pthread_join
- C course
- Advanced
- Lesson 20/83
- 5 min
Learn to wait for threads with pthread_join.
pthread_join in detail
pthread_join(tid, &result) retrieves the value returned by the thread. If the function returned a pointer with return, that value is written to result.
The thread function returns void *. For example, return an int * with return r;, then in the parent use int *result; pthread_join(tid, (void **)&result);. You then access the value with *result. This is the standard way to get a thread's result back.
Detached threads
Sometimes you let a thread run independently without waiting. With pthread_detach(tid) you detach it; when it ends, resources are freed automatically.
A detached thread cannot be joined. A thread is either joined or detached; not both. Background logger or monitor threads are usually detached. The main flow does not care about them or wait for their end.
Practice Get the thread result
Write void *worker(void *arg) with int *r = malloc(sizeof(int)); *r = 99; return r;. Create with pthread_create(&tid, NULL, worker, NULL);. Retrieve with int *result; pthread_join(tid, (void **)&result);. Print with printf("result=%d\n", *result); and free(result);.
Starter code
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
// Write worker function (return value with malloc)
// Create and join to get result
// Print result
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
void *worker(void *arg) {
int *r = malloc(sizeof(int));
*r = 99;
return r;
}
int main(void) {
pthread_t tid;
pthread_create(&tid, NULL, worker, NULL);
int *result;
pthread_join(tid, (void **)&result);
printf("result=%d\n", *result);
free(result);
return 0;
}Expected output
result=99
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match join concepts
| Term | Meaning |
|---|---|
pthread_join |
Gets return value |
pthread_detach |
Makes independent |
void ** |
Pointer to pointer |
return |
Ends function |
2 What does the second parameter of pthread_join take?
- The thread ID
- Attributes
- Wait duration
- Return value pointer Correct answer
Explanation: The second parameter is the pointer for the return value.
3 Get value with join
pthread_join(tid, (void **)&____);pthread_join(tid, (void **)&result);Explanation: The return value is written to the result pointer.