Print passed argument to pthread_create [duplicate] - c

Question

This question already has an answer here:
Unexpected output in a multithreaded program
(1 answer)
Closed 4 years ago.
I want to print the passed to create_thread argument. I create an array with 10 int's to simulate the number of each thread -
void* thr_func(void *arg){
printf("\nTHREAD ID %d", *(int*)arg);
return 0;
}
int main(){
int arr[10]={0,1,2,3,4,5,6,7,8,9};
pthread_t thread_id;
for(int i=0;i<10;i++){
pthread_create(&thread_id, NULL, &thr_func,&arr[i]);
}
return 0;
}
Here is the output from two runs
1:
THREAD ID 1
THREAD ID 5
THREAD ID 6
THREAD ID 2
THREAD ID 3
THREAD ID 4
TTHREAD ID 0
THREAD ID 3
2:
THREAD ID 1
THREAD ID 2
THREAD ID 5
THREAD ID 6
THREAD ID 3
THREAD ID 7
THREAD ID 4
THREAD ID 8
THREAD ID 8
I know I can't rely on consecutive numbers, but why does it print some of them twice ?

Answer 1

As #Some programmer dude mentioned, you should always wait for the termination of your single child threads in your main function (except you detach your threads). The reason why some of your arrayelements are printed twice or more often is because before your buffer is cleared the thread may gets killed. (In your case the thread 8). To avoid that the processes are shut-down before they have finished their work you have to waitfor your threads, or in other words you have to join your threads.
That is done by adding the function: pthread_join(pthread_t thread, void **retvalue);
The pthread_join() function waits for the thread specified by thread to terminate.
Just put it in a separate loopafter your pthread_create() function and the parent thread (main) will wait for his childs.
Edit:
To wait for each thread you could change your pthread_tvariable to an array of threads: pthread_t thread[10] so you could wait for each thread in relation to the index.
int arr[10]={0,1,2,3,4,5,6,7,8,9};
pthread_t thread[10];
for(int i=0;i<10;i++){
pthread_create(&thread[i], NULL, &thr_func,&arr[i]);
}
for(int i=0;i<10;i++){
pthread_join(thread[i],NULL);
}