Re: [Boost-users] [Thread] finding a good expression to wait...

8 Sep 2009


      AMDG

Julien Claassen wrote:
...
Hello everyone!
  I'm trying to build a simple user-interface. It reads events from 
the keyboard using cin in one thread and then yields to a controller, 
which does some work.
  I'm doing the waiting part in the threads using condition_variables. 
The input-threads "main-function" looks something like this:
*** CODE SNIPPET ***
condition input_cond, output_cond;
// some other code
void main_loop(condition& in, condition& out)
{
  char input_key;
  boost::mutex::scoped_lock lk(my_global_mutex);
  while (input_key != 'q')
  {
    while (in.timed_wait(lk,system_time(second_clock::local() + 
seconds(1)) \
        == false) // DEFINITELY NOT! good
    { }
    cin >> input_key;
    out.notify_one();
  }
}
*** END OF CODE ***
  The controller/output main_loop looks similar only it uses 
out.timed_wait(...) and in.notify_one(), when the crucial task is 
done. Result is, both main_loops, when passed to a thread start and 
then it's deadlocked.
  I have seen examples of the producer/consumer pattern, which worked 
with buffers and they used conditions like:
  while (buffer_full == 0)
  or similar. I only have one key, no queueing, so what to do here?
  Please any good pointer/help/example is appreciated.
You effectively need a blocking queue that holds at most one element.

#include <boost/thread/thread.hpp>
#include <boost/thread/condition.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/lambda/lambda.hpp>
#include <iostream>

using boost::lambda::var;

boost::condition in, out;
boost::mutex m;
char input_key;
int size;

void reader() {
    boost::unique_lock<boost::mutex> l(m);
    while(input_key != 'q') {
        in.wait(l, var(size) < 1);
        std::cin >> input_key;
        size = 1;
        out.notify_one();
    }
}

void controller() {
    boost::unique_lock<boost::mutex> l(m);
    while(input_key != 'q') {
        out.wait(l, var(size) > 0);
        std::cout << input_key << std::endl;
        size = 0;
        in.notify_one();
    }
}

int main() {
    boost::thread t1(&reader);
    boost::thread t2(&controller);
    t1.join();
    t2.join();
}

In Christ,
Steven Watanabe