Synchronizing Threads with POSIX Semaphores

  1. Why semaphores?
  2. Posix semaphores are easy to use
  3. Exercises   1   2   3


Now it is time to take a look at some code that does something a little unexpected. The program badcnt.c creates two new threads, both of which increment a global variable called cnt exactly NITER, with NITER = 1,000,000. But the program produces unexpected results.


Exercise 1. Create a directory called posixsem in your class Unix directory. Download in this directory the code badcnt.c and compile it using
     gcc badcnt.c -o badcnt -lpthread 
Run the executable badcnt and observe the ouput. Try it on both tanner and felix.

Quite unexpected! Since cnt starts at 0, and both threads increment it NITER times, we should see cnt equal to 2*NITER at the end of the program. What happens?


Threads can greatly simplify writing elegant and efficient programs. However, there are problems when multiple threads share a common address space, like the variable cnt in our earlier example.

To understand what might happen, let us analyze this simple piece of code:

      THREAD 1                THREAD 2
      a = data;               b = data;
      a++;                    b--;
      data = a;               data = b;

Now if this code is executed serially (for instance, THREAD 1 first and then THREAD 2), there are no problems. However threads execute in an arbitrary order, so consider the following situation:

Thread 1Thread 2
data
a = data;
---
0
a = a+1;
---
0
---
b = data;  // 0
0
---
b = b + 1;
0
data = a;  // 1
---
1
---
data = b;  // 1
1

So data could end up +1, 0, -1, and there is NO WAY to know which value! It is completely non-deterministic!

The solution to this is to provide functions that will block a thread if another thread is accessing data that it is using.

Pthreads may use semaphores to achieve this.


Posix semaphores

All POSIX semaphore functions and types are prototyped or defined in semaphore.h. To define a semaphore object, use

      sem_t sem_name;

To initialize a semaphore, use sem_init():

      int sem_init(sem_t *sem, int pshared, unsigned int value);

Example of use:
      sem_init(&sem_name, 0, 10);

To wait on a semaphore, use sem_wait:
      int sem_wait(sem_t *sem);
Example of use:
      sem_wait(&sem_name); 


To increment the value of a semaphore, use sem_post:
      int sem_post(sem_t *sem);
Example of use:
      sem_post(&sem_name); 


To find out the value of a semaphore, use
      int sem_getvalue(sem_t *sem, int *valp);

Example of use:
      int value; 
      sem_getvalue(&sem_name, &value); 
      printf("The value of the semaphors is %d\n", value);

To destroy a semaphore, use
      int sem_destroy(sem_t *sem);
Example of use:
     sem_destroy(&sem_name); 

Using semaphores - a short example

Consider the problem we had before and now let us use semaphores:

 
      Declare the semaphore global (outside of any funcion):

            sem_t mutex;

      Initialize the semaphore in the main function:
     
            sem_init(&mutex, 0, 1);

Thread 1Thread 2
data
sem_wait (&mutex);
---
0
---
sem_wait (&mutex);
0
a = data;
/* blocked */
0
a = a+1;
/* blocked */
0
data = a;
/* blocked */
1
sem_post (&mutex);
/* blocked */
1
/* blocked */
b = data;
1
/* blocked */
b = b + 1;
1
/* blocked */
data = b;
2
/* blocked */
sem_post (&mutex);
2
[data is fine. The data race is gone.]


Exercise 2.Use the example above as a guide to fix the program badcnt.c, so that the program always produces the expected output (the value 2*NITER). Make a copy of badcnt.c into goodcnt.c before you modify the code.

To compile a program that uses pthreads and posix semaphores, use

     gcc -o filename filename.c -lpthread -lrt 

Exercise 3. Download this incomplete producer-consumer code in your posixsem directory. Complete the downloaded code to implement a solution to the Producer-Consumer problem using Posix threads and semaphores.

Comment well your code. Compile and run your program and observe the output. Label each line in the output by the identifier for each producer and consumer (P1, P2, P3, C1, C2, C3). The output of your program should be similar to the following:

      [P1] Producing A ...
      [P1] Producing B ...
       ------> [C1] Consuming A ...
       ------> [C1] Consuming B ...
      [P2] Producing A ...
      [P2] Producing B ...
       ------> [C2] Consuming A ...
       ------> [C2] Consuming B ...
      [P3] Producing A ...
      [P3] Producing B ...
       ------> [C3] Consuming A ...
       ------> [C3] Consuming B ...
      [P1] Producing C ...
      [P1] Producing D ...
       ------> [C1] Consuming C ...
       ------> [C1] Consuming D ...
      [P2] Producing C ...
      [P2] Producing D ...
       ------> [C2] Consuming C ...
       ------> [C2] Consuming D ...
      [P3] Producing C ...
      [P3] Producing D ...
       ------> [C1] Consuming C ...
      [P2] Producing E ...
       ------> [C2] Consuming D ...
      [P3] Producing E ...
       ------> [C3] Consuming E ...
       ------> [C3] Consuming E ...
      [P1] Producing E ...
       ------> [C3] Consuming E ...
      [P2] Producing F ...
       ------> [C2] Consuming F ...
      [P3] Producing F ...
       ------> [C1] Consuming F ...
      [P1] Producing F ...
       ------> [C3] Consuming F ...

To compile a program that uses pthreads and posix semaphores, use

     gcc -o filename filename.c -lpthread -lrt