Re: Problem with threaded program

[Andrew Cagney <ac131313@cygnus.com>]

> Greetings,
> 
> The problem below was originally reported to the Linux Kernel Mailing List.  It looks to me to be a gdb problem.
> 
> I used a freshly compiled and installed copy of gdb-5.1 (configured as "i686-pc-linux-gnu") for this test on a Pentium III 500mhz running the 2.4.16 kernel.  The same problem happens with gdb-5.0.  gdb-4.18 appears to work fine.
> 
> Here's the test program, test.c:
> 
> #include <stdlib.h>
> int main() {
>   char *t="1.0";
>   double d=0;
>   d=strtod(t,(char **)NULL);
>   printf( "%f\n", d );
>   return 0;
> }
> 
> Build using "gcc -g -lpthread test.c"; run using "gdb a.out".
> 
> If you step through the program one line at a time and display variable d after each assignment, the strtod() call seems to return "nan(0x8000000000000)", which is also shown by print().
> 
> If you restart the program with a breakpoint at printf(), let it run, and display d at the breakpoint, the value shown is "1.000000" which is correct.
> 
> Is this a defect in gdb, or is my analysis wrong?

Ah, looks like the GDB is corrupting a threaded programs FP registers 
problem.

I'm 99% certain this is in the thread-db/kernel interface that GDB is 
using.  Each time this crops up, the problem gets resolved with a 
kernel/library update.

If someone can point out a definitive explination I'll add it to the 
5.1.1 PROBLEMS file.  That way it is at least clearly documented.

The apparent 4.18 -> 5.0 ``breakage'' would have occured because GDB 
switched to using the thread-db/kernel interface.

enjoy,
Andrew