Multithreading and concurrency are essential concepts in modern programming, allowing multiple tasks to run simultaneously, thereby improving performance and efficiency. This module will cover the basics of multithreading, thread management, synchronization techniques, and the use of mutexes and locks to handle shared resources.
1. Introduction to Multithreading
Multithreading refers to the concurrent execution of more than one sequential set of instructions, or thread. A thread is the smallest unit of CPU execution, and multithreading allows a program to perform multiple operations at the same time.
Key Concepts:
- Thread: A single sequence of execution within a program.
- Process: A program in execution, which can contain one or more threads.
- Concurrency: The ability of a system to handle multiple tasks at once, even if not simultaneously.
- Parallelism: Actual simultaneous execution of tasks using multiple processors or cores.
Benefits of Multithreading:
- Better Resource Utilization: Leverages CPU time efficiently, especially on multi-core systems.
- Improved Performance: Faster execution of tasks, especially for CPU-bound operations.
- Responsiveness: Keeps applications responsive, particularly in user interfaces (UI).
Example:
#include <iostream>
#include <thread>
void printMessage() {
std::cout << “Hello from the thread!” << std::endl;
}
int main() {
std::thread t(printMessage); // Create a new thread
t.join(); // Wait for the thread to finish
return 0;
}
2. Thread Management
Thread management involves creating, controlling, and terminating threads within a program. The C++ Standard Library provides tools to manage threads.
Creating Threads:
In C++, threads are created using the std::thread class from the <thread> library.
Example:
std::thread t1(function_name); // Create a thread
Joining and Detaching Threads:
- join(): Blocks the calling thread until the thread finishes its execution.
- detach(): Detaches the thread and allows it to execute independently of the calling thread. It’s useful for background tasks.
Example:
std::thread t1(printMessage); // Create thread
t1.join(); // Wait for thread to finish
Handling Multiple Threads:
You can create multiple threads and wait for all of them to complete using join() for each thread.
Example:
std::thread t1(printMessage);
std::thread t2(printMessage);
t1.join();
t2.join();
Thread Safety:
- When multiple threads access shared resources, you need to ensure thread safety to prevent issues like race conditions.
3. Synchronization Techniques
When multiple threads access shared resources, synchronization is needed to ensure the integrity of the data. Synchronization prevents race conditions, where the outcome depends on the order of execution.
Types of Synchronization:
- Mutual Exclusion (Mutex): Prevents multiple threads from accessing the same resource simultaneously.
- Condition Variables: Allows threads to wait for certain conditions to be met before continuing execution.
- Atomic Operations: Ensures that operations on shared data are indivisible.
Example:
#include <iostream>
#include <thread>
#include <mutex>
std::mutex mtx; // Mutex to protect shared resource
void printMessage() {
mtx.lock(); // Lock the mutex
std::cout << “Message from thread!” << std::endl;
mtx.unlock(); // Unlock the mutex
}
int main() {
std::thread t1(printMessage);
std::thread t2(printMessage);
t1.join();
t2.join();
return 0;
}
In this example, the mutex mtx ensures that only one thread can access printMessage at a time.
4. Mutexes and Locks
A mutex (short for mutual exclusion) is a synchronization primitive used to prevent multiple threads from simultaneously accessing a critical section of code.
Mutexes:
A mutex is used to ensure that only one thread can access a shared resource at a time. Once a thread locks a mutex, other threads that try to lock it will block until the mutex is unlocked.
- std::mutex: The basic mutex class in C++.
- std::lock_guard: A wrapper that automatically locks and unlocks the mutex, ensuring proper resource management.
Example: Using std::mutex and std::lock_guard
#include <iostream>
#include <thread>
#include <mutex>
std::mutex mtx; // Mutex
void printMessage(int id) {
std::lock_guard<std::mutex> lock(mtx); // Automatically locks and unlocks
std::cout << “Message from thread ” << id << std::endl;
}
int main() {
std::thread t1(printMessage, 1);
std::thread t2(printMessage, 2);
t1.join();
t2.join();
return 0;
}
Here, std::lock_guard locks the mutex when the function is called and automatically releases it when the function scope ends.
std::unique_lock:
- Provides more flexibility compared to std::lock_guard.
- Allows manual locking and unlocking of mutexes.
- Can be used with condition variables.
Example:
#include <iostream>
#include <thread>
#include <mutex>
std::mutex mtx;
void printMessage() {
std::unique_lock<std::mutex> lock(mtx); // Locking mutex
std::cout << “Thread message.” << std::endl;
} // Automatically unlocks when leaving the scope
int main() {
std::thread t1(printMessage);
t1.join();
return 0;
}
Summary
- Multithreading allows simultaneous execution of tasks, improving performance and responsiveness.
- Thread Management in C++ includes creating, joining, and detaching threads using the std::thread
- Synchronization Techniques ensure thread safety by controlling access to shared resources.
- Mutexes and Locks are fundamental tools for synchronizing threads and ensuring that critical sections of code are accessed by only one thread at a time.
Practice Exercises
- Write a program that creates multiple threads to perform different tasks and uses join() to wait for all threads to finish.
- Modify the program to include synchronization with a mutex to ensure that shared data is not modified by multiple threads simultaneously.
- Implement a program that uses condition variables to control when threads should start executing.
- Create a program that demonstrates the use of std::unique_lock for manual locking and unlocking of mutexes.
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