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;
}