POSIX thread problem

[maluk]

Hi all! I got a problem regarding POSIX threads specifically pthread_mutex_lock and pthread_mutex_unlock.

Here is the code:

#include <iostream>
#include <pthread.h>
#include <unistd.h>

using namespace std;

void* doRunA(void* arg);

class A
{
  public:
	A() : a(0) { }
	void Run()
	{
	    pthread_t thr1;
	    pthread_create(&thr1, reinterpret_cast<pthread_attr_t*>(0), doRunA, this);
	    pthread_detach(thr1);
	}

	void IncrementA()
	{
	  pthread_mutex_lock(&mut);
	  this->a++;
	  pthread_mutex_unlock(&mut);
	  // sleep(5);
	}
		  
	void ShowA()
	{
	  cout << "Value of a = " << this->a << endl;
	}
  private:
	int a;
	pthread_mutex_t mut;
};

void* doRunA(void* arg)
{
	A* pA = reinterpret_cast<A*>(arg);

	while(true)
	   pA->IncrementA();

}

int main(int argc, char** argv)
{
	A myA;

		
	myA.Run();
	while(true)
	   myA.ShowA();
		
	return 0;
}

Ok. Try running the code above.
I was expecting to see an output similar to the one below: (an incrementing value)

Value of a = 0;
Value of a = 1;
Value of a = 2;
...
...

But instead, what got are all zeros.

Value of a = 0;
Value of a = 0;
Value of a = 0;
...
...

Can someone help me out?
Thanks!

Rome did not create a great empire by having meetings, they did it by
killing all those who opposed them.

- janvier -

 

[maluk]

Ok. As a follow up, using some logging mechanism I discovered that making the mutex variable a member variable has caused the problem. It seems that the the call to pthread_mutex_lock(&mut) in A::IncrementA() never returns.

Can someone explain the theory behind this? I am really at a loss.

Rome did not create a great empire by having meetings, they did it by
killing all those who opposed them.

- janvier -

 

[Salem]

Unless ShowA() also uses the same mutex then both threads (doRunA and main) are going to run in free-run mode, which basically means they get a block of CPU time before control is passed to the other thread.

So your output would be

Value of a = 0;
Value of a = 0;
...
Value of a = 2345;
Value of a = 2345;
...
...
Value of a = 7890;
Value of a = 7890;

Basically, long runs of identical values, then a big jump to another set of identical values.

Also, how did you create the mutex?

--

 

[maluk]

I think if you declare a mutex as I did in the class definition, it will be created also it will be a 'fast mutex' by default so there is no need to initialize it too.

Rome did not create a great empire by having meetings, they did it by
killing all those who opposed them.

- janvier -