Reusable barrier implementation using POSIX semaphores - c

Question

Need a solution that creates 5 pthreads. Each pthread executes a function that involves iterating through a loop 10 times. In each iteration of the loop, a thread increments an int from 0 to 0.9*MAX_INT and then prints the iteration number. Make sure that each of the 5 threads finish the ith iteration of the loop before they can start the (i+1)th iteration (i.e. all threads synchronize/rendezvous towards the end of each iteration). I need to use a two-phase barrier implemented using POSIX semaphores to enforce the synchronization constraint
I wrote following code am I correct ?
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int thread_count;
void* MyThread(void* rank);
int main()
{
long thread;
pthread_t* thread_handles;
thread_count = 5;
thread_handles = malloc (thread_count*sizeof(pthread_t));
for (thread = 0; thread < thread_count; thread++)
pthread_create(&thread_handles[thread],NULL,MyThread,(void*) thread);
for (thread = 0; thread < thread_count; thread++)
pthread_join(thread_handles[thread], NULL);
free(thread_handles);
return 0;
}
void* Hello(void* rank)
{
long my_rank = (long) rank;
int a,i;
a=0;
for(i=0;i<10;i++)
{
int n = 5;
int count = 0;
pthread_mutex_t mutex = Semaphore(1)
barrier = Semaphore(0)
a = a + 0.9*MAX_INT;
printf("this is %d iteration\n",i);
mutex.wait()
count = count + 1
mutex.signal()
if count == n: barrier.signal() # unblock ONE thread
barrier.wait()
barrier.signal()
}
}
Edit:
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>
#include <semaphore.h>
typedef struct {
int n;
int count;
sem_t mutex;
sem_t turnstyle;
sem_t turnstyle2;
} barrier_t;
void init_barrier(barrier_t *barrier, int n)
{
barrier->n = n;
barrier->count = 0;
sem_init(&barrier->mutex, 0, 1);
sem_init(&barrier->turnstyle, 0, 0);
sem_init(&barrier->turnstyle2, 0, 0);
}
void phase1_barrier(barrier_t *barrier)
{
sem_wait(&barrier->mutex);
if (++barrier->count == barrier->n) {
int i;
for (i = 0; i < barrier->n; i++) {
sem_post(&barrier->turnstyle);
}
}
sem_post(&barrier->mutex);
sem_wait(&barrier->turnstyle);
}
void phase2_barrier(barrier_t *barrier)
{
sem_wait(&barrier->mutex);
if (--barrier->count == 0) {
int i;
for (i = 0; i < barrier->n; i++) {
sem_post(&barrier->turnstyle2);
}
}
sem_post(&barrier->mutex);
sem_wait(&barrier->turnstyle2);
}
void wait_barrier(barrier_t *barrier)
{
phase1_barrier(barrier);
phase2_barrier(barrier);
}
#define NUM_THREADS 5
void *myThread(void *);
int main(int argc, char **argv)
{
pthread_t threads[NUM_THREADS];
barrier_t barrier;
int i;
init_barrier(&barrier, NUM_THREADS);
for (i = 0; i < NUM_THREADS; i++) {
pthread_create(&threads[i], NULL, myThread, &barrier);
}
for (i = 0; i < NUM_THREADS, i++) {
pthread_join(threads[i], NULL);
}
return 0;
}
void *myThread(void *arg)
{
barrier_t *barrier = arg;
int i,a;
for(i=0;i<10;i++)
{
a = a + 0.9*MAX_INT;
printf("this is %d iteration\n",i);
}
return NULL;
}

Answer 1

OK, if we examine the Barrier object in, "The Little Book of Semaphores", section 3.7.7, we see that we need a mutex and 2 semaphores called turnstile and turnstile2 (a mutex can be a semaphore that is initialized to 1).
Since we have to use POSIX semaphores, pthreads, and INT_MAX, we start by including the requisite header files:
#include <pthread.h>
#include <semaphore.h>
#include <limits.h>
The book makes the Barrier an object; however, in C, we don't really have objects, but we can create a struct with some functions to operate on it:
typedef struct {
int n;
int count;
sem_t mutex;
sem_t turnstile;
sem_t turnstile2;
} barrier_t;
We can create a function to initialize a barrier:
void init_barrier(barrier_t *barrier, int n)
{
barrier->n = n;
barrier->count = 0;
sem_init(&barrier->mutex, 0, 1);
sem_init(&barrier->turnstile, 0, 0);
sem_init(&barrier->turnstile2, 0, 0);
}
And implement the phase1 function, as in the book:
void phase1_barrier(barrier_t *barrier)
{
sem_wait(&barrier->mutex);
if (++barrier->count == barrier->n) {
int i;
for (i = 0; i < barrier->n; i++) {
sem_post(&barrier->turnstile);
}
}
sem_post(&barrier->mutex);
sem_wait(&barrier->turnstile);
}
Note that the sem_post function only posts a single time, so a loop is required to post the turnstile n times.
The phase2 function also follows directly in the same manner:
void phase2_barrier(barrier_t *barrier)
{
sem_wait(&barrier->mutex);
if (--barrier->count == 0) {
int i;
for (i = 0; i < barrier->n; i++) {
sem_post(&barrier->turnstile2);
}
}
sem_post(&barrier->mutex);
sem_wait(&barrier->turnstile2);
}
Finally, we can implement the wait function:
void wait_barrier(barrier_t *barrier)
{
phase1_barrier(barrier);
phase2_barrier(barrier);
}
Now, in your main function, you can allocate and initialize a barrier and pass it to your spawned threads:
#define NUM_THREADS 5
void *myThread(void *);
int main(int argc, char **argv)
{
pthread_t threads[NUM_THREADS];
barrier_t barrier;
int i;
init_barrier(&barrier, NUM_THREADS);
for (i = 0; i < NUM_THREADS; i++) {
pthread_create(&threads[i], NULL, myThread, &barrier);
}
for (i = 0; i < NUM_THREADS, i++) {
pthread_join(threads[i], NULL);
}
return 0;
}
And finally, implement the thread:
void *myThread(void *arg)
{
barrier_t *barrier = arg;
int i;
int a;
for (i = 0; i < 10; i++) {
for (a = 0; a < 0.9*INT_MAX; a++);
printf("this is %d iteration\n", i);
wait_barrier(barrier);
}
return NULL;
}