pthread和pthread_join函数的行为

问题描述

我是新的多线程编程，并有一个关于pthreads的问题。

这是我运行的测试代码，我不明白它的行为。

  void * t1（void * args）{
 printf from t1\\\
）; 
 return; 
} 
 
 void * t2（void * args）{
 printf（从t2\\\
返回）; 
 return; 
} 
 
 int main（）{
 pthread_t thread1，thread2; 
 int r1，r2; 
 r1 = pthread_create（& thread1，NULL，t1，NULL）; 
 r2 = pthread_create（& thread2，NULL，t2，NULL）; 
 
 pthread_join（thread1，NULL）; 
 // pthread_join（thread2，NULL）; 
 
 return 0; 
} 
  

此程序的行为如下所示5。

  murtuza @ murtuza：FFTW $ ./ptest 
从t2返回
从t1返回
 murtuza @ murtuza ：FFTW $ ./ptest 
从t1返回
从t2返回
 murtuza @ murtuza：FFTW $ ./ptest 
从t1返回
 murtuza @ murtuza：FFTW $ ./ptest 
从t2返回
从t2返回
 murtuza @ murtuza：FFTW $ ./ptest 
从t1返回
从t2返回
从t2 
  

我不明白第4和第5个输出。为什么线程t2执行两次？当然，如果我取消注释 pthread_join（& thread2，NULL，t2，NULL）
程序将正常运行，但我特别感兴趣的情况下，

感谢，
Mir

解决方案

我想您希望我们在这里解释未定义的行为。在您离开 main（）后，不应使用任何C库函数。我认为你正在看到的是 main（）线程刷新缓冲区，而它正在关闭C库。我认为它可能在它关闭时忽略任何流锁。

I am new to multi-threaded programming and have a question about pthreads.

This is the test code that I run and I don't understand its behaviour. Can someone throw some light on it please.

void *t1(void *args){
    printf("returning from t1\n");
    return;
}

void *t2(void *args){
    printf("returning from t2\n");
    return;
}

int main(){
    pthread_t thread1,thread2;
    int r1,r2;
    r1=pthread_create(&thread1,NULL,t1,NULL);
    r2=pthread_create(&thread2,NULL,t2,NULL);

    pthread_join(thread1,NULL);
 // pthread_join(thread2,NULL);

    return 0;
}

The behaviour of this program is either of the 5 shown below

murtuza@murtuza:FFTW$ ./ptest
returning from t2
returning from t1
murtuza@murtuza:FFTW$ ./ptest
returning from t1
returning from t2
murtuza@murtuza:FFTW$ ./ptest
returning from t1
murtuza@murtuza:FFTW$ ./ptest
returning from t2
returning from t2
murtuza@murtuza:FFTW$ ./ptest
returning from t1
returning from t2
returning from t2

I don't understand the 4th and 5th output. Why is thread t2 executing twice? Of course, if I uncomment pthread_join(&thread2,NULL,t2,NULL)the program will behave correctly but I am specifically interested in the case where only one thread joins the main() thread.

thanks,Mir

解决方案

I think you want us to explain undefined behaviour here. You should never use any C library function after you leave main(). I think that what you are seeing is the main() thread flushing buffers while it is shutting down the C library. I think it might be ignoring any streams locks at the time it is shutting down.

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