How to turn off Glibc run-time protections?

Question

I am trying to learn about code vulnerabilities, and am testing some simple programs I wrote. However, many of the issues Glibc catches during runtime (e.g. Stack-Smashing,

Answer 1

You can overload operator new and operator delete, but that isn't going to help with a program that uses malloc and free. You can of course write your own implementations of those, also, but overloading C-library functions can be a bit challenging on some OSs.

What, conceptually, is the difference between a double free and free (unallocated_pointer) ?