gdb_related.md

How to avoid optimized out at variable evaluation

• Sample cpp code

#include <iostream>
#include <cmath>
int main()
{
    int x=1;
    std::cin >> x;
    int y=0;
    if (x==1 || x==2)
    {
	y = 3;
    } else
    {
	y = 4;
    }
    x = 2;
    int z = 3;
    if (x==1 || x==2)
    {
	z = 13;
    } else
    {
	z = 14;
    }
     std::cout << z << std::endl;
    return 0;
}

• By the end of the code, -O3 -g will yield optimized out for variable y

gdb-peda$ p y
$1 = <optimized out>

• -O1 yields same results
• Applying -O0 will show the value of y but in -O3 state, we can use volatile int y=0;
  • This yields:

gdb-peda$ p y
$1 = 0x4

when std::bad_cast exception

• bt may not work. Need to stop throw
• catch throw then run the execuble within gdb, to stay in the stack memory
• bt will work now