The differences between pthread_cond_t and std::condition_variable

Question

I am now testing std::condition_variable recently , and find it is quite different with pthread_cond_t after test , I like to know if anything in my test wrong ? or std::condition_variable is really quite different with pthread_cond_t ?

The pthread_cond_t source is the following , compiled at gcc 4.4.6 :

pthread_cond_t  condA  = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int ProcessRow = 0 ;
#define LOOPCNT 10

void *producer()
{
    int idx ;
    for(idx=0;idx<LOOPCNT;idx++)
    {
        //pthread_mutex_lock(&mutex);
        __sync_add_and_fetch(&ProcessRow,1) ;
        pthread_cond_signal(&condA);
        printf("sending signal...(%d)\n",ProcessRow) ;
        //pthread_mutex_unlock(&mutex);
    }
    printf("I am out ... \n") ;
}

void *consumer()
{
    int icnt = 0 ;
    while(1)
    {
        pthread_mutex_lock(&mutex);
        while (ProcessRow <= 0)
            pthread_cond_wait(&condA, &mutex);
        pthread_mutex_unlock(&mutex); // I forget to add unlock to fail this test
        __sync_sub_and_fetch(&ProcessRow,1) ;
        ++icnt ;
        printf("receving=(%d)\n",ProcessRow) ;
        usleep(10000) ;
    }
    printf("(%d)\n",ProcessRow) ;
}

The output :

sending signal...(1)
sending signal...(2)
sending signal...(3)
sending signal...(4)
sending signal...(5)
sending signal...(6)
sending signal...(7)
sending signal...(8)
sending signal...(9)
sending signal...(10)
I am out ...
receving=(9)

Look like comsumer thread block in pthread_cond_wait , so that "receving" only print one time !!!!

and then the following test is for std::condition_variable !!!!

The following binsem.hpp comes from https://gist.github.com/yohhoy/2156481
with a little modification , compiled at g++ 4.8.1

class binsem {
public:    
    explicit binsem(int init_count = count_max)      
   : count_(init_count) {}     
// P-operation / acquire    
void wait()    
{        
    std::unique_lock<std::mutex> lk(m_);        
    cv_.wait(lk, [this]{ return 0 < count_; });
    --count_;    
}    
bool try_wait()    
{        
    std::lock_guard<std::mutex> lk(m_);        
    if (0 < count_) 
    {            
        --count_;            
        return true;        
    } else 
    {            
        return false;        
    }    
}
// V-operation / release    
void signal()    
{        
    std::lock_guard<std::mutex> lk(m_);

    //if (count_ < count_max)  // I mark here
    //{  // I mark here
            ++count_; 
            cv_.notify_one();        
    //}    // I mark here
}     
// Lockable requirements    
void lock() { wait(); }    
bool try_lock() { return try_wait(); }    
void unlock() { signal(); } 
private:    
    static const int count_max = 1;    
    int count_;    
    std::mutex m_;    
    std::condition_variable cv_;
}; 

and my source :

#define LOOPCNT 10
atomic<int>  ProcessRow  ;

void f4()
{
    for(int i=0;i<LOOPCNT;i++)
    {
        sem2.unlock() ;
        ++ProcessRow ;
    }
    cout << "i am out" << endl ;
}

void f5()
{
    int icnt = 0 ;
    std::chrono::milliseconds sleepDuration(1000);
    while(1)
    {
        sem2.lock() ;
        ++icnt ;
        std::this_thread::sleep_for(sleepDuration);
        cout << ProcessRow << "in f5 " << endl ;
        --ProcessRow ;
        if(icnt >= LOOPCNT)
            break ;
     }
     printf("(%d)\n",icnt) ;
}

The output :

i am out
10in f5
9in f5
8in f5
7in f5
6in f5
5in f5
4in f5
3in f5
2in f5
1in f5
(10)

Look like signal only effect if the pthread_cond_wait is waiting!! if not , signal is losted !!

And for std::condition_variable , look like std::condition_variable.wait() will wake up the times notify_one() are called ,if you call notify_one() 10 seconds ago and then call wait() , std::condition_variable.wait() still will get that notify_one() message , quite different with pthread_cond_t !!

Am I miss something in this test ? Or just like my test , std::condition and pthread_cond_t just act like the test showes ?

Edit :

I think the following will showes more easier for this test , sorry to forget to unlock so that the test failed , they are the same behavior !!!!

int main()
{
    //pthread_mutex_lock(&mutex);
    ++ProcessRow ;
    pthread_cond_signal(&condA);
    //pthread_mutex_unlock(&mutex);
    printf("sending signal...\n") ;
    sleep(10) ;

    pthread_mutex_lock(&mutex);
    while (ProcessRow <= 0)
        pthread_cond_wait(&condA, &mutex);
    pthread_mutex_unlock(&mutex);
    printf("wait pass through\n") ;
}

This will showes :

sending signal...
wait pass through

And for std::condition_variable

int main()
{
sem2.unlock() ;
std::chrono::milliseconds sleepDuration(10000);
cout << "going sleep" << endl ;
std::this_thread::sleep_for(sleepDuration);
sem2.lock() ;
cout << "lock pass through " << endl ;

} 

Will showes :

going sleep
lock pass through

So it is my fault to do the test wrong , cause to deadlock !!! Thanks for all great advice!

Answer 1

In your pthread code, you never unlock the mutex, The consumer() function deadlocks on the second iteration. Also, the outer while loop should break out when some condition is satisfied. I suggest that it should break out when icnt reaches the LOOPCNT. This sort of matches how you break the loop in f5().

void *consumer(void *x)
{
    int icnt = 0 ;
    while(1)
    {
        pthread_mutex_lock(&mutex);
        while (ProcessRow <= 0)
            pthread_cond_wait(&condA, &mutex);
        __sync_sub_and_fetch(&ProcessRow,1) ;
        ++icnt ;
        printf("receving=(%d) icnt=(%d)\n",ProcessRow, icnt) ;
        pthread_mutex_unlock(&mutex);
        if (icnt == LOOPCNT) break;
        usleep(10000) ;
    }
    printf("(%d)\n",ProcessRow) ;
}

It doesn't seem like your std::thread version of the code closely matches the pthread version at all, so I don't think you can compare their executions in this way. Instead of mimicking a semaphore, I think it better to just use the std::condition_variable exactly like you use it in the pthread version of the code. This way, you can really compare apples to apples.

std::condition_variable condA;
std::mutex mutex;
volatile int ProcessRow = 0 ;
#define LOOPCNT 10

void producer()
{
    int idx ;
    for(idx=0;idx<LOOPCNT;idx++)
    {
        std::unique_lock<std::mutex> lock(mutex);
        __sync_add_and_fetch(&ProcessRow,1) ;
        condA.notify_one();
        printf("sending signal...(%d)\n",ProcessRow) ;
    }
    printf("I am out ... \n") ;
}

void consumer()
{
    int icnt = 0 ;
    while(icnt < LOOPCNT)
    {
        if(icnt > 0) usleep(10000);
        std::unique_lock<std::mutex> lock(mutex);
        while (ProcessRow <= 0)
            condA.wait(lock);
        __sync_sub_and_fetch(&ProcessRow,1) ;
        ++icnt ;
        printf("receving=(%d) icnt=(%d)\n",ProcessRow, icnt) ;
    }
    printf("(%d)\n",ProcessRow) ;
}

Answer 2

Both pthread_cond_t and std::condition_variable work the same way. They are stateless and a signal can only get "lost" if no thread is blocked, in which case no signal is needed because there is no thread that needs one.