setting a breakpoint in gdb gets a complaint about trying to set a
> breakpoint in /lib/libc.so.7.
Setting a breakpoint in a llibc should work fine, since time() is a regular
function and not a syscall stub. Have you built a libc with debugging
symbols? ( easy way: add DEBUG_FLAGS=-g to the top of
/usr/src/lib/libc/Makefile, and run "make obj && make clean && make depend
&& make && make install" in that directory )
> Any ideas what the problem was?
--
Dan Nelson
dnelson@allantgroup.com
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