libformatstr VS peda

I still don't understand why would you prefer raw memory manipulation to bytestring manipulation. If you want, just make a Swift library that will implement the memory like you want but without unsafe raw access (but just a few methods over a byte array). Back in the days when I did CTFs, I used Python for writing binary exploits, never C.

https://github.com/hellman/libformatstr

You can do something like this, no need to work with raw memory.

libformatstr - Simplify format string exploitation.

Looks like they are using PEDA.

There are a lot of these types of tools already in the reverse engineering community (in order of lowest chance of breaking when you throw really weird stuff at it):

GEF: https://gef.readthedocs.io/en/master/

PWNDBG: https://github.com/pwndbg/pwndbg

PEDA: https://github.com/longld/peda

They also come with a slew of different features to aid in RE/exploit dev, but many of them are also useful for debugging really weird issues.

PEDA - GDB plugin (only python2.7).

peda - Python Exploit Development Assistance for GDB.

Did you follow the instructions? Step 2 adds PEDA to your ~/.gdbinit so it will/should load every time you open gdb. So if it doesn't work check your ~/.gdbinit file.