CODEWITHSUNDEEP
c++multithreadingc++11
Martin Drozdik
Martin Drozdik

Reputation: 13303

How to find out if we are running in main thread?

We can get the id of the main thread by calling std::this_thread::get_id() just at the start of the main function like this answer suggests. We can then store this id in a global variable and compare against a call of std::this_thread::get_id().

However, this forces us to change the main function. Is there a way to create a library function that does this? I was thinking about using a global variable initialized with std::this_thread::get_id() expression. Since global variables (variables with static duration) are initialized relatively early it is unlikely (but not impossible, see: deferred dynamic initialization) that threads are spawned before these variables are initialized.

I could also initialize the global variable with a helper function which enumerates all threads and picks the one with the earliest creation time (based on this answer).

I am very new to multithreading so any advice or guidance is extremely welcome.

Upvotes: 9

Views: 12608

Answers (4)

itaych
itaych

Reputation: 674

Here's a solution for Linux:

#include <stdio.h>
#include <string>
#include <thread>
#include <unistd.h>
#include <sys/syscall.h>

void check_thread() {
    if (syscall(SYS_gettid) == getpid()) {
        printf("Hello from main thread\n");
    }
    else {
        printf("Hello from child thread\n");
    }
}

int main() {
    check_thread();
    std::thread t(check_thread);
    t.join();
}

Upvotes: 1

Tom Tom
Tom Tom

Reputation: 1209

If you are using MFC. You can check AfxGetMainWnd(). If it return a pWnd then you have the (MFC) main thread. Disclaimer: If you have not manipulatued the m_pMainWnd Pointer.

Upvotes: 0

SergeyA
SergeyA

Reputation: 62583

There is no such thing as main thread. There is a thread which was launched first, but all threads are first-class citizens. By tinkering with linker flags, I can easily create a program where the thread executing main() would not be the the thread launched first.

Rethink your design.

Upvotes: 17

Neijwiert
Neijwiert

Reputation: 1025

EDIT

This is not a solid way of getting the main thread's ID considering what @ta.speot.is and @David Schwartz said.

You could make a static variable somewhere that initializes with the current thread's ID.

const std::thread::id MAIN_THREAD_ID = std::this_thread::get_id();

And then somewhere else:

if (std::this_thread::get_id() == MAIN_THREAD_ID)
{
    std::cout << "main thread\n";
}
else
{
    std::cout << "not main thread\n";
}

Upvotes: 10

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