All threads are executing at once - multithreading

Question

I try to find out the number of CPU cores using threads in C. Someone told me to try to execute 40 threads at once, make every thread sleep for one second, and see how many are executed simultaneously. I really like his approach, the problem is after executing my code, the program is sleeping for 1 second, and after that all threads are launced at once(no sleeping included).
Can someone please help me out?
void func(void* arg)
{
int n=(int*)arg;
sleep(1);
printf("Exec nr:%d\n",n);
}
int main(void) {
int i;
time_t rawtime;
struct tm * timeinfo;
time ( &rawtime );
timeinfo = localtime ( &rawtime );
printf ( "Current local time and date: %s", asctime (timeinfo) );
for(i=0;i<N;i++)
{
pthread_create(&th[i],NULL,func,(int*)i);
}
for(i=0;i<N;i++)
{
pthread_join(th[i],NULL);
}
time_t rawtime2;
struct tm* timeinfo2;
time ( &rawtime2 );
timeinfo2 = localtime ( &rawtime2 );
printf ( "Current local time and date: %s", asctime (timeinfo2) );
return 0;
}

Answer 1

You won't be able of discovering the number of CPUs in this way, because the scheduler may choose to run all your threads on the same core and leave the other cores for (more important) stuff.
Therefore, you should rely on some functionality provided by your OS.
For example, on Linux the file /proc/cpuinfo provides information about the CPU. This file can be opened and parsed by any user-level program. Other operating systems provide different mechanisms.