C++11 : How to get a Thread ID ?
In this article we will discuss how to get thread Id in different scenarios.
Every thread has an unique Id associated with it. c++11 provides a type to store this id i.e.
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std::thread::id |
Objects of std::thread::id is comparable, copy-able and default implementation of std::hash() is also provided by the standard. Therefore, std::thread::id objects can be used as keys in both map and unordered_map.
std::thread::get_id()
std::thread provides a member function get_id() i.e.
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std::thread::id get_id() const noexcept; |
It returns the thread id of associated object.
Let’s use this function to fetch the thread id i.e.
Get thread ID from a Join-able Thread Object
Let’s create a thread i.e.
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// Starting Thread std::thread th(threadFunction); |
Now get it’s thread id from thread object.
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// Fetching thread ID from Thread Object using get_id() member function std::thread::id threadID = th.get_id(); |
Get thread ID from a Detached Thread object
Let’s create a thread i.e.
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// Starting Thread std::thread dThObj(threadFunction); |
Detach the thread from thread object i.e.
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// Detached the thread dThObj.detach(); |
Now thread object has no associated thread with it’s id. Therefore, get_id() on detached thread object will return default constructed value i.e.
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// Fetching thread ID from Thread Object using get_id() member function std::thread::id dThreadID = dThObj.get_id(); // Detached thread's get_id() function will return default constructed thread::id only assert(dThreadID == std::thread::id()); |
Get current thread ID inside thread function
Inside a function, which is currently executed by some thread, we can access the current thread object through,
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std::this_thread |
So, to get the current thread ID inside thread function, we can call get_id() with this_thread i.e.
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// Fetch the thread ID of the thread which is executing this function std::thread::id threadID = std::this_thread::get_id(); |
Complete example is as follows,
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#include <thread> #include <iostream> #include <assert.h> #include <chrono> using namespace std::chrono_literals; void threadFunction() { std::cout << "Func Start" << std::endl; // Fetch the thread ID of the thread which is executing this function std::thread::id threadID = std::this_thread::get_id(); std::cout << "Inside Thread :: Thread ID : " << threadID << "\n"; std::cout << "Func End" << std::endl; } int main() { // Starting Thread std::thread th(threadFunction); // Fetching thread ID from Thread Object using get_id() member function std::thread::id threadID = th.get_id(); // Join the Thread if its Joinable if (th.joinable()) th.join(); std::cout << "Thread from Main : " << threadID << std::endl; /** Fetching Thread ID from Detached Thread ****/ // Starting Thread std::thread dThObj(threadFunction); // Detached the thread dThObj.detach(); // Fetching thread ID from Thread Object using get_id() member function std::thread::id dThreadID = dThObj.get_id(); // Detached thread's get_id() function will return default constructed thread::id only assert(dThreadID == std::thread::id()); std::this_thread::sleep_for(2s); std::cout << "Thread from Main : " << dThreadID << std::endl; return 0; } |
Output:
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Func Start Inside Thread :: Thread ID : 139924979255040 Func End Thread from Main : 139924979255040 Func Start Inside Thread :: Thread ID : 139924979255040 Func End Thread from Main : thread::id of a non-executing thread |
To Compile the above example on linux use following command,
g++ –std=c++14 example.cpp -lpthread
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