C code freezes/Pauses while execution

fflush() is declared in <stdio.h> Not sure what your problem is compiling with it.

Your screenshot shows garbage characters in the output after "hi". Your screenshot also shows this is a multi-core project. Those two facts make me think that your problem is more likely something like memory corruption causing the progrm you are trying to debug to crash, or possibly multiple cores accessing the same serial port? I would try the basic debugging technique of removing large chunks of code until your printf()'s start working again to get an idea of where the issue might be. I would start with just a single core executing.