I was reading threads and concurrency and hit this quote:
std::this_thread::get_id() return the id of the thread calling it.
So, I wrote the following code to test the function and see if it's doing something else than what Uniux's gettid() does:
#include <iostream>
#include <thread>
#include <chrono>
#include <unistd.h>
using namespace std;
void someFunction(){
int i = 0;
while(i++ < 2){
cout<<"TID using gettid(): "<< gettid()<<endl;
cout<<"TID using this_thread::get_id: "<< this_thread::get_id()<<endl<<endl;
std::this_thread::sleep_for(chrono::milliseconds(900));
}
}
int main(){
cout<<"Hello from main"<<endl;
cout<<"TID using gettid(): "<<gettid()<<endl;
cout<<"TID using this_thread::get_id(): "<<this_thread::get_id()<<endl<<endl;
thread testThread(someFunction);
testThread.join();
return 0;
}
The output shows that the two functions are returning different values and my question is why? is this related to pths and pthreads?
The gettid() is a Linux specific system call returning the thread ID of the thread calling it. The returning type is a pid_t (an integer) as written in the man page https://man7.org/linux/man-pages/man2/gettid.2.html
There's no guarantee that this system call return the same number as the POSIX thread ID.
The C++ function std::this_thread::get_id() is part of the C++ standard and it's completely different. It's purpose is to identify threads in different systems (not specifically Linux). Moreover, the type returned by this function is not an integer but it's a std::thread::id and you can print it to your console only because its << operator is overloaded.