How do i properly call a function while creating pthread? - c

Question

I am creating a pthread in the main function and calling another function called "PrintHello". the "PrintHello" function should print some messages. My threads are being created but i am guessing my function "PrintHello" isn't being called properly as the messages are not printing.
I have put another print command in "PrintHello" function and that is printing. That means the function is being called. but i can't figure out why the messages aren't printing.
char *messages[NUM_THREADS];
struct thread_data
{
int thread_id;
int sum;
char *message;
};
struct thread_data thread_data_array[NUM_THREADS];
void *PrintHello(void *threadarg)
{
printf("I am in PrintHello");
int taskid , sum;
char *hello_msg;
struct thread_data *my_data;
Sleep(1);
my_data = (struct thread_data *) threadarg;
taskid = my_data ->thread_id;
sum = my_data ->sum;
hello_msg = my_data ->message;
printf("Thread %d: %s Sum=%d\n", taskid , hello_msg , sum) ;
pthread_exit(NULL);
}
int main(int argc , char *argv[])
{
pthread_t threads[NUM_THREADS];
int *taskids[NUM_THREADS];
int rc, t, sum;
sum=0;
messages[0] = "English: Hello World!";
..............
messages[7] = "Latin: Orbis , te saluto!";
for(t=0;t<NUM_THREADS;t++)
{
sum = sum + t;
thread_data_array[t].thread_id = t;
thread_data_array[t].sum = sum;
thread_data_array[t].message = messages[t];
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t], NULL , PrintHello , (void *) &thread_data_array[t]);
//rc = pthread_create(&threads[t], NULL , PrintHello , NULL);
if (rc)
{
printf("ERROR; return code from pthread_create() is %d \n", rc);
exit(-1);
}
}
pthread_exit(NULL);
}
The code should print out the hello messages.

Answer 1

I assume what's happening is your application is exiting before your threads have the chance to actually execute fully. Since you are storing each thread handle in the array of pthread_t threads[NUM_THREADS]; what you will need to do is upon creation a call to
pthread_join should be made which will allow the caller thread to be blocked until the thread has executed and returned. You can call pthread_join right after you call pthread_create or loop through all your handles and call pthread_join on each index in your array. If a call to pthread_join is made after the creation of each thread individually then a new thread will not spawn until the previous finishes. If you wish to execute them simultaneously then a loop after all the threads have been created would be the better option.
int main(int argc , char *argv[])
{
pthread_t threads[NUM_THREADS];
int *taskids[NUM_THREADS];
int rc, t, sum;
sum=0;
messages[0] = "English: Hello World!";
..............
messages[7] = "Latin: Orbis , te saluto!";
for(t=0;t<NUM_THREADS;t++)
{
sum = sum + t;
thread_data_array[t].thread_id = t;
thread_data_array[t].sum = sum;
thread_data_array[t].message = messages[t];
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t], NULL , PrintHello , (void *) &thread_data_array[t]);
//rc = pthread_create(&threads[t], NULL , PrintHello , NULL);
if (rc)
{
printf("ERROR; return code from pthread_create() is %d \n", rc);
exit(-1);
}
}
for(int index = 0; index < NUM_THREADS; ++index){
pthread_join(threads[index],NULL);//Wait for execution of each thread
}
}
You also don't need to call pthread_exit in your main. Typically that should be called in the thread you wish to terminate early where the value passed into pthread_exit can be retrieved from the second argument of pthread_join

Answer 2

Sorry for the late reply. I should have mentioned that i am using Windows. Anyway, i found out the problem. It is happening because of the "sleep" argument. For windows apparently it is different. so i changed my sleep argument to Sleep(1000), which apparently means 1 second in windows and that solved my problem. Thank you for all the replies.