A thread is defined as an independent stream of instructions that can be scheduled to run as such by the operating system. Each program contains a number of threads which can be running in parallel.

The program  intro.c is a demonstration of using  pthread_create(thread,attr,start_routine,arg) function to create a new thread. The parameter thread is used as a way to pass variable between the main programs to the thread. The  attr parameter is used to set the thread attributes, which is usually set to NULL. The start_routine parameter is the C routine being executed when the thread starts. The arg parameter is the argument passed to the routine.

A thread is terminated using the function pthread_exit(NULL).
 
In the beginning of the main program a  thread_id of 1 is assigned to the first thread via the thread_data structure that is declared at the beginning of the program. This is used to identify which process is being run by the processor. Then the thread is created using the pthread_create() function with the rc variable which is used to identified the problem if the thread fail to be created. The first thread will execute the PrintProcess1 routine when the thread is created. After the first thread is created we create the second thread with a thread_id of 2 using the same procedure, but this time the PrintProcess2 routine will be used for the second thread.

In each of the PrintProcess routine the thread_id will be passed onto a process_id variable via the threadid data structure. Then the process will print a message saying the process is running, then delay by the number of seconds that is decided by the process_id, and display a message saying the process is finished. The pthread_exit(NULL) function will be used to terminate the current thread.