What does the thread_local mean in C++ 11 ?

thread_local is a storage class specifier introduced in C++11, used to declare variables that have an independent instance in each thread. This means that even if multiple threads access the same global variable, each thread has its own independent copy, and variables between threads do not interfere with each other.

In multithreaded programming, thread_local is highly beneficial as it reduces data races and the need for locks, thereby enhancing program performance. Using thread_local, each thread has its own copy of the variable, which avoids synchronization issues when sharing data in a multithreaded environment.

Example

Assume we have a function that aims to record the number of times it is called. If this function runs in a multithreaded environment and we use a simple global counter, multiple threads will concurrently update this counter, which may result in race conditions and incorrect counts.

Using thread_local, we can provide an independent counter for each thread:

cpp
#include <iostream>
#include <thread>

thread_local int counter = 0;  // Each thread has its own copy of counter

void incrementCounter() {
    ++counter;
    std::cout << "Counter in thread " << std::this_thread::get_id() << ": " << counter << std::endl;
}

int main() {
    std::thread t1(incrementCounter);
    std::thread t2(incrementCounter);
    std::thread t3(incrementCounter);

    t1.join();
    t2.join();
    t3.join();

    return 0;
}

In this example, even though three different threads call the incrementCounter function, each thread prints its own value of counter, which is 1. This is because each thread has its own copy of counter, and they do not interfere with each other. Thus, we can safely maintain state in a multithreaded environment without using mutex locks or other synchronization mechanisms.