pthreads program doesn't behave well - c

Question

I'm trying to make a simple program using pthreads, want to make 6 threads and pass index to all of them. here's the code:
#include <pthread.h>
#include <stdio.h>
#define num_students 6
void thread_starter();
int main() {
pthread_t thread1[num_students];
int i = 0;
for(i = 0; i<num_students; i++) {
int q = i;
pthread_create(&thread1[i], NULL, (void *) &thread_starter, (void *)&q);
}
sleep(1);
}
void thread_starter(void* a) {
printf("Thread %i \n", *((int*)a));
}
And the output:
Thread 2
Thread 3
Thread 2
Thread 4
Thread 5
Thread 5
why do they have commmon names? what's wrong?
Thanks

Answer 1

You're passing a stack address from the main thread into all the child threads. There is no guarantee when these threads will be scheduled so you have no way of knowing whether the main thread will have updated its stack variable by the time each child gets around to reading it.
To avoid this, you need to allocate memory for the data being passed to each thread.
The easiest way to do this in your example is to use another automatic variable to store the data being passed to the new threads
void thread_starter(void* a);
int main() {
pthread_t thread1[num_students];
int thread_data[num_students];
int i = 0;
for(i = 0; i<num_students; i++) {
thread_data[i] = i;
pthread_create(&thread1[i], NULL, thread_starter, &thread_data[i]);
}
sleep(1);
}
Note also that you can avoid having to cast thread_starter if you give it the correct signature in your forward declaration.
For more complex programs you may need to dynamically allocate memory for each thread instead, passing ownership of that memory to the new threads.
int main() {
pthread_t thread1[num_students];
int i = 0;
for(i = 0; i<num_students; i++) {
int* data = malloc(sizeof(*data));
*data = i;
pthread_create(&thread1[i], NULL, thread_starter, data);
}
sleep(1);
}
void thread_starter(void* a) {
printf("Thread %i \n", *((int*)a));
free(a);
}
Finally, using sleep(1) isn't a very rigorous way of ensuring that all your threads will be run. It'd be better to use pthread_join instead
for(i = 0; i<num_students; i++) {
pthread_join(thread1[i], NULL);
}

Answer 2

sleep is not the correct tool to wait for spawned threads, use pthread_join for this.
Your main function terminating is equivalent to calling exit for the whole program and killing all the other threads.

Answer 3

Try This for Variation
void thread_starter(void* a) {
// Put a sleep(1) Here and see you will get even bizarre results
printf("Thread %i \n", *((int*)a));
}
Ok the problem here is of course
the race condition at this point here
int q = i;
pthread_create(&thread1[i], NULL, (void *) &thread_starter, (void *)&q);
lets say the first thread is created and the value at q is 0
now assume that before executing the statement Here
printf("Thread %i \n", *((int*)a));
if the main thread loops further and executes this statement here
int q = i;
again, then the q value changes (because its a reference) hence the problem
well one way to avoid this is to copy this reference variable in a local variable in thread routine also use mutex.
Sorry I was on a Hurry some more tips from my side
#define num_students 6
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; // A Global Mutex
Will aquire a lock Here
pthread_mutex_lock( &mutex );
printf("\n Got Mutex %d\n", i);
int q = i;
pthread_create(&thread1[i], NULL, (void *) &thread_starter, (void *)&q);
and will release the lock Here in child Routine
int i = *((int*)a);
sleep(1);
pthread_mutex_unlock( &mutex );
printf("Thread %i \n", i);
P.S - Remove un necessary Prints and Sleeps where ever not applicable