System Thread Interface

• std::thread uses the system's thread implementation
• We may need to use the thread implementation directly
• Some functionality is not available in standard C++:
  • Thread priority
    • Gives a thread higher or lower share of processor time
  • Thread affinity
    • "Pin" a thread on a specific processor core

native_handle()

• Each execution thread has a "handle"

  • Used internally by the system's thread implementation
  • Needed when making calls into the implementation's API

• Returned by the native_handle() member function

// Get the handle associated with an std::thread object
thr.native_handle()

#include <iostream>
#include <thread>

// Task function
void hello() { std::cout << "Hello, thread!" << std::endl; }

int main() {
  // create thread object
  std::thread thr(hello);

  // Display child thread's native handle
  std::cout << "Hello thread has native handle " << thr.native_handle() << '\n';

  // Wait for the thread to complete
  thr.join();

  std::cout << "Hello thread now has native handle " << thr.native_handle()
            << '\n';
}

std::thread ID

• Each execution thread has a thread identifier

  • Used internally by the system's implementation

• Guaranteed to be unique

  • If two thread identifiers are equal, the related objects are identical
  • Could be used to store std::thread objects in associative containers
  • A new thread may get the ID of an earlier thread which has completed

// Return the identifier of the current thread
std::this_thread::get_id();

// Return the identidier associated with an std::thread object
thr.get_id();

#include <iostream>
#include <thread>

// Task function
// Display thread ID
void hello() {
  std::cout << "Hello from thread with ID " << std::this_thread::get_id()
            << '\n';
}

int main() {
  // Display main thread ID
  std::cout << "Main thread has ID " << std::this_thread::get_id() << '\n';

  // create thread object
  std::thread thr(hello);

  // Display child thread's native handle
  std::cout << "Hello thread has ID " << thr.get_id() << '\n';

  // Wait for the thread to complete
  thr.join();

  std::cout << "Hello thread now has id " << thr.get_id() << '\n';
}

Pausing Threads

• We can pause a thread or make it "sleep"

std::this_thread::sleep_for()

• Takes an argument of type std::chrono::duration

// C++14
std::this_thread::sleep_for(2s);

// C++11
std::this_thread::sleep_for(std::chrono::seconds(2));

• This also works with single-threaded programs
  • Pauses the thread which executes main()

#include <iostream>
#include <thread>

using namespace std::literals;

void hello() {
  std::this_thread::sleep_for(2s);
  std::cout << "hello, thread!\n";
}

int main() {
  std::cout << "Starting thread...\n";
  std::thread thr(hello);

  thr.join();
}