Pthread in C basic print - c

Question

I'm writing a C program using Pthreads that creates a child thread. After creating the child thread, the parent thread should ouput two messages: "parent:begin" then it should print "parent:done". Same for child thread "child:begin" and "child:done". I have to make sure that the main thread prints his second message before the spawned (child) thread does. I have to following implementation but it only prints in the wrong order. I assume I should use flags. Any help would be appreciated.
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
volatile int done = 0;
void *child(void *arg) {
printf("child\n");
done = 1;
printf("child:done");
return NULL;
}
int main(int argc, char *argv[]) {
printf("parent: begin\n");
pthread_t c;
pthread_create(&c, NULL, child, NULL); // create child
while (done == 0); // spin
printf("parent: end\n");
return 0;
}

Answer 1

If you want the parent to print done first, then you should have the child thread spin until the parent is done. (Right now the parent spins waiting for the child.) You should also use pthread_join to make sure the child is done before the main thread returns:
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
volatile int done = 0;
void *child(void *arg) {
printf("child: begin\n");
while (done == 0); // spin
printf("child: done\n");
return NULL;
}
int main(int argc, char *argv[]) {
printf("parent: begin\n");
pthread_t c;
pthread_create(&c, NULL, child, NULL); // create child
printf("parent: done\n");
done = 1;
pthread_join(c, NULL);
return 0;
}
On my machine I get this output:
parent: begin
parent: done
child: begin
child: done

Answer 2

At the moment done is accessed without any synchronization by both threads. Proper way is to signal the child that the parent has completed printing using a conditional variable. This leads to data race.
Moreover, main() thread complete the execution before the does. In that case, the whole process will die. You can either call pthread_join() or simply exit the main thread with pthread_exit().
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
pthread_cond_t cond=PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex=PTHREAD_MUTEX_INITIALIZER;
volatile int done = 0;
void *child(void *arg) {
printf("child\n");
pthread_mutex_lock(&mutex);
while(done == 0)
pthread_cond_wait(&cond, &mutex);
pthread_mutex_unlock(&mutex);
printf("child:done");
return NULL;
}
int main(int argc, char *argv[]) {
printf("parent: begin\n");
pthread_t c;
pthread_create(&c, NULL, child, NULL); // create child
pthread_mutex_lock(&mutex);
done = 1;
printf("parent: end\n");
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
pthread_exit(0);
}