Re: another conditional breakpoint question

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Sep 20, 2002 at 03:36:06PM -0700, Peter Jay Salzman wrote:
> begin Daniel Jacobowitz <drow@mvista.com> 
> > On Fri, Sep 20, 2002 at 01:13:04PM -0700, Peter Jay Salzman wrote:
> > > in the program:
> > > 
> > >    #include <math.h>
> > >    int main(void)
> > >    {
> > >         double i = 0.0;
> > >         double j = cos(i);
> > >         return 0;
> > >    }
> > > 
> > > i set a breakpoint which should never be reached:
> > > 
> > > p@satan% gdb mymath
> > > (gdb) break main if cos(0.0) > 1000.0
> > > Breakpoint 1 at 0x8048426: file mymath.c, line 5.
> > > 
> > > (gdb) run
> > > Breakpoint 1, main () at mymath.c:5
> > > 5               double i = 0.0;
> > > 
> > > considering that cosine should be greater than 1 for a real variable,
> > > this is quite strange!
> > > 
> > > 
> > > i don't know if this is related, but i notice that cos(0) doesn't have a
> > > value of 1.0.  not even close:
> > > 
> > >    (gdb) p cos(0)
> > >    $3 = 14368
> > > 
> > > but just in case gdb doesn't know that cos() returns a double, i also
> > > tried:
> > > 
> > >    (gdb) p/f cos(0.0)
> > >    $1 = 2.00890148e-41
> > > 
> > > neither of these are even close to being right.
> > > 
> > > 
> > > can someone tell me why gdb is reaching that breakpoint which should
> > > never be reaced?  and is there any way of getting gdb to tell me that
> > > cos(0) is supposed to be equal to 1?   :*)
> > 
> > You don't have debug information in your libraries.  Try:
> > 
> > (gdb) ptype cos
> > 
> > to see what I mean.  You need debugging info if you want that to wokr.
>  
> hi daniel,
> 
> i see what you mean.  but then i'm confused why strlen works.
> 
>    18      int main(void)
>    19      {
>    20              char *string = "GNU/Linux";
>    21              char revstring[255];
>    22
>    23              reverse(string, revstring);
>    24              printf("%s\n", revstring);
>    25
>    26              return 0;
>    27      }
>    
>    (gdb) break 21 if strlen(string) == 9
>    Breakpoint 3 at 0x8048440: file string_reverse.c, line 21.

It returns an int.  That's what GDB assumes when it has no better
information.  You get lucky.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer