3. Threads

3. Threads
	Chapter 10. Individual Assignments - Laboratory

Threads

The following functions are from the Linux threads library based on the POSIX 1003.b standard:

  pthread_create - Adds a new thread of control to the current process.

  sched_yield - Causes the current thread to yield its execution in 
                favor of another thread with the same or greater priority.

  pthread_exit - Terminates a thread.

  pthread_join - Blocks the calling thread until a specified thread
                terminates.

The following program creates two threads. Each prints the name of a city a number of times, with a random delay between the prints. Run it a number of times.

#include <pthread.h>
/* 
* Code of the threads 
*/ 
  void* printPlace(void* name) 
  { 
     int aux1, aux2, aux3, count; 
    /* Find number of loops */ 
     if (strcmp(name, "Jamaica")==0) 
        count= 5; 
     else count=10; 
        /* Main loop */ 
        for (aux1=0; aux1 < count; aux1++) 
        { 
            /* Print Name */ 
            printf("%d %s\n", aux1, name); 
            /* Create a variable delay */ 
            for (aux2= rand()/10000; aux2 > 0; aux2--) 
                for(aux3=10; aux3 > 0; aux3--); 
            /* Yields control */ 
            /* sched_yield(); */ 
        } pthread_exit(0); 
  }
  
  void main()
  { 
      /* Variables that keep the threads ids */ 
         pthread_t simpleThread1, simpleThread2; 
      /* Maximum concurrence level */ 
         pthread_setconcurrency(100); 
         printf("Concurrency %d\n", pthread_getconcurrency()); 
      /* Create first thread */ 
         pthread_create(&simpleThread1, 0, printPlace, "Java"); 
      /* Create second thread */ 
         pthread_create(&simpleThread2, 0, printPlace, "Fiji"); 
      /* Wait until all threads stop */ 
         pthread_join(simpleThread1, 0); pthread_join(simpleThread2, 0); 
      /* Wait until a specific thread stops */ 
      /* pthread_join(simpleThread1, 0); */ 
  }

To compile the program:

cc threads.c -o threads -lpthread

Each time you run the program it prints out a random list with the cities. Explain how it works What is the function of lines:
```
pthread_join(simpleThread1, 0);
pthread_join(simpleThread2, 0);          
```
Uncomment the line:
```
/* sched_yield(); */          
```
Run the program. What happens? Why?
Undo the change you have done.

Change the name "Java" on the following line for "Jamaica":

pthread_create(simpleThread1, 0, printPlace,"Java");

Run the program. Now comment the lines:

pthread_join(simpleThread1, 0);
pthread_join(simpleThread2, 0);

And uncomment the line:

/*   pthread_join(simpleThread1, 0);       */

Run the program. What happens to simpleThread1?


2. Pipes		4. Threads Synchronization