thread_local vs local variable in C++

Question

Can someone clearly and simply explain what is the difference between i and j variables?

#include <thread>

using namespace std;

void f(int n)
{
    thread_local int i = n;
    int j = n;
}

int main()
{
    thread t1(f, 1);
    thread t2(f, 2);

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

    return 0;
}

Answer 1

thread_local implies static when static is omitted.

A local static variable preserves its value when a given function is called multiple times. You may read about static variable elsewhere.

Now, I assume you do know what static variable is - the important things are just:

• static variables have local scope
• (but) static variables have global existence

The second point makes a static variable's memory accessible to other functions by C++ references and pointers - it proves that the static variable has just one copy - across all threads of the process. You need to know what threads are and how to create/program threads.

Now, to the question. You know that Global variables have global scope and global accessibility, but when you run two or more instances of your program, both/all of them would have a SEPARATE copy of that global variable. That means each process would have a separate copy of that global variable.

So, what if you wanted to have a SEPARATE copy of static variable per thread? You use thread_local. Each thread will have a separate copy of that static variable.

Interestingly, you can apply thread_local to global variables also - thence each thread will receive a separate copy of those global variables also!

// Globals
int counter;
thread_local int this_thread_counter; // Each thread will have separate copy

Now, think how strtok would work - think about the (concurrent) calls to strtok !