主线程退出 子线程会退出吗？子线程崩溃 主线程会退出吗？

记得以前刚接触多线程编程时，作为一个新手遇到的一个问题就是：我创建的子线程为什么没有跑起来？如以下最简单的一个程序：

#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <pthread.h>#include <string.h>void *thread_function(void *arg);char message[] = "THREAD_TEST";struct test{int a;int b;int c;};struct test outpara;int main(){int res;pthread_t a_thread;void *thread_result;struct test para = {.a = 10,.b = 20,.c = 30,};res = pthread_create(&a_thread, NULL, thread_function, (void *)&para);if(res != 0){perror("Pthread_create");exit(EXIT_FAILURE);}printf("the thread has finish, return %d\n", ((struct test*)thread_result)->a);exit(EXIT_SUCCESS);}void *thread_function(void *arg){struct test inpara = *(struct test*)arg;printf("thread is running, the parameter is %d, %d, %d\n", inpara.a,inpara.b, inpara.c);sleep(300);outpara.a = 100, outpara.b = 200,outpara.c = 300,pthread_exit(&outpara);}

执行结果却是诡异的：

大部分情况子线程就没有执行起来，偶尔会有一次子线程执行了。这是为什么呢？这是因为进程退出时，其创建的子线程都会被终止。那有没有办法让主线程退出时，子线程继续执行呢？有一个函数可以实现：pthread_exit()。

我们 man pthread_exit 可以看到：

pthread_exit 函数会终止调用线程，但在线程终止时，进程共享资源不会被释放，只有进程中的最后一个线程终止时，进程共享资源才会释放。

#include "Thread.h"#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <string.h>#include <sys/syscall.h>#include <signal.h>namespace CurrentThread{__thread int tCachedTid = 0;void cacheTid(){if(tCachedTid == 0){printf("first time get tid\n");tCachedTid = (int)syscall(SYS_gettid);}}inline int tid(){if(tCachedTid == 0){cacheTid();}return tCachedTid;}}class CMyThread: public CThread {public:CMyThread();~CMyThread();virtual void threadProc();};CMyThread::CMyThread(): CThread("myThread"){}CMyThread::~CMyThread(){}void CMyThread::threadProc(){int count = 5;while(count--){sleep(1);printf("in CMyThread, threadID = %d\n", CurrentThread::tid());}pthread_exit(NULL);}int main(){CMyThread *myThread = new CMyThread();myThread->createThread();printf("exit main thread!\n");pthread_exit(NULL);return 0;}

执行结果：

若是主线程调用 pthread_exit() 进行退出，则其创建的子线程会继续执行，直到整个进程终止。假如主线程用 return 或 exit() 进行退出，则整个进程就退出了，子线程可能还没来得及执行。

那如果子线程崩溃了，整个进程会结束吗？ 如我在子线程里对NULL指针进行操作，这样会产生一个 SIGSEGV 信号：

#include "Thread.h"#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <string.h>#include <sys/syscall.h>#include <signal.h>namespace CurrentThread{__thread int tCachedTid = 0;void cacheTid(){if(tCachedTid == 0){printf("first time get tid\n");tCachedTid = (int)syscall(SYS_gettid);}}inline int tid(){if(tCachedTid == 0){cacheTid();}return tCachedTid;}}class CMyThread: public CThread {public:CMyThread();~CMyThread();virtual void threadProc();};CMyThread::CMyThread(): CThread("myThread"){}CMyThread::~CMyThread(){}void CMyThread::threadProc(){int count = 5;while(count--){sleep(1);printf("in CMyThread, threadID = %d\n", CurrentThread::tid());}char *p = NULL;memcpy(p, "test", 4);pthread_exit(NULL);}int main(){CMyThread *myThread = new CMyThread();myThread->createThread();sleep(10);printf("exit main thread!\n");return 0;}

可以看到进程结束了，没跑用例前我以为子线程崩溃主线程不会有影响，但是错误的。因为这里涉及到了信号的处理行为，子线程产生的信号应该也算是此进程产生的信号，那信号对应的处理方式是什么呢？这里找到 kernel 相关头文件里有说明signal.h