栅栏的功能与其字面含义大体相同，阻塞到达栅栏的线程，直到指定数目的线程全部到达。

1 栅栏属性类型

#define __SIZEOF_PTHREAD_BARRIERATTR_T 4
typedef union
{
  char __size[__SIZEOF_PTHREAD_BARRIERATTR_T];
  int __align;
} pthread_barrierattr_t;

2 初始化与销毁栅栏属性

int pthread_barrierattr_destroy(pthread_barrierattr_t *attr);
int pthread_barrierattr_init(pthread_barrierattr_t *attr);

3 栅栏属性之进程共享属性

int pthread_barrierattr_getpshared(const pthread_barrierattr_t
                                   *restrict attr, int *restrict pshared);
int pthread_barrierattr_setpshared(pthread_barrierattr_t *attr,int pshared);

• 
PTHREAD_PROCESS_SHARED


  能够访问栅栏内存的所有线程

• 
PTHREAD_PROCESS_PRIVATE


  创建栅栏所在的进程内的所有线程

4 栅栏结构

# define __SIZEOF_PTHREAD_BARRIER_T 20
typedef union
{
  char __size[__SIZEOF_PTHREAD_BARRIER_T];
  long int __align;
} pthread_barrier_t;

5 初始化与销毁栅栏

int pthread_barrier_destroy(pthread_barrier_t *barrier);
int pthread_barrier_init(pthread_barrier_t *restrict barrier,
                         const pthread_barrierattr_t *restrict attr,
                         unsigned count);

6 栅栏等待

int pthread_barrier_wait(pthread_barrier_t *barrier);

7 案例：栅栏的使用

• 源码

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <pthread.h>
  #include <time.h>
  
  pthread_barrier_t barrier;
  
  void *start_routine_01(void *ptr)
  {
      printf("子线程(%lu)开始运行...\n", pthread_self());
      printf("子线程(%lu)栅栏等待状态前\n", pthread_self());
      pthread_barrier_wait(&barrier);
      printf("子线程(%lu)栅栏等待状态后\n", pthread_self());
      printf("子线程(%lu)即将退出...\n", pthread_self());
      return (void*)"9999";
  }
  
  void *start_routine_02(void *ptr)
  {
      printf("子线程(%lu)开始运行...\n", pthread_self());
      printf("子线程(%lu)栅栏等待状态前\n", pthread_self());
      pthread_barrier_wait(&barrier);
      printf("子线程(%lu)栅栏等待状态后\n", pthread_self());
      printf("子线程(%lu)即将退出...\n", pthread_self());
      return (void*)"9999";
  }
  
  void *start_routine_03(void *ptr)
  {
      printf("子线程(%lu)开始运行...\n", pthread_self());
      printf("子线程(%lu)栅栏等待状态前\n", pthread_self());
      pthread_barrier_wait(&barrier);
      printf("子线程(%lu)栅栏等待状态后\n", pthread_self());
      printf("子线程(%lu)即将退出...\n", pthread_self());
      return (void*)"9999";
  }
  
  int main(int argc, char const *argv[])
  {
      printf("主线程(%lu)开始运行...\n", pthread_self());
  
      pthread_barrierattr_t attr;
      pthread_barrierattr_init(&attr);
      pthread_barrier_init(&barrier, &attr, 3);
      pthread_barrier_destroy(&attr);
      
      pthread_t thread_id_01;
      pthread_create(&thread_id_01, NULL, start_routine_01, NULL);
  
      pthread_t thread_id_02;
      pthread_create(&thread_id_02, NULL, start_routine_02, NULL);
  
      pthread_t thread_id_03;
      pthread_create(&thread_id_03, NULL, start_routine_03, NULL);
  
      pthread_join(thread_id_01, NULL);
      pthread_join(thread_id_02, NULL);
      pthread_join(thread_id_03, NULL);
  
      pthread_barrier_destroy(&barrier);
      printf("主线程(%lu)即将退出...\n", pthread_self());
      exit(EXIT_SUCCESS);
  }
  

• 输出

  主线程(140073106831168)开始运行…
  
  子线程(140073106827008)开始运行…
  
  子线程(140073106827008)栅栏等待状态前
  
  子线程(140073098434304)开始运行…
  
  子线程(140073098434304)栅栏等待状态前
  
  子线程(140073090041600)开始运行…
  
  子线程(140073090041600)栅栏等待状态前
  
  子线程(140073090041600)栅栏等待状态后
  
  子线程(140073090041600)即将退出…
  
  子线程(140073098434304)栅栏等待状态后
  
  子线程(140073098434304)即将退出…
  
  子线程(140073106827008)栅栏等待状态后
  
  子线程(140073106827008)即将退出…
  
  主线程(140073106831168)即将退出…