了解POSIX屏障机制 [英] Understanding posix barrier mechanism

问题描述

下面是一块（非常简化，用全局变量的等气味）C code，它使用POSIX屏障原语sincronize线程启动的。

Here is piece (very simplified, with global var's and other "smells") of C code, which uses posix barrier primitive to sincronize thread start.

#include <pthread.h>
#include <stdio.h>

pthread_barrier_t barrier;

void* thread_func(void* aArgs)
{
  pthread_barrier_wait(&barrier);

  printf("Entering thread %p\n", (void*)pthread_self());
  int i;
  for(i = 0 ; i < 5; i++)
    printf("val is %d in thread %p \n", i, (void*)pthread_self());
}

int main()
{
  pthread_t thread_1, thread_2;
  pthread_barrier_init(&barrier, NULL, 2);

  pthread_create(&thread_1, NULL, (void*)thread_func, NULL);
  printf("Thread %p created\n", (void*)thread_1);

  usleep(500);

  pthread_create(&thread_2, NULL, (void*)thread_func, NULL);
  printf("Thread %p created\n", (void*)thread_2);

  pthread_join(thread_1, NULL);
  pthread_join(thread_2, NULL);

  pthread_barrier_destroy(&barrier);

  return 0;
}

我不明白，为什么输入线......字符串同时发生不输出？当我在上面运行PROGRAMM，通常会得到一样的输出：

I can't understand, why "Entering thread ..." string occurs not simultaneously in the output? When i run above programm, usually get alike output:

Thread 0xb74fdb40 created
Thread 0xb6cfcb40 created
Entering thread 0xb6cfcb40
val is 0 in thread 0xb6cfcb40 
val is 1 in thread 0xb6cfcb40 
val is 2 in thread 0xb6cfcb40 
val is 3 in thread 0xb6cfcb40 
val is 4 in thread 0xb6cfcb40 
Entering thread 0xb74fdb40
val is 0 in thread 0xb74fdb40 
val is 1 in thread 0xb74fdb40 
val is 2 in thread 0xb74fdb40 
val is 3 in thread 0xb74fdb40 
val is 4 in thread 0xb74fdb40 

我有什么期望，同时启动两个线程，而出现的输入线......在输出序列串。
与编译：gcc的barrier.c -pthread

What i expect, is simultaneously starting of two threads, and appearing of "Entering thread ..." strings in sequence in the output. Compile it with: gcc barrier.c -pthread

我在做什么错呢？

推荐答案

你的障碍正在做的是preventing线程1从创建线程2之前任何输出。

What your barrier is doing is preventing Thread 1 from producing any output before Thread 2 is created.

创建两个线程后，两个越过障碍和畅通。 之后，你是不是主张任何控制至于他们是如何交织的所以线程1偶然得到它的时间片，并产生所有的输出;然后线程2获得其时间片并产生所有的输出。

After both threads are created, both cross the barrier and are unblocked. After that you are not asserting any control as to how they are interleaved. So, thread 1 happens to get its time slice and produces all its output; then thread 2 gets its time slice and produces all its output.

尝试移动屏障的之后的输入输出到更好地理解。

Try moving the barrier after the "Enter" output to get a better understanding.

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