How to evaluate functions in GDB?

Question

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

(gdb) p pow(3,         


        

Answer 1

Actually, at least on my LINUX implementation of gcc, many of the math functions are replaced with variants specific to the types of their arguments via some fancy substitutions pulled in by math.h and bits/mathcalls.h (included from within math.h). As a consequence, functions like pow and exp are called instead as __pow or *__GI___exp (your results may vary depending on the types of the arguments and perhaps the particular version).

To identify what exactly the function is that is linked in to my code I put a break at a line where just that function is called, e.g. have a line in my code with b=exp(c);. Then I run in gdb up till that break point and then use the "step" command to enter the call from that line. Then I can use the "where" command to identify the name of the called routine. In my case that was *__GI___exp.

There are probably cleverer ways to get this information, however, I was not able to find the right name just by running the preprocessor alone (the -E option) or by looking at the assembly code generated (-s).