pthread_barrier_wait 创建所有线程后挂起

Question

我正在尝试编写一个简单的程序来使用屏障在从主线程打印消息之前等待多个线程的创建。

这是我的代码：

#include <iostream>
#include <pthread.h>
#include <stdio.h>
#include <cstdlib>
#include <cstdint>

#define NUM_THREADS 8

pthread_barrier_t barrier;

void *threadFun(void *tid)
{
    intptr_t temp = (intptr_t) tid;
    printf("Hello from thread %d\n", temp);
}

int main()
{
    pthread_t threads[NUM_THREADS];
    int rc;
    pthread_barrier_init(&barrier, NULL, NUM_THREADS);

    for(int i = 0; i < NUM_THREADS; ++i) {
        rc = pthread_create(&threads[i], NULL, threadFun, (void *) i);
        if(rc)  {
            printf("Error creating thread %d\n", i);
            exit(-1);
        }
    }

    pthread_barrier_wait(&barrier);
    printf("Hello from main!\n");

    for(int i = 0; i < NUM_THREADS; ++i) {
        pthread_join(threads[i], NULL);
    }
    pthread_barrier_destroy(&barrier);

    return 0;
}

目前，我的程序打印了一些不确定的 "Hello from thread " 语句，并在打印 "Hello from main!" 之前挂起；但是，它总是打印 8 个线程消息。因此，创建了所有线程。

为什么还挂着？

Answer 1

屏障预计被 wait 编辑 NUM_THREADS 次，但实际上只有一个线程（主线程）调用 pthread_barrier_wait.

如果要将主线程与工作线程同步，则需要初始化 NUM_WORKER_THREADS + 1 的屏障。

Answer 2

//  q30941549.cpp
// 
#include <iostream>
#include <pthread.h>
#define NUM_THREADS 8
pthread_barrier_t barrier;

void *threadFun(void *tid) {
  printf("Work done by thread %ld BEFORE the barrier.\n", (long)tid);
  pthread_barrier_wait(&barrier);
  printf("Hello from thread %ld AFTER the barrier.\n", (long)tid);
  pthread_exit(NULL);
}

int main() {
  pthread_t threads[NUM_THREADS];
  int rc;
  pthread_barrier_init(&barrier, NULL, NUM_THREADS+1);
  for(int i = 0; i < NUM_THREADS; ++i) {
    rc = pthread_create(&threads[i], NULL, threadFun, (void *)(long)i);
    if(rc)  {
      printf("Error creating thread %d\n", i);
      exit(-1);
    }
  }
  printf(" X- - - - - - - - - - - - - - - -X\n");
  printf("main thread BEFORE the barrier.\n");
  printf(" = = BEFORE barrier = BEFORE barrier = =\n");
  pthread_barrier_wait(&barrier);
  printf(" = = AFTER barrier = = AFTER barrier = =\n");
  printf("Hello from main! AFTER the barrier.\n");
  printf(" Y- - - - - - - - - - - - - - - -Y\n");
  for(int i = 0; i < NUM_THREADS; ++i) {
    pthread_join(threads[i], NULL);
  }
  printf("* - * - * - * - * - * - * - * - * - * - * - * - * - *\n");
  printf("This line is ALWAYS last. Only main lives after join.\n");
  return EXIT_SUCCESS;
}

编译和运行:

g++ -pthread q30941549.cpp && ./a.out # Linux
g++ -pthread q30941549.cpp && ./a.exe # MSYS2, Windows

如果您尝试运行几次，您可能会看到如下内容：

Work done by thread 0 BEFORE the barrier.
Work done by thread 3 BEFORE the barrier.
Work done by thread 6 BEFORE the barrier.
Work done by thread 4 BEFORE the barrier.
Work done by thread 1 BEFORE the barrier.
Work done by thread 5 BEFORE the barrier.
Work done by thread 7 BEFORE the barrier.
Work done by thread 2 BEFORE the barrier.
 X- - - - - - - - - - - - - - - -X
main thread BEFORE the barrier.
 = = BEFORE barrier = BEFORE barrier = =
 = = AFTER barrier = = AFTER barrier = =
Hello from main! AFTER the barrier.
 Y- - - - - - - - - - - - - - - -Y
Hello from thread 6 AFTER the barrier.
Hello from thread 4 AFTER the barrier.
Hello from thread 1 AFTER the barrier.
Hello from thread 5 AFTER the barrier.
Hello from thread 7 AFTER the barrier.
Hello from thread 2 AFTER the barrier.
Hello from thread 0 AFTER the barrier.
Hello from thread 3 AFTER the barrier.
* - * - * - * - * - * - * - * - * - * - * - * - * - *
This line is ALWAYS last. Only main lives after join.

pthread_barrier_wait 创建所有线程后挂起

pthread_barrier_wait hangs after creation of all threads

c++

multithreading

pthreads

barrier