Can't understand this basic threads behaviour - c++

Question

I am studying about threads and concurrent programming. Tried this basic example from what was provided in class:
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 8
void *printGoGators(void *noArg)
{
printf("Go Gators!\n");
return NULL;
}
int main(int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc, t;
for (t = 0; t < NUM_THREADS; t++)
{
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t],
NULL,
printGoGators,
NULL);
if(rc)
{
printf("ERROR %d", rc);
exit(-1);
}
}
pthread_exit(NULL);
}
This code produces the following output. :
Creating thread 0
Creating thread 1
Go Gators!
Creating thread 2
Go Gators!
Creating thread 3
Go Gators!
Creating thread 4
Go Gators!
Creating thread 5
Go Gators!
Creating thread 6
Go Gators!
Creating thread 7
Go Gators!
Go Gators!
Why is Go Gators! not printed directly after its corresponding Creating thread # for all threads?
Please help!

Answer 1

If your code looked like this, then the output would be in the order you expect:
for (t = 0; t < NUM_THREADS; t++)
{
printf("Creating thread %d\n", t);
printGoGators(NULL);
}
So, you're making an assumption that threads will execute in the same order that they are created. However, that assumption is incorrect - threads may execute in any order.

Answer 2

After you create a thread it is up to the OS to decide what order threads are executed. You can control it using mutexes and conditions to lock a thread let another thread to run and then unlock it.
Unlike accepted answer, this example uses threads and not just print something in a loop.
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 8
pthread_mutex_t myMutex; // Declere global mutex
pthread_cond_t myCondition; // Declere global condition
void *printGoGators(void *arg) {
printf("Go Gators! %i\n", *(int*)arg);
delete (int*) arg;
pthread_cond_signal(&myCondition); // Signal that a condition is met
return NULL;
}
int main(int argc, char *argv[]) {
pthread_mutex_init(&myMutex, NULL); // Initialize mutex
pthread_cond_init (&myCondition, NULL); // Initialize condition
pthread_t threads[NUM_THREADS];
int rc, t;
for (t = 0; t < NUM_THREADS; t++) {
int* n = new int;
*n = t;
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t],
NULL,
printGoGators,
n);
if(rc) {
printf("ERROR %d", rc);
exit(-1);
}
pthread_cond_wait(&myCondition, &myMutex); // waite for condition
}
pthread_exit(NULL);
}
result:
Creating thread 0
Go Gators! 0
Creating thread 1
Go Gators! 1
Creating thread 2
Go Gators! 2
Creating thread 3
Go Gators! 3
Creating thread 4
Go Gators! 4
Creating thread 5
Go Gators! 5
Creating thread 6
Go Gators! 6
Creating thread 7
Go Gators! 7
Main thread creation loop: Create a thread and then wait for the condition.
New thread: Print a message then signal that condition is met.
This way you manage the order of execution.