$4.$ A mutex is an exclusive lock to avoid non$-$atomic operation on a shared resource. Atomic

operations are ensured by setting a mutex before using the resource and releasing it after using

the resource. Research the mutex and review the multithreaded program written in C language

on Week $7$ Lecture Note page $33$ Three Conceptual Pieces: Concurrency. Note that the lecture

code can be found on Moodle.

$($a$)$ Add a mutex in the code so that only one thread can access the variable counter at a

time and the final value of counter is always twice of the input. Assume the same header

files common.h and common threads.h are given, and only supply your modified code of

main.c with corresponding comments.

$($b$)$ Use either a locally installed compiler or an online compiler to test your code with $4$

different inputs, including your student number and three larger integers than it$.$ Provide

screenshots of your $4$ tests$/$outputs$.$