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Question

I am playing around with semaphores and get myself into this problem. What I am trying to do is updating the semaphore inside the pthread from the main thread. I created the struct , passed the semaphore with that struct into the pthread and used the struct in another function to do sem_post . But the following code is not working. It seems that semaphore I passed into the pthread and the one I used in start function are different. There are a lot of examples which create semaphores globally and use in threads. But I would like to create semaphore locally so that if needed, I can create multiple semaphores and multiple threads to synchronize. How should I approach this problem?

#include <iostream> 
#include <semaphore.h> 
#include <pthread.h> 
#include <signal.h>
#include "unistd.h"

using namespace std; 

class m { 
    public: 
        struct my_sem { 
            sem_t sem_lock; 
        };  
        m(); 
        ~m(); 

        void create_threads(); 
        void start(my_sem*); 
        static void *hello(void *);  
}; 

m::m() { 
    create_threads(); 
} 

m::~m() {}

void m::create_threads() { 
    pthread_t t1, t2; 
    sem_t s; 

    sem_init(&s, 0, 0); 

    my_sem *sem1 = new my_sem; 
    sem1->sem_lock = s; 

    pthread_create(&t1, null, hello, sem1); 
    start(sem1); 
} 

void *m::hello(void *arg) { 
    my_sem* a = (my_sem*)arg; 
    sem_t t = a->sem_lock; 

    while(1) { 
        sem_wait(&t); 
        cout << "hello" << endl; 
    } 
} 

void m::start(my_sem* s) { 
    sem_t h = s->sem_lock; 
    while(1) { 
        sleep(10); 
        sem_post(&h); 
    } 
} 

int main() { 

    m m;     
    return 0; 
} 

The Output I am expecting here is:

HELLO 
<wait 10 seconds> 
HELLO 
<wait 10 seconds> 
.
.
.

Answer 1

You don't copy semaphores as they rely on their address (location in the memory).

Assigning one sem_t to another, will not share the state between them. The assignment will copy some values, but you will have two distinct semaphore objects.

Instead, you create a single semaphore as a member of some class and pass pointers to it, like this:

void m::create_threads() { 
    pthread_t t1, t2; 

    sem_init(&this->sem.sem_lock, 0, 0); 

    pthread_create(&t1, nullptr, hello, &this->sem); 
    start(&this->sem);
} 

void *m::hello(void *arg) { 
    my_sem* a = (my_sem*)arg; 

    while(1) { 
        sem_wait(&a->sem_lock); 
        cout << "hello" << endl; 
    } 
} 

void m::start(my_sem* s) { 
    while(1) { 
        sleep(1); 
        sem_post(&s->sem_lock); 
    }
}