Re: question about conditional breaking

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Sep 20, 2002 at 12:36:48AM -0700, Peter Jay Salzman wrote:
> dear gdb mailing list,
> 
> the gdb user's manual doesn't have a lot to say about conditional
> breaking, but i think i'm beginning to understand it more thoroughly.
> 
> first, we know that the boolean expressions you'd expect to be able to
> use, work as advertised.
> 
> break 1 if x > 0
> break 2 if y <= x
> 
> parenthesis seem to be purely optional.
> 
> break 3 if (! x >= -1)
> 
> 
> but there seems to be more to the story.  you can use functions too.
> here's a test program:
> 
> #include <math.h>
> 
> int main(void)
> {
>    double l = cos(i);
>    double m = y0(i);
> 
>    return 0;
> }
> 
> 
> where y0 is a bessel function of the 2nd kind (a von neumann function)
> of the zeroth order.
> 
> it seems like you can use functions, like math library functions.  but
> with a privisio:
> 
>    (gdb) break main if cos(3) == 0
>    Breakpoint 1 at 0x8048456: file math.c, line 5.
>    
>    (gdb) break main if sin(3) == 0
>    No symbol "sin" in current context.
>    
>    (gdb) break main if exp(3) == 0
>    No symbol "exp" in current context.
>    
>    (gdb) break main if y0(3) == 0
>    Note: breakpoint 1 also set at pc 0x8048456.
>    Breakpoint 2 at 0x8048456: file math.c, line 5.
>    
>    (gdb) break main if y1(3) == 0
>    No symbol "y1" in current context.
> 
> 
> so it appears that here's the rule:
> 
> you're allowed to use any function, even library functions, provided that:
> 
> 1. the library is linked to your application
> 2. you actually _use_ the function somewhere in your code.
> 
> to be honest, i'm not the least surprised by condition 1, but i'm
> shocked by condition 2.

You can use any C expression that GDB can evaluate to a value; see the
'print' command.  (2) should not be necessary if you have shared
libraries; but if you don't, then the function is likely to not be
present in the application unless you used it.  GDB can't call
functions that aren't there.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer