I have some questions about multi threading .

[nirmalshravanthk]

I ve been given this question in a test and i failed to answer it because i wasnt that adept at multi threading . Can any of u guys explain it to me. THe question is below.

class Buffer {
   static int dataBuffer[100];
   static int head = 0;
   static int tail = 0;
 
   public:
   static void Produce(int data) {
      dataBuffer[head++] = data;
      if (head == 100) head = 0;
   }
   static bool CanConsume() {
      if (head != tail) return true;
      return false;
   }
   static int Consume() {
      int pos = tail++;
      if (tail == 100) tail = 0;
      return dataBuffer[pos];
   }
}
void Producer() {
   while(1) {
      … some stuff …
      Buffer.Produce(x);
      … some stuff …
   }
}
void Consumer() {
   while(1) {
      … some stuff …
      if(Buffer.CanConsume() ) {
         int x = Buffer.Consume();
         … do something with x …
      }
      … some stuff …
   }
}
 

a. If we have a multithreaded system with one thread executing ‘Producer’ and one thread executing ‘Consumer’ is there anything wrong with the code above? Give details.




b. If we have two threads running ‘Producer’ and one thread running ‘Consumer’ is there anything wrong with the code above? Give details.




c. If we have one thread Running ‘Producer’ and two threads running ‘Consumer’ is there anything wrong with the code above? Give details.

[sicarie]

So, due to this site's policy on homework questions, let's start with your answers.

What do you think the answers to these are?

[nirmalshravanthk]

So, due to this site's policy on homework questions, let's start with your answers.

What do you think the answers to these are?

I think if we just use a single thread then te producer and consumer will get into a deadlock .

But if we use two threads of producer and one thread of consumer it will be fine.

Am i right ??

[SpecialKay]

I as well dont know the answer, but i understand threading some what. If i had to guess. i think that you are right, single threaded the code would block in the endless loop. if both function were running at the same time, i think it would work alright. As for having 2 threads running consumer and likewise for producer, i could only guess it would work, but you might get some wierd values.

interesting tho, i look forward to the answer.

[weaknessforcats]

Any code using static variables cannot be multi-threaded. Those variables will be visible to every thread and their values cannot be relied upon unless thay are protected by a critical section, mutex, or some such.

I don't see any such protections in the code.

If you intend those static variables to be unique per thread, then don't make thenm static. Instead, have the thread create them on the heap and use the pointers in the thread code. Then just delete the pointers before you exit the thread.

[JosAH]

That buffer is the shared resource so it should be protected by a mutex (a binary
semaphore). A producer should wait as long as the buffer is full, a consumer should
wait as long as the buffer is empty. When any of them passes the mentioned
wait-loop, they should notify all the other waiting threads so they are able to do
whatever they want to do. It is multi-threading 101.

kind regards,

Jos

[SpecialKay]

no where in the code is there a signal to the other thread. So that, plus the Static decliarations are the problems?