Condition Variables in C++ Multithreading

In this article, we will discuss the condition variable in C++ multithreading. But before discussing its condition variable, we must know about multithreading.

What is Multithreading?

Multithreading is a fundamental concept in computer science and software development. It involves running several threads concurrently inside of a single process. The smallest units of execution in a program are called threads, and they are essentially separate, concurrent sequences of instructions. A program can carry out several activities at once thanks to multithreading, which improves responsiveness, performance, and resource efficiency.

Important Elements of Multithreading:

Thread:

• A thread is a single set of instructions in a process. Several threads inside a process can effectively communicate and interact with one another because they share the same memory and resources.

Process vs. Thread:

• A process is a single instance of a program that is now running, including its data, code, and system resources.
• Multiple threads that share the same memory space can exist within a process.
• Compared to threads in separate processes, which have their own memory space, threads within the same process can communicate more readily.

Concurrency vs. Parallelism:

• Concurrency is the capacity of a system to manage several tasks at once.
• The operating system's quick context switching may cause threads to appear to operate concurrently, even if they may not execute simultaneously.
• Conversely, parallelism entails the actual simultaneous execution of threads, frequently necessitating the use of several CPU cores.

Overview of Condition Variables:

Condition variables are primitives for synchronization that allow threads to pause in the middle of their execution until a given condition is met. They efficiently manage shared resources and synchronize threads when used in conjunction with mutexes.

Condition variables in C++ are usually used with the std::condition_variable class from the C++ Standard Library's <condition_variable> header. They give threads an effective means of waiting for a condition to change without wasting CPU cycles.

Working Mechanism:

The core operations related to condition variables are the wait, notify_one, and notify_all methods. These routines are used to coordinate threads according to predefined circumstances in conjunction with a mutex.

1. Wait:

• When the calling thread receives a notify_one or notify_all call from another thread, the wait function atomically releases the associated mutex and puts it to sleep.
• The waiting for thread returns from the wait function after regaining possession of the mutex.

2. notify_one and notify_all:

• The notify_one and notify_all functions allow one of the waiting for threads (if any) that are stuck on the condition variable to resume its execution by waking it up.
• On the other hand, Notify_all awakens every waiting thread that has been stopped on the condition variable.

Program:

Let us take an example to illustrate the condition variables in C++.

Output:

Condition met. Proceeding with execution.

Variable Condition:

• The goal of condition variables is to assist threads in coordinating their execution under specific circumstances.

Usage:

• They are usually used in conjunction with a mutex to synchronize access to shared data.
• Fundamental functions related to condition variables are wait, notify_one, and notify_all.

Mutex:

• The goal of mutex (mutual exclusion) is to guarantee threads' exclusive access to shared resources.

Use:

• It guarantees that only one thread reads crucial portions of the code at a time and stops data races.

Wait Logic:

• In the waitForCondition function, a single thread awaits the fulfillment of a condition.
• It makes use of a mutex and a condition variable (cv.wait(lock)) to wait effectively.
• While it waits, the thread releases the mutex, enabling other threads to use the shared resources.

Set Condition Logic:

• After working for a while, a different thread (in the setCondition function) modifies the condition (ready flag).
• It uses a mutex to ensure exclusive access (std::lock_guard) when making changes to the shared variable.
• It alerts one waiting for thread (cv.notify_one()) to wake it up after changing the condition.

Main Function:

• It creates two threads, one that sets the condition and the other that waits for it.
• After that, a mutex is used to synchronize threads' access to the shared variable (ready), and the condition variable is used to coordinate thread execution.
• Essentially, the reasoning is built on threads coordinating their behavior according to a common condition.
• Threads altering the condition notify waiting threads (cv.notify_one) after updating the shared variable inside a crucial region protected by the mutex.
• Threads waiting for a condition to change employ condition variables (cv. wait) in conjunction with a mutex.

In summary:

Writing concurrent C++ code that is both efficient and thread-safe requires an understanding of condition variables and how they interact with mutexes. On the other hand, improper use of condition variables might result in undetectable errors like deadlocks or missed signals. Therefore, careful design and implementation are crucial when using condition variables in multithreaded programs.

In conclusion, mastering condition variables empowers C++ developers to create robust and responsive multithreaded applications, harnessing the full potential of concurrent programming.