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Print numbers in sequence using thread synchronization
  • Difficulty Level : Expert
  • Last Updated : 08 Oct, 2020

The problem is to synchronize n number of threads using pthread library. The idea is take thread count and print 1 in first thread, print 2 in second thread, print 3 in third thread, ….. print n in nth thread and again repeat from thread 1 infinitely. 
Prerequisite : Multithreading

Examples : 

Input : Thread count
Output : 1 2 3 ... thread count 1 2 3 ... thread 
count 1 2 3 ... thread count ....

Input : 5
Output : 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 
1 2 3 4 5 1 2 3 4 5 ....

 Below is the implementation:

CPP




// C code to synchronize threads
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
 
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t* cond = NULL;
 
int threads;
volatile int cnt = 0;
 
// function to synchronize threads
void* foo(void* arg)
{
    // turn is a basically to identify a thread
    int turn = *(int*)arg;
 
    while (1) {
        pthread_mutex_lock(&mutex);
 
        // cnt is used to determne which thread should
        // enter into critical section(printf() statement)
        if (turn != cnt) {
 
            // put all thread except one thread in waiting
            // state
            pthread_cond_wait(&cond[turn], &mutex);
        }
 
        // it's a time to print turn can have
        // values starting from 0. Hence + 1
        printf("%d ", turn + 1);
 
        // detemine which thread need to be scheduled now
        if (cnt < threads - 1) {
            cnt++;
        }
        else {
            cnt = 0;
        }
 
        // weak up next thread
        pthread_cond_signal(&cond[cnt]);
        pthread_mutex_unlock(&mutex);
    }
 
    return NULL;
}
 
// Driver code
int main()
{
    pthread_t* tid;
    volatile int i;
    int* arr;
 
    printf("\nEnter number of threads: ");
    scanf("%d", &threads);
 
    // allocate memory to cond (conditional variable),
    // thread id's and array of size threads
    cond = (pthread_cond_t*)malloc(sizeof(pthread_cond_t)
                                   * threads);
    tid = (pthread_t*)malloc(sizeof(pthread_t) * threads);
    arr = (int*)malloc(sizeof(int) * threads);
 
    // Initialize cond (conditional variable)
    for (int i = 0; i < threads; i++) {
        if (pthread_cond_init(&cond[i], NULL) != 0) {
            perror("pthread_cond_init() error");
            exit(1);
        }
    }
 
    // create threads
    for (i = 0; i < threads; i++) {
        arr[i] = i;
        pthread_create(&tid[i], NULL, foo, (void*)&arr[i]);
    }
 
    // waiting for thread
    for (i = 0; i < threads; i++) {
        pthread_join(tid[i], NULL);
    }
 
    return 0;
}

Output : 
 

 

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