线程回收

pthread_join函数

阻塞等待线程退出，获取线程退出状态其作用，对应进程中 waitpid() 函数。

int pthread_join(pthread_t thread, void **retval); 成功：0；失败：错误号

参数：thread：线程ID （【注意】：不是指针）；retval：存储线程结束状态。

对比记忆：

进程中：main返回值、exit参数-->int；等待子进程结束 wait 函数参数-->int *

线程中：线程主函数返回值、pthread_exit-->void *；等待线程结束 pthread_join 函数参数-->void **

【练习】：参数 retval 非空用法。【pthrd_exit_join.c】

调用该函数的线程将挂起等待，直到id为thread的线程终止。thread线程以不同的方法终止，通过pthread_join得到的终止状态是不同的，总结如下：

如果thread线程通过return返回，retval所指向的单元里存放的是thread线程函数的返回值。
如果thread线程被别的线程调用pthread_cancel异常终止掉，retval所指向的单元里存放的是常数PTHREAD_CANCELED。
如果thread线程是自己调用pthread_exit终止的，retval所指向的单元存放的是传给pthread_exit的参数。
如果对thread线程的终止状态不感兴趣，可以传NULL给retval参数。

【练习】：使用pthread_join函数将循环创建的多个子线程回收。

/***
thread_join.c
***/
#include<stdio.h>
#include<error.h>
#include<string.h>
#include<unistd.h>
#include<stdlib.h>
#include<pthread.h>typedef struct 
{char ch;int var;char str[64];
}exit_t;void *thrd_func(void *arg)
{pthread_exit((void *)1);
}int main()
{pthread_t tid;int ret;int *retval;printf("In main 1 : thread id = %lu, pid = %u\n",pthread_self(),getpid());ret = pthread_create(&tid,NULL,thrd_func,NULL);if(0 != ret){fprintf(stderr,"pthread_create error : %s \n",strerror(ret));exit(1);}pthread_join(tid,(void **)&retval);printf("--------------%d\n",(int)retval);pthread_exit((void*)1);
}

运行结果：

ubuntu1604@ubuntu:~/wangqinghe/linux/20190819$ ./thread_join

In main 1 : thread id = 139721544345344, pid = 12974

--------------1

/***
pthread_join_struct.c
***/
#include<stdio.h>
#include<error.h>
#include<string.h>
#include<unistd.h>
#include<stdlib.h>
#include<pthread.h>typedef struct 
{char ch;int var;char str[64];
}exit_t;void *thrd_func(void *arg)
{exit_t *retvar = (exit_t *)malloc(sizeof(exit_t));retvar->ch = 'm';retvar->var = 200;strcpy(retvar->str,"my thread\n");pthread_exit((exit_t *)retvar);
}int main()
{pthread_t tid;int ret;exit_t *retval;printf("In main 1 : thread id = %lu, pid = %u\n",pthread_self(),getpid());ret = pthread_create(&tid,NULL,thrd_func,NULL);if(0 != ret){fprintf(stderr,"pthread_create error : %s \n",strerror(ret));exit(1);}pthread_join(tid,(void **)&retval);printf("ch = %c,var = %d,str = %s\n--------------%d\n",retval->ch,retval->var,retval->str);free(retval);pthread_exit((void*)1);
}

运行结果：

ubuntu1604@ubuntu:~/wangqinghe/linux/20190819$ ./thread_join

In main 1 : thread id = 140316487100160, pid = 13121

ch = m,var = 200,str = my thread

--------------202

/***
thread_join_loop.c
***/
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<pthread.h>int var = 100;void *tfn(void *arg)
{int i;i = (int)arg;sleep(i);if(1 == i){var = 333;printf("var = %d\n",var);return (void*)var;}else if( 3 == i){var = 888;printf("i'm %dth pthread,pthread id = %lu ,var = %d\n",i+1,pthread_self(),var);pthread_exit((void*)var);}else{printf("i'm %dth pthread,pthread id = %lu ,var = %d\n",i+1,pthread_self(),var);pthread_exit((void*)var);        }return NULL;    
}int main()
{pthread_t tid[5];int i;int *ret[5];for(i = 0; i < 5; i++){pthread_create(&tid[i],NULL,tfn,(void*)i);}for(i = 0; i < 5; i++){pthread_join(tid[i],(void**)&ret[i]);printf("-----------%d's ret = %d\n",(int)ret[i]);}printf("I'm main pthread tid = %lu var = %d\n",pthread_self(),var);sleep(i);return 0;
}