为什么这会导致Seg故障？

Question

我正在做一个任务，用pthread处理线程和同步。在示例代码中，主线程创建了另外两个执行良好的线程。主线程被阻塞，直到这两个“子”线程终止。至少，我是这样理解的。当主线程恢复执行时，当它调用AvionicsTask的析构函数时，似乎得到了一个分段错误。老实说，我不知道为什么，除了我可能没有正确初始化一些东西。无论如何，代码如下：

Task.h：

class Task {
protected:
    /* -- NAME */
    static const int MAX_NAME_LEN = 15;
    char name[MAX_NAME_LEN];

    /* -- IMPLEMENTATION */
    pthread_t thread_id;

public:
    /* -- CONSTRUCTOR/DESTRUCTOR */
    Task(const char _name[]) {

        std::strncpy(name, _name, MAX_NAME_LEN);
    }
    ~Task(){}
    /* -- ACCESSORS */
    char * Name();

    virtual void Start();    
    virtual void Run()= 0;
    static void GracefullyExitMainThread();
};

Task.cpp：

#include "task.h"

std::vector<pthread_t> tasklist;        //keep track of tasks created

void * thunkfunc(void * args) {
        Task * task_instance = (Task *)args;
        task_instance->Run();
        return NULL;
}

void Task::Start(){
    pthread_t threadmachine;
    void * start_arg = NULL;
    pthread_create(&threadmachine,  NULL, thunkfunc, this);
    tasklist.push_back(threadmachine);
}

void Task::GracefullyExitMainThread() {
    void** return_value;                //unused
    for(int i = 0; i < tasklist.size(); i++){
        pthread_join(tasklist[i], return_value);
    }
}

char * Task::Name(){
    return name; 
}

Task_Test_step1.cpp：

#include <iostream>
using namespace std;
#include "task.h"


class RudderController : public Task {
public:
  RudderController(char _name[]) : Task(_name) {}

  void Run() {
    cout << "Rudder Controller [" << name << "] running\n" << flush;
    for (int i = 0; i < 10; i++) {
      cout << name << " waiting for next sensor input\n" << flush;
      usleep(1000000);
      cout << name << " issueing rudder control command" << i << "\n" << flush;
      usleep(10000);
    }
  }
};

class AvionicsTask : public Task {
public:
  AvionicsTask(char _name[]) : Task(_name) {}

  void Run() {
    cout << "Avionics System [" << name << "] running\n" << flush;
    for (int i = 0; i < 10; i++) {
      cout << name << " waiting for next refresh interval\n" << flush;
      usleep(700000);
      cout << name << " refreshing avionics screen " << i << "\n" << flush;
      usleep(12000);
    }
  }
};

int main(int argc, char * argv[]) {

  /* -- CREATE TASKS */
  RudderController task1("rudder control");
  AvionicsTask     task2("avionics task");

  /* -- LAUNCH TASKS */
  task1.Start();
  task2.Start();

  Task::GracefullyExitMainThread();
}

使用gdb的程序输出：

Starting program: /home/ben/Desktop/Part 1/test 
[Thread debugging using libthread_db enabled]
[New Thread 0x7ffff707e700 (LWP 6797)]
Rudder Controller [rudder control] running
rudder control waiting for next sensor input
[New Thread 0x7ffff687d700 (LWP 6798)]
Avionics System [avionics task] running
avionics task waiting for next refresh interval
avionics task refreshing avionics screen 0
...
//more messages from the threads
...
avionics task refreshing avionics screen 9
[Thread 0x7ffff687d700 (LWP 6798) exited]
rudder control issueing rudder control command7
rudder control waiting for next sensor input
rudder control issueing rudder control command8
rudder control waiting for next sensor input
rudder control issueing rudder control command9
[Thread 0x7ffff707e700 (LWP 6797) exited]

Program received signal SIGSEGV, Segmentation fault.
0x0000000000401ffa in AvionicsTask::~AvionicsTask (this=0xffffffffffffffc0, 
    __in_chrg=<optimized out>) at task_test_step1.cpp:21
21  class AvionicsTask : public Task {

Answer 1

你使用return_value的方式是错误的：

void** return_value;               
for(int i = 0; i < tasklist.size(); i++){
    pthread_join(tasklist[i], return_value); // <== return_value has an indeterminate value
                                             // who knows where pthread_join() will write to?
}

如果你不关心你加入的线程传递给pthread_exit()的是什么(或者等价的线程函数返回了什么)，那么就传入NULL：

for(int i = 0; i < tasklist.size(); i++){
    pthread_join(tasklist[i], NULL);   // <== ignore what the thread returns
}

如果您决定要获取线程返回的值，请使用如下所示的内容：

void* return_value;               
for(int i = 0; i < tasklist.size(); i++){
    pthread_join(tasklist[i], &return_value);

    // return_value contains the data the thread returned...
}