Synchronisation before pthread_cond_broadcast - c++

Question

I want to send a broadcast signal from the main thread to all the other threads waiting for a condition. It seems to me that the broadcast signal comes to early to the threads.
#include <iostream>
#include <pthread.h>
#define NUM 4
#define SIZE 256
using namespace std;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_barrier_t barrier;
class cache{
int lv1;
public:
int write(int i){
lv1=i;
pthread_cond_broadcast(&cond);
}
};
cache c[NUM];
void *thread(void *arg){
int i = (int)arg;
for(;;){
pthread_mutex_lock(&mutex);
pthread_cond_wait(&cond,&mutex);
cout << "Thread: "<< i << endl;
//do some work
pthread_mutex_unlock(&mutex);
}
}
int main()
{
pthread_t tid[NUM];
pthread_barrier_init(&barrier,NULL,NUM+1);
for(int i=0;i<NUM;i++){
pthread_create(&tid[i],NULL,thread,(void*)i);
}
//Sleep(2);
c[0].write(55); //broadcast signal
//Sleep(2);
c[1].write(44); //broadcast signal
for(int i=0;i<NUM;i++){
pthread_join(tid[i],NULL);
}
cout << "Hello world!" << endl;
return 0;
}
If I insert Sleep(2) in the main function, it works, but I do not want to wait a time but a synchronisation before calling pthread_broadcast.
I thought of a barrier, but pthread_cond_wait is blocking, right?

Answer 1

You need to read up on how condition variables are used. You also handled the integer arguments to your threads erroneously. Here is a fixed version, hopefully similar to what you wanted:
#include <iostream>
#include <pthread.h>
#include <unistd.h>
#define NUM 4
#define SIZE 256
using namespace std;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_barrier_t barrier;
class cache{
int lv1;
public:
int write(int i) { lv1=i; }
const int val() { return lv1; }
};
cache c[NUM];
void *thread(void *arg)
{
int i = *(int*)arg;
for(;;) {
pthread_mutex_lock(&mutex);
// Check predicate, do not go to sleep if predicate is fulfilled
if (c[0].val() > 0 && c[1].val() > 0) {
cout << "Thread " << i << " leaving...\n";
pthread_mutex_unlock(&mutex);
return 0;
}
pthread_cond_wait(&cond, &mutex);
cout << "Thread wakeup: "<< i << endl;
// do some work
pthread_mutex_unlock(&mutex);
}
}
int main()
{
pthread_t tid[NUM];
int arg[NUM];
for(int i=0; i<NUM; i++) {
arg[i] = i; // make a copy of i used by only one thread
pthread_create(&tid[i], NULL, thread,(void*)&arg[i]);
}
pthread_mutex_lock(&mutex);
c[0].write(55);
c[1].write(44);
pthread_mutex_unlock(&mutex);
pthread_cond_broadcast(&cond); // Signal all threads that predicate is fulfilled
for(int i=0; i<NUM; i++) {
pthread_join(tid[i],NULL);
cout << "Joined " << i << '\n';
}
cout << "Hello world!" << endl;
return 0;
}

Answer 2

Sequence how I think it is
This is how it is right now I think, I still need a way to synchronise, before I send the broadcast signal.
In that picture, the first pthread_broadcast comes too early.