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; } 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:
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 !
std::cout << i++ << " " << j++ << std::endl;into yourffunction and run a (single) thread as follows:thread t([]{ f(1); f(1); f(1); });. Live demo: wandbox.org/permlink/ZbWGoWOiEDsXkUCE.