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Synchronizing Threads with POSIX Semaphores
- Why semaphores?
- Posix semaphores are easy to use
Now it is time to take a look at some code that does something a little unexpected. The program creates two new threads, both of which increment a global variable called
count exactly NITER, with
NITER = 1,000,000.
But the program produces unexpected results.
and compile it using
gcc threadadd.c -o threadadd -lpthreadRun the executable threadadd and observe the ouput. Try it on both tanner and felix.
Quite unexpected! Since count starts at 0, and both
threads
increment it NITER times, we should see count equal
to 2*NITER at the end of the program. Something fishy is going on here.
Threads can greatly simplify writing elegant and efficient programs. However,
there are problems when multiple threads share a common address
space, like the variable count 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 1 | Thread 2 | |
| a = data; | ||
| a = a+1; | ||
| b = data; // 0 | ||
| b = b + 1; | ||
| data = a; // 1 | ||
| data = b; // 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.
To initialize a semaphore, use sem_init():
int sem_init(sem_t *sem, int pshared, unsigned int value);
- sem points to a semaphore object to initialize
- pshared is a flag indicating whether or not the semaphore should be shared with fork()ed processes. LinuxThreads does not currently support shared semaphores
- value is an initial value to set the semaphore to
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);
- If the value of the semaphore is negative, the calling process blocks; one of the blocked processes wakes up when another process calls sem_post.
To increment the value of a semaphore, use sem_post:
int sem_post(sem_t *sem);Example of use:
sem_post(&sem_name);
- It increments the value of the semaphore and wakes up a blocked process waiting on the semaphore, if any.
To find out the value of a semaphore, use
int sem_getvalue(sem_t *sem, int *valp);
- gets the current value of sem and places it in the location pointed to by valp
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);
- destroys the semaphore; no threads should be waiting on the semaphore if its destruction is to succeed.
sem_destroy(&sem_name);
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 1 | Thread 2 | |
| sem_wait (&mutex); | ||
| sem_wait (&mutex); | ||
| a = data; | ||
| a = a+1; | ||
| data = a; | ||
| sem_post (&mutex); | ||
| b = data; | ||
| b = b + 1; | ||
| data = b; | ||
| sem_post (&mutex); | ||
, so that the program always produces the expected output (the value 2*NITER).
To compile a program that uses pthreads and posix semaphores, use
gcc -o filename filename.c -lpthread -lrt
Exercise 3. Download this (incomplete) code in your posixsem directory (call it
PC.c).
The producer and the consumer share a buffer with four character slots.
Extend this code to implement a solution to the producer
consumer problem using Posix threads and semaphores. Assume
that there is only one producer and one consumer. The output of your code should be
similar to the following:
Producing A ...
Producing B ...
Producing C ...
Producing D ...
------> Consuming A ...
------> Consuming B ...
------> Consuming C ...
------> Consuming D ...
Producing E ...
Producing F ...
Producing G ...
Producing H ...
------> Consuming E ...
------> Consuming F ...
------> Consuming G ...
------> Consuming H ...
Producing I ...
Producing J ...
------> Consuming I ...
------> Consuming J ...
To compile a program that uses pthreads and posix semaphores, use
gcc -o filename filename.c -lpthread -lrt
Exercise 4. Modify the code from exercise 3 to work with multiple producers and multiple consumers. Create three producers and three consumers in the main function and try to interleave their execution by making them sleep for a while. 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 ...