What's wrong with this code?

Robert Bonomi bonomi at mail.r-bonomi.com
Tue Dec 13 06:36:39 UTC 2011

From: "John Levine" <johnl at iecc.com> wrote:
> 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.
> 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.

how about a tiny .so that includes -only- that routine, and a 3-line or so
main() that links against -that- .so?

> 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.
> Any ideas what the problem was?

The errorno value you report is *NOT* a defined return code for date(3).
The libc date(3) will return only EFAULT, or EPERM, per the manpage. 

This lends credence to the possibility of a run-time linker issue.

HOWEVER, there is also the possiblity of memory getting trashed -- in just
the 'right' wrong way -- *elsewhere* in the code.   OR a corrupted .so

Mentioning O/S release level, and CPU architecture would be a good idea :)

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