C程序在创建几个线程后终止

So, I have a project in school in which we need to use C and multithreading etc.. I am a newbie in C, and how pointers work, so I will need your help here. I wrote the program, I execute it, and it creates two or three threads and then says (core dumped). Additionally, I ran it through valgrind, and it obviously has a null pointer, but I cannot solve it. Below is the code and the results when I ran it trough valgrind. So, I have a project in school in which we need