CODEWITHSUNDEEP
clinuxmultithreadingpthreads
Mohammad Tayyab
Mohammad Tayyab

Reputation: 696

pthread_join() mixing threads outputs

I studied about pthread_join() I get to know that it run thread until thread processing is done, But in my case it is not working like this, I've to create 4 threads using 2 functions, I was expecting output as fun1,fun2,fun1 and fun2 again, because I'm calling threads in this sequence. Here is my code to describe my question more clearly.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *thread1()
{
int c=0;
while(c++ < 10)
printf("%i : Hello!!\n",c);
}
void *thread2()
{
int c=0;
while(c++ < 10)
printf("%i : How Are You!!\n",c);
}
int main( int argc, char *argv[], char *env[] )
{
pthread_t tid1,tid2,tid3,tid4;
pthread_create(&tid1,NULL,thread1,NULL);
pthread_create(&tid2,NULL,thread2,NULL);
pthread_create(&tid3,NULL,thread1,NULL);
pthread_create(&tid4,NULL,thread2,NULL);
pthread_join(tid1,NULL);
pthread_join(tid2,NULL);
pthread_join(tid3,NULL);
pthread_join(tid4,NULL);
return 0;
}

Output variate every time, give different output every time, I don't understand whats happening. Output : 

1 : Hello!!
2 : Hello!!
3 : Hello!!
4 : Hello!!
5 : Hello!!
6 : Hello!!
7 : Hello!!
1 : How Are You!!
1 : How Are You!!
2 : How Are You!!
3 : How Are You!!
4 : How Are You!!
8 : Hello!!
9 : Hello!!
10 : Hello!!
2 : How Are You!!
3 : How Are You!!
4 : How Are You!!
5 : How Are You!!
6 : How Are You!!
7 : How Are You!!
8 : How Are You!!
9 : How Are You!!
10 : How Are You!!
5 : How Are You!!
6 : How Are You!!
7 : How Are You!!
8 : How Are You!!
9 : How Are You!!
10 : How Are You!!
1 : Hello!!
2 : Hello!!
3 : Hello!!
4 : Hello!!
5 : Hello!!
6 : Hello!!
7 : Hello!!
8 : Hello!!
9 : Hello!!
10 : Hello!!

what I was expecting

1 : Hello!!
2 : Hello!!
3 : Hello!!
4 : Hello!!
5 : Hello!!
6 : Hello!!
7 : Hello!!
8 : Hello!!
9 : Hello!!
10 : Hello!!
1 : How Are You!!
2 : How Are You!!
3 : How Are You!!
4 : How Are You!!
5 : How Are You!!
6 : How Are You!!
7 : How Are You!!
8 : How Are You!!
9 : How Are You!!
10 : How Are You!!
1 : Hello!!
2 : Hello!!
3 : Hello!!
4 : Hello!!
5 : Hello!!
6 : Hello!!
7 : Hello!!
8 : Hello!!
9 : Hello!!
10 : Hello!!
1 : How Are You!!
2 : How Are You!!
3 : How Are You!!
4 : How Are You!!
5 : How Are You!!
6 : How Are You!!
7 : How Are You!!
8 : How Are You!!
9 : How Are You!!
10 : How Are You!!

Can anyone describe what's happening here ? Thanks!

Upvotes: 1

Views: 347

Answers (2)

Ahmad Tamsal
Ahmad Tamsal

Reputation: 1

Because threads run in a parallel manner they run independently of each other. If you want them to be in a sequence using this way:

    pthread_create(&tid1,NULL,thread1,NULL);
    pthread_join(tid1,NULL);
    pthread_create(&tid2,NULL,thread2,NULL);
    pthread_join(tid2,NULL);
    pthread_create(&tid3,NULL,thread1,NULL);
    pthread_join(tid3,NULL);
    pthread_create(&tid4,NULL,thread2,NULL);
    pthread_join(tid4,NULL);

Now when we have created thread t1 it will execute and pthread_join() will wait for finishing its execution. after thread t1 will finish its execution now we are creating another thread t2 and now it will run. So in this way we can get output in a sequence.

Upvotes: 0

Andrew Henle
Andrew Henle

Reputation: 1

I was expecting output as fun1,fun2,fun1 and fun2 again, because I'm calling threads in this sequence.

Threads aren't sequenced - they run independently. If you want the different threads to be synchronized or sequenced in any way, you have to code that yourself using things like mutexes, semaphores, condition variables, or some other explicit synchronization method.

You can't expect any particular execution order between separate threads without such explicit synchronization.

Upvotes: 6

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