\input{configpres}

\def\lximg{/usr/share/lx/icons/fueller.png}

\subsection{C++ Threads}

\title{Threading in C++}
\maketitle

% stop displaying 'fueller.png' on the following slides
\def\lximg{none}

%\begin{frame}
%	\tableofcontents
%\end{frame}

\subsubsection{Threading}
\begin{frame}
\frametitle{Threads in C++}
\begin{itemize}
\item Platform independet concurrency available since C++11
\item Prior to C++11: platform-specific extensions like OpenMP, PThreads
\item Classes for thread management, manipulation and synchronisation
\end{itemize}
\end{frame}

\begin{frame}[fragile]
\frametitle{C++ Thread Management}
Creating and starting a thread:
\begin{lstlisting}
#include <iostream>
#include <thread>

void thread_fn()
{
  std::cout << "This output was generated in a thread." << std::endl;
}

int main()
{
  std::thread my_thread(thread_fn); // instantiate thread object

  my_thread.join();                 // wait for my_thread to finish
}
\end{lstlisting}
\end{frame}

\begin{frame}[fragile]
\frametitle{C++ Thread Management}
Passing parameters to a thread function:
\begin{lstlisting}
#include <iostream>
#include <thread>

void thread_add(int a, int b)
{
  int c = a + b;
  
  std::cout << "Addition result: " << c << std::endl;
}


int main()
{
  std::thread my_thread_add(thread_add, 10, 20);

  my_thread_add.join();             // wait for addition to finish
}
\end{lstlisting}
\end{frame}

\subsubsection{Synchronisation}
\begin{frame}
\frametitle{Synchronisation Mechanisms}
\begin{itemize}
\item Synchronisation of threads is mandatory
\item Basic synchronisation primitives:
\begin{itemize}
\item mutex objects
\item condition variables
\end{itemize}
\item Advanced synchronisation primitives:
\begin{itemize}
\item futures
\item atomics
\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}[fragile]
\frametitle{Mutex}
Using mutex objects:
\begin{lstlisting}
#include <mutex>

std::mutex my_mutex;
unsigned int up_counter = 0;

unsigned int counter_inc()
{
  std::lock_guard<std::mutex> lock(my_mutex);
  return ++up_counter;
}

unsigned int counter_read()
{
  std::lock_guard<std::mutex> lock(my_mutex);
  return up_counter;
}
\end{lstlisting}
Using direct calls to the methods lock() and unlock() is possible too.
We must ensure to unlock the mutex at each exit from protected regions
including the launch of exceptions. So it's more safe to to use the
lockguard template!
\end{frame}

\begin{frame}[fragile]
\frametitle{Condition variables}
Using condition variables:
\begin{lstlisting}
#include <condition_variable>

std::mutex m; std::condition_variable cv; bool processed = false;
 
void thread_fn()
{
    processed = true;
    std::cout << "Thread signals data processing completed\n";
    cv.notify_one();
}
 
int main()
{
  std::thread my_thread(thread_fn);
  std::unique_lock<std::mutex> lk(m);
  cv.wait(lk, []{return processed;});
  if(processed)
    std::cout << "Main thread got the signal\n";
  else
    std::cout << "Oops, something went wrong\n";
  my_thread.join();
}
\end{lstlisting}
\end{frame}

\begin{frame}[fragile]
\frametitle{Futures}
Futures are used to launch asynchronous operations whose results are
not required immediately. 
\begin{lstlisting}
#include <future>

int long_time_computation()
{
  int a = 5, b = 3;
  
  return a + b;
}

void do_other_stuff()
{
  std::cout << "We print this while long_time_computation is ongoing\n";
}

int main()
{
  std::future<int> the_result = std::async(long_time_computation);

  do_other_stuff();

  std::cout << "The result is " << the_result.get() << std::endl;
}
\end{lstlisting}
\end{frame}

\begin{frame}[fragile]
\frametitle{Atomics}
\begin{verbatim}
#include <atomic>

std::atomic<int> counter = 1;
counter++;
\end{verbatim}
\begin{itemize}
\item An atomic type can be used with any template T type
\item Operations on atomics will be executed in its entirety or not at all
\item Good performance due to lock free implementation
\end{itemize}
\end{frame}

\subsubsection{C++-Threads going realtime}

\begin{frame}[fragile]
\frametitle{C++-Threads going realtime}
C++-Threads can be brought to realtime context using the PThread-API
\begin{lstlisting}
#include <thread>
#include <pthread.h>
 
int main()
{
  std::thread my_thread; 
  sched_param sch;
  int policy;

  pthread_getschedparam(my_thread.native_handle(), &policy, &sch);
  sch.sched_priority = 20;
  if(pthread_setschedparam(my_thread.native_handle(), SCHED_FIFO, &sch))
  {
    std::cout << "Failure: " << std::strerror(errno) << std::endl;
  }
}
\end{lstlisting}
\end{frame}

\input{tailpres}