Multiple threads are expected to run in parallel to share the workload, i.e. suppose 3 threads to process 30 files totally, then each thread should process 10 files;

When a thread is created, a message should be print out showing which files this thread will process, for example:

Thread id = 274237184 starts processing files with index from 0 to 10!

When a file is being processed, a message should be print out showing which thread (thread_id = xxx) is processing this file, for example:

Thread id = 265844480 is processing file input_11.txt

When a thread is done with its workload, a message should be print out showing which files this thread has done with work, for example:

Thread id = 274237184 is done !

The array long specialfreq[ ] should always be up-to-date, i.e. it always has the result of all the threads counted so far. [You may need to use mutexes to protect this critical region.]