POSIX的pthread_join
POSIX的pthread_join
join
- join是三种同步线程的方式之一。另外两种分别是互斥锁(mutex)和条件变量(condition variable)。
- 调用pthread_join()将阻塞自己,一直到要等待加入的线程运行结束。
- 可以用pthread_join()获取线程的返回值。
- 一个线程对应一个pthread_join()调用,对同一个线程进行多次pthread_join()调用是逻辑错误。
join or detach
- 线程分两种:一种可以join,另一种不可以。该属性在创建线程的时候指定。
- joinable线程可在创建后,用pthread_detach()显式地分离。但分离后不可以再合并。该操作不可逆。
- 为了确保移植性,在创建线程时,最好显式指定其join或detach属性。似乎不是所有POSIX实现都是用joinable作默认。
- #include <pthread.h>
- #include <stdio.h>
- #include <stdlib.h>
- #define NUM_THREADS 4
- void *BusyWork(void *t)
- {
- double result=0.0;
- long tid = (long)t;
- printf("Thread %ld starting...\n",tid);
- for (int i=0; i<1000000; i++)
- {
- result = result + sin(i) * tan(i);
- }
- printf("Thread %ld done. Result = %e\n",tid, result);
- pthread_exit((void*) t);
- }
- int main (int argc, char *argv[])
- {
- pthread_t thread[NUM_THREADS];
- pthread_attr_t attr;
- // 1/4: init
- pthread_attr_init(&attr);
- // 2/4: explicitly specify as joinable or detached
- pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
- int rc;
- long t;
- for(t=0; t<NUM_THREADS; t++)
- {
- printf("Main: creating thread %ld\n", t);
- // 3/4: use thread attribute
- rc = pthread_create(&thread[t], &attr, BusyWork, (void *)t);
- if (rc) {
- printf("ERROR; return code from pthread_create() is %d\n", rc);
- exit(-1);
- }
- }
- // 4/4: release thread attribute
- pthread_attr_destroy(&attr);
- void *status;
- for(t=0; t<NUM_THREADS; t++)
- {
- rc = pthread_join(thread[t], &status);
- if (rc) {
- printf("ERROR; return code from pthread_join() is %d\n", rc);
- exit(-1);
- }
- printf("Main: completed join with thread %ld having a status of %ld\n",t,(long)status);
- }
- printf("Main: program completed. Exiting.\n");
- return 0;
- }
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