As we all know, C++11 introduced the thread_local keyword to replace the __thread provided by the compiler,
or the specific key related functions provided by the pthread library.
Here is a typical example of using thread_local.
#include <thread>
static thread_local int i = 0;
int main() {
auto th = std::thread([]{
i = 1;
});
th.join();
assert(i == 0);
}
Photon begins to support TLS for coroutines since version 0.4.0. Due to some limitations, Photon cannot achieve the
same syntax as thread_local, but implements it in a close way.
#include <photon/thread/std-compat.h>
static photon::thread_local_ptr<int, int> pI(0);
int main() {
if (photon::init())
abort();
DEFER(photon::fini());
auto th = photon_std::thread([]{
*pI = 1;
});
th.join();
assert(*pI == 0);
}
In this code above, thread_local_ptr is a template class that provides pointer-like operators.
You need to pass the appropriate constructor type to its template parameter, which in this example, is also a int.
When users access it in different coroutines, they will always get a separate value.
Below is a more complicated example:
class Value {
public:
explicit Value(std::string s) : m_s(std::move(s)) {}
size_t size() { return m_s.size(); }
private:
std::string m_s;
};
class A {
public:
void func();
private:
static photon::thread_local_ptr<Value, std::string> m_value;
};
static photon::thread_local_ptr<Value, std::string> m_value("123");
void A::func() {
std::cout << "Value size " << m_value->size() << std::endl;
}