gdb evaluation of a function

Question

Hi, I wonder why evaluate function doesn't work in gdb? In my source file I include, when debugging in gdb, these examples are wrongly evaludations.

(gdb) p pow(3,2)

$10 = 1

(gdb) p pow(3,3)

$11 = 1

(gdb) p sqrt(9)

$12 = 0

Thanks and regards!

Answer 1

The syntax for calling a function in gdb is

call pow(3,2)

Type

help call

at the gdb prompt for more information.

Answer 2

NAME
   pow, powf, powl - power functions

SYNOPSIS
   #include <math.h>

   double pow(double x, double y);

You shouldn't pass an int in the place of a double

 call pow( 3. , 2. )

Also, passing a single argument is not enough, you need two arguments just like the function expects

 wrong: call pow ( 3. )

Answer 3

My guess is that the compiler and linker does some magic with those particular functions. Most likely to increase performance.

If you absolutely need pow() to be available in gdb then you can create your own wrapper function:

double mypow(double a, double b)
{
    return pow(a,b);
}

Maybe also wrap it into a #ifdef DEBUG or something to not clutter the final binary.

BTW, you will notice that other library functions can be called (and their return value printed), for instance:

(gdb) print printf("hello world")
$4 = 11