pthread_join和pthread_detach的用法 【】转载】 C/C++：pthread_join()和pthread_detach()的区别
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简单来说：pthread_detach()和pthread_join()就是控制子线程回收资源的两种不同的方式。同一进程间的线程具有共享和独立的资源，其*享的资源有堆、全局变量、静态变量、文件等公用资源。而独享的资源有栈和寄存器，这两种方式就是决定子线程结束时如何回收独享的资源。
pthread_detach()即主线程与子线程分离，两者相互不干涉，子线程结束同时子线程的资源自动回收。pthread_join()即是子线程合入主线程，主线程会一直阻塞，直到子线程执行结束，然后回收子线程资源，并继续执行。
代码编译过程为（文件取名main.cc）
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/** 编译 -lpthread为链接pthread**/
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g++ -o a.out main.cc -lpthread
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/** 执行 **/
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./a.out
linux下内存占用查看指令为
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/** 先找到想要查看的程序的进程号 **/
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ps -ef
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/** -d 1 表示数据刷新频率1秒 -p xxx 表示想查看的程序的进程号 **/
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top -d 1 -p xxx
测试程序如下
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// for test
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#include <iostream>
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#include <unistd.h>
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#include <pthread.h>
-
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using namespace std;
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void* svc(void* args)
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{
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sleep(3);
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cout << "111111111111111" << endl;
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return nullptr;
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}
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int main()
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{
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do
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{
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pthread_t threadId = 0;
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const int ret = pthread_create(&threadId, 0, &svc, nullptr);
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if (ret == -1)
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{
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cout << "error while create thread!" << endl;
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return 0;
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}
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sleep(1)
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// pthread_join(threadId, nullptr);
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// cout << "222222222222222" << endl;
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// pthread_detach(threadId);
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// cout << "333333333333333" << endl;
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}while (false);
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getchar();
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return 0;
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}
先注销掉pthread_join()和pthread_detach()两个函数，代码中每隔1s创建一个子线程，并且子线程在执行结束后资源并没有被释放，变成了僵尸线程，此时用top明明查看进程的内存占用，发现进程占用的内存不断增大。此时若取消注释pthread_join()或者是pthread_detach()中的任意一个，再重新编译并启动程序，发现进程内存占用不会增大。因为子线程资源被释放掉了（无论是子线程自动释放还是主线程回收，都是释放）。
然后在一个子线程中观察两种释放的区别（此时不再创建多个子线程）
-
// for test
-
#include <iostream>
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#include <unistd.h>
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#include <pthread.h>
-
-
using namespace std;
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void* svc(void* args)
-
{
-
sleep(3);
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cout << "111111111111111" << endl;
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return nullptr;
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}
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int main()
-
{
-
do
-
{
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pthread_t threadId = 0;
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const int ret = pthread_create(&threadId, 0, &svc, nullptr);
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if (ret == -1)
-
{
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cout << "error while create thread!" << endl;
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return 0;
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}
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sleep(1)
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pthread_join(threadId, nullptr);
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cout << "222222222222222" << endl;
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// pthread_detach(threadId);
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// cout << "222222222222222" << endl;
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} while (false);
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// getchar();
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return 0;
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}
主线程调用pthread_join()时，主线程将阻塞在这条调用语句上，不再执行下面的cout，直到子线程执行完毕后，主线程回收了子线程的资源，才会继续向下执行。这段代码的测试结果是
先输出子线程休眠三秒后打印的111111111111111，后输出pthread_join()语句之后的222222222222222，这样我们发现，在子线程执行完毕之后，主线程才继续执行pthread_join()之后的语句。而如果我们使用pthread_detach()，代码如下
-
// for test
-
#include <iostream>
-
#include <unistd.h>
-
#include <pthread.h>
-
-
using namespace std;
-
void* svc(void* args)
-
{
-
sleep(3);
-
cout << "111111111111111" << endl;
-
return nullptr;
-
}
-
int main()
-
{
-
do
-
{
-
pthread_t threadId = 0;
-
const int ret = pthread_create(&threadId, 0, &svc, nullptr);
-
if (ret == -1)
-
{
-
cout << "error while create thread!" << endl;
-
return 0;
-
}
-
sleep(1)
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// pthread_join(threadId, nullptr);
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// cout << "222222222222222" << endl;
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pthread_detach(threadId);
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cout << "222222222222222" << endl;
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} while (false);
-
// getchar();
-
return 0;
-
}
结果如下
主线程不会等待子线程执行结束就已经执行结束了，因此子线程根本就没有来得及执行，程序就结束了。
我觉得这里可以理解为如果子线程的资源需要主线程来回收的话，那么主线程就一定要等子线程结束，因为子线程还没用完呢你就不能拿去回收，但是如果子线程资源要自动回收的话，那么主线程就不必等你了。
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