sing counting semaphores can simplify thread synchronization, in particular, when some resource that has multiple instances.

In the following example, 16 concurrent threads (executing the run function) attempt to access 4 virtual printers. We use a counting semaphore initialized to 4 in order to represent the number of available printers.

Each thread will do the following, forever:

Try and obtain an available printer

Sleep for a random number of seconds (This simulates using the printer.)

Release the printer

Sleep for a random number of seconds (This simulates other actions of a typical thread before needing the printer again.)

Threads will simply be use Decrement and Wait before accessing a printer and Increment and Signal when they have finished using a printer.

Create the file: counting_semaphore.c

#include

#include

#include //compile and link with -pthread

#include

const int NUM_THREADS = 16;

const int NUM_PRINTERS = 4;

sem_t s;

void *run( void *ptr ) {

int tid = (int) pthread_self();

int value;

while(1){

printf("%d needs a printer now! ", tid);

sem_getvalue(&s, &value);

printf("\tsemaphore value = %d ", value);

sem_wait(&s);

printf("%d is printing now! ", tid);

usleep(rand() % 1000);

printf("%d is done printing! ", tid);

sem_getvalue(&s, &value);

printf("\tsemaphore value = %d ", value);

sem_post(&s);

printf("%d is doing something else... ", tid);

usleep(rand() % 1000);

}

}

int main() {

pthread_t threads[NUM_THREADS];

sem_init(&s, 0, NUM_PRINTERS);

srand(time(NULL));

int i;

for(i = 0; i < NUM_THREADS; i++){

pthread_create( &threads[i], NULL, run, NULL);

}

for(i = 0; i < NUM_THREADS; i++){

pthread_join( threads[i], NULL);

}

exit(0);

}

Compile the file:

student@Ubuntu:~/labs/lab3$ gcc -pthread counting_semaphore.c -o cs

Run the counting_semaphore program for a few seconds. Then press <Ctrl+ C> to stop the program:

student@Ubuntu:~ /labs/lab3 $ ./cs

Copy and paste the output below. Check the output for any inconsistency.