如何从循环外部杀死无限循环中的pthread？

Question

我创建了一个线程,并将其置于无限循环中.使用valgrind检查代码时出现内存泄漏.这是我的代码:

#include <pthread.h>
#include <time.h>

void thread_do(void){
    while(1){}
}

int main(){
    pthread_t th;   
    pthread_create(&th, NULL, (void *)thread_do, NULL);

    sleep(2);
    /* I want to kill thread here */
    sleep(2);
    return 0;
}

所以在main中创建一个线程,并且一直运行thread_do().有没有办法在2秒后从主内部杀死它？我试过了两次pthread_detach(th),pthread_cancel(th)但我仍然有泄漏.

Answer 1

正如@sarnold指出的那样,默认情况下,pthread_cancel()如果不调用任何作为取消点的函数,就不能取消你的线程......但是这可以通过使用pthread_setcanceltype()将线程的取消类型设置为异步而不是延迟来更改.要做到这一点,你需要pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS,NULL);在开始循环之前添加类似于线程函数开头附近的东西.然后你就可以通过调用来终止线程pthread_cancel(th)的main().

但请注意,以这种方式取消线程(无论是否异步)都不会清除线程函数中分配的任何资源(如评论中Kevin所述).为了干净利落,您可以:

• 确保线程在退出之前不执行任何需要清理的操作(例如,使用malloc()分配缓冲区)
• 在线程退出后,确保在线程之后有一些清理方法
• 使用pthread_cleanup_push()和pthread_cleanup_pop()添加清理处理程序以在取消线程时清理资源.请注意,如果取消类型是异步的,这仍然存在风险,因为可以在分配资源和添加清理处理程序之间取消线程.
• 避免使用pthread_cancel()并让线程检查某些条件以确定何时终止(将在长时间运行的循环中检查).由于您的线程本身会检查终止,因此它可以在检查后执行所需的任何清理.

实现最后一个选项的一种方法是使用互斥锁作为标志,并pthread_mutex_trylock()使用函数包装进行测试以在循环测试中使用:

#include <pthread.h>
#include <unistd.h>
#include <errno.h>

/* Returns 1 (true) if the mutex is unlocked, which is the
 * thread's signal to terminate. 
 */
int needQuit(pthread_mutex_t *mtx)
{
  switch(pthread_mutex_trylock(mtx)) {
    case 0: /* if we got the lock, unlock and return 1 (true) */
      pthread_mutex_unlock(mtx);
      return 1;
    case EBUSY: /* return 0 (false) if the mutex was locked */
      return 0;
  }
  return 1;
}

/* Thread function, containing a loop that's infinite except that it checks for
 * termination with needQuit() 
 */
void *thread_do(void *arg)
{
  pthread_mutex_t *mx = arg;
  while( !needQuit(mx) ) {}
  return NULL;
}

int main(int argc, char *argv[])
{
  pthread_t th;
  pthread_mutex_t mxq; /* mutex used as quit flag */

  /* init and lock the mutex before creating the thread.  As long as the
     mutex stays locked, the thread should keep running.  A pointer to the
     mutex is passed as the argument to the thread function. */
  pthread_mutex_init(&mxq,NULL);
  pthread_mutex_lock(&mxq);
  pthread_create(&th,NULL,thread_do,&mxq);

  sleep(2);

  /* unlock mxq to tell the thread to terminate, then join the thread */
  pthread_mutex_unlock(&mxq); 
  pthread_join(th,NULL);

  sleep(2);
  return 0;
}

如果线程未分离(通常不是默认情况下),则应pthread_join()在停止线程后调用.如果线程已分离,则不需要加入它,但是您不会确切地知道它何时终止(或者甚至是大约,除非您添加另一种方式来指示其退出).

Answer 2

一些小小的想法:

1. 您正在尝试取消您的主题,但如果适用的取消政策是延期取消,您thread_do()将永远不会被取消,因为它永远不会调用任何取消点的功能:

    A thread's cancellation type, determined by
    pthread_setcanceltype(3), may be either asynchronous or
    deferred (the default for new threads).  Asynchronous
    cancelability means that the thread can be canceled at any
    time (usually immediately, but the system does not guarantee
    this).  Deferred cancelability means that cancellation will
    be delayed until the thread next calls a function that is a
    cancellation point.  A list of functions that are or may be
    cancellation points is provided in pthreads(7).

2. 您没有加入简单示例代码中的线程; pthread_join(3)在程序结束前致电:

    After a canceled thread has terminated, a join with that
    thread using pthread_join(3) obtains PTHREAD_CANCELED as the
    thread's exit status.  (Joining with a thread is the only way
    to know that cancellation has completed.)