**`_Thread_local`** is a *storage specifier* introduced in the **C11** version of C. It indicates *thread storage duration*.

# Purpose

Variables declared with the `_Thread_local` keyword are *local* to the thread in which they are declared. This means that these variables are allocated when the thread begins and de-allocated when the thread ends.

Every thread has its own copy of the variable declared with the `_Thread_local` keyword.

#include <stdio.h>
#include "threads.h"
#define SIZE 5

int func(void *id)
{
    //_Thread_local variable 
    static thread_local int var = 5;
    var += 5;

    //Print id of current thread and addr of var
    printf("Thread ID:[%d], Value of var: %d\n", *(int*)id, var);

    return 0;
}

int main(void)
{
    thrd_t id[SIZE];

    //thread ID arr
    int arr[SIZE] = {1, 2, 3, 4, 5};

    //Creating 5 threads
    for(int i = 0; i < SIZE; i++) {
        thrd_create(&id[i], func, &arr[i]);
    }

    //Wait for threads to complete
    for(int i = 0; i < SIZE; i++) {
        thrd_join(id[i], NULL);
    }
}

We declare a variable ( `var` ) using `_Thread_local` in the `func()` function.

The `func()` function prints the value of `var` and the `id` of the current thread.

The same value for `var` in each thread implies that the variables were local and distinct, despite the use of `static`. The value is never updated, as the variable is de-allocated once the thread goes out of scope.

> We may also use the `_Thread_local` keyword with `static` or `extern`.

What is the _Thread_local keyword in C?

_Thread_local in C11 creates thread-local storage, ensuring variables are unique to each thread's lifecycle.

What is the _Thread_local keyword in C?

Purpose

Code