Threads

Threads are a fundamental concept in concurrent and parallel programming, allowing multiple sequences of instructions to run simultaneously. In C++, threads enable a program to perform multiple tasks at the same time, potentially improving performance, especially on multi-core processors.

What is a Thread?

• Thread: A thread is the smallest sequence of programmed instructions that can be managed independently by a scheduler. It is a single path of execution within a program.

• Multithreading: This is the ability of a CPU, or a single core in a multi-core processor, to execute multiple threads concurrently.

Why Use Threads?

Threads are used to:

1. Improve Performance: By dividing tasks into smaller sub-tasks and running them concurrently, a program can better utilize the available CPU cores.

2. Responsiveness: In GUI applications, threads allow the program to remain responsive while performing long operations in the background.

3. Modularity: Separating tasks into different threads can make a program's structure cleaner and easier to understand.

Threads in C++17

C++11 introduced a standard thread library, which continued in C++17 with some enhancements. The <thread> header provides facilities to manage and use threads.

Basic Usage of Threads in C++17

Here's a simple example to demonstrate how to create and use threads in C++17:

Example: Basic Thread Creation and Usage

#include <iostream>
#include <thread>

// A simple function that will be executed by a thread
void print_message(const std::string& message) {
    std::cout << message << std::endl;
}

int main() {
    // Creating a thread that runs the print_message function
    std::thread t1(print_message, "Hello from Thread 1!");

    // Lambda function to be executed by a second thread
    std::thread t2([]() {
        std::cout << "Hello from Thread 2!" << std::endl;
    });

    // Wait for both threads to finish execution
    t1.join();
    t2.join();

    std::cout << "Main thread finished!" << std::endl;
    return 0;
}

Explanation:

1. #include <thread>: Includes the thread library.

2. std::thread t1(print_message, "Hello from Thread 1!");: Creates a thread t1 that runs the print_message function with the provided argument.

3. Lambda Expression: Demonstrates another way to create threads using lambda functions.

4. t1.join() and t2.join(): join() is called to wait for the threads t1 and t2 to finish execution before the main function continues. If join() is not called, the program may terminate while the threads are still running, leading to undefined behavior.

Key Points about Threads in C++17

1. Detaching Threads: Instead of joining, you can detach a thread using t1.detach(). This allows the thread to run independently from the main thread. However, this can lead to issues if not managed properly, as the detached thread will run independently and can't be joined later.

2. Thread Safety: When multiple threads access shared resources, there is a risk of race conditions. Synchronization primitives like std::mutex, std::lock_guard, std::unique_lock, and others are used to ensure thread safety.

3. Thread Lifetime: A thread object is joinable after it has been started and before it has been joined or detached. A joinable thread should either be joined or detached before the thread object is destroyed, or it will terminate the program.

4. Passing Arguments: Arguments are passed to the thread function by value. If you need to pass by reference, use std::ref().

Example: Thread with Mutex for Synchronization

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mtx;

void print_message(const std::string& message) {
    // Locking the mutex
    std::lock_guard<std::mutex> lock(mtx);
    std::cout << message << std::endl;
}

int main() {
    std::thread t1(print_message, "Hello from Thread 1!");
    std::thread t2(print_message, "Hello from Thread 2!");

    t1.join();
    t2.join();

    std::cout << "Main thread finished!" << std::endl;
    return 0;
}

In this example:

• std::mutex mtx;: Declares a mutex.

• std::lock_guard<std::mutex> lock(mtx);: Creates a lock_guard object that manages the mutex, ensuring that it is locked when the guard is created and unlocked when the guard goes out of scope. This prevents race conditions when threads are trying to access shared resources.

Conclusion

Threads are a powerful feature in C++ for performing concurrent programming. With the standard thread library introduced in C++11 and further improved in C++17, creating and managing threads has become more straightforward and safer. However, with the power of multithreading comes the responsibility to manage synchronization and avoid race conditions effectively.