2020-02-12

如何取得 C++11 thread id

本篇介紹如何取得 C++11 的 std::thread::id，有時候在 C++ 多執行緒的情況下，我們會需要印出 thread id 以方便判斷各自是哪個執行緒，以下範例就是簡單的取得 std::thread::id 的範例。

範例1. 取得 std::thread::id

std-thread-get-thread-id1.cpp

// g++ std-thread-get-thread-id1.cpp -o a.out -std=c++11 -pthread
#include <iostream>
#include <thread>
#include <chrono>
#include <assert.h>

using namespace std;

void foo() {
    std::thread::id tid = std::this_thread::get_id();
    cout << "foo thread id : " << tid << "\n";
}

int main() {
    std::thread t1(foo);

    std::thread::id tid = t1.get_id();
    std::thread::id mainTid = std::this_thread::get_id();

    if (t1.joinable())
        t1.join();

    cout << "t1 tid: " << tid << endl;
    cout << "main tid: " << mainTid << endl;

    return 0;
}

輸出

1
2
3

foo thread id : 140532863575808
t1 tid: 140532863575808
main tid: 140532880869184

範例2. join 與 detach 的 std::thread::id

std-thread-get-thread-id2.cpp

// g++ std-thread-get-thread-id2.cpp -o a.out -std=c++11 -pthread
#include <iostream>
#include <thread>
#include <chrono>
#include <assert.h>

using namespace std;

void foo() {
    cout << "foo start\n";
    std::thread::id tid = std::this_thread::get_id();
    cout << "foo thread id : " << tid << "\n";
    cout << "foo end\n";
}

int main() {
    std::thread t1(foo);

    std::thread::id tid = t1.get_id();

    if (t1.joinable())
        t1.join();

    cout << "Thread from Main : " << tid << endl;

    std::thread t2(foo);

    t2.detach();

    std::thread::id tid2 = t2.get_id();

    assert(tid2 == std::thread::id());

    cout << "sleep 2s\n";
    std::this_thread::sleep_for(std::chrono::seconds(2));

    cout << "Thread from Main : " << tid2 << endl;

    return 0;
}

輸出

foo start
foo thread id : 140447362262784
foo end
Thread from Main : 140447362262784
sleep 2s
foo start
foo thread id : 140447362262784
foo end
Thread from Main : thread::id of a non-executing thread

參考
How to get a Thread ID ?
https://thispointer.com/c11-how-to-get-a-thread-id/