Question

I wrote a small program to see the amount of overhead there is to create a thread.

Here is the program (I wrote it quickly so it isn’t the best):

#include <iostream>
#include <pthread.h>

void * lala(void * cake) {
 int * hi = (int *)cake;
 std::cout << *hi << '\n';
}

int main(void) {
 pthread_t thread;
 for (int i = 0;i < 10000;i = i + 1) {
  pthread_create(&thread,0,lala,&i);
 }
}

It basically launches 10000 threads and passes them their thread number, and each thread outputs its number.

The output changes each time I run the program, however I noticed that there was one part that never changed:

At the end of the output, I always find this:

...
9994
9995
9996
9997
9998
9999
0

Which means that the first thread finishes last…

So, my friends, does anyone have a possible explanation for this phenomenon?

Answer 1

First thing, you are passing the address-of a local variable to the thread, which is constantly changing. So, when the thread gets time to read it, the content of i would already be changed.
Why can’t you just pass i instead of &i, where variable is just of 4-bytes (i.e. fits in pointer)?

Secondly, you shouldn’t be concerned how OS schedules your threads.