LIBUV学习笔记 uv_barrier_xxx与pthread_barrier_xxx相关
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在学习libuv的线程相关API的时候碰到了uv_barrier_xxx相关函数，虽然说libuv的线程库总的来说和pthread差不多，但在看APUE的时候都没碰到过线程同步相关里面这个概念，于是网上查找了一下，总结如下：
pthread_barrier_xxx系列函数在<pthread.h>中定义，用于多线程的同步，它包含下列三个函数：
1. pthread_barrier_init();
2. pthread_barrier_wait();
3. pthread_barrier_destroy();
pthread_barrier_xxx其实只做且只能做一件事，就是充当栏杆（barrier意为栏杆）形象的说就是把先后到达的多个线程挡在同一栏杆前，直到所有线程到齐，然后撤下栏杆同时放行。
1)、init函数负责指定要等待的线程个数；
2)、wait()函数由每个线程主动调用，它告诉栏杆“我到起跑线前了”。wait()执行末尾栏杆会检查是否所有人都到栏杆前了，如果是，栏杆就消失所有线程继续执行下一句代码；如果不是，则所有已到wait()的线程停在该函数不动，剩下没执行到wait()的线程继续执行；
3)、destroy函数释放init申请的资源。
1、函数原型
#include <pthread.h>
int pthread_barrier_init(pthread_barrier_t *restrict barrier, const pthread_barrierattr_t *restrict attr, unsigned count);
int pthread_barrier_wait(pthread_barrier_t *barrier);
int pthread_barrier_destroy(pthread_barrier_t *barrier);
参数解释：
pthread_barrier_t：是一个计数锁，对该锁的操作都包含在三个函数内部，我们不用关心也无法直接操作，只需要实例化一个对象丢给它就好。
pthread_barrierattr_t：锁的属性设置，设为NULL让函数使用默认属性即可。
count：你要指定的等待个数。
2、使用场景
这种“栏杆”机制最大的特点就是最后一个执行wait的动作最为重要，就像赛跑时的起跑枪一样，它来之前所有人都必须等着。所以实际使用中，pthread_barrier_xxx常常用来让所有线程等待“起跑枪”响起后再一起行动。比如我们可以用pthread_create()生成100个线程，每个子线程在被create出的瞬间就会自顾自的立刻进入回调函数运行。但我们可能不希望它们这样做，因为这时主进程还没准备好，和它们一起配合的其它线程还没准备好，我们希望它们在回调函数中申请完线程空间、初始化后停下来，一起等待主进程释放一个“开始”信号，然后所有线程再开始执行业务逻辑代码。
为了解决上述场景问题，我们可以在init时指定n+1个等待，其中n是线程数。而在每个线程执行函数的首部调用wait()。这样100个pthread_create()结束后所有线程都停下来等待最后一个wait()函数被调用。这个wait()由主进程在它觉得合适的时候调用就好。最后这个wait()就是鸣响的起跑枪。
3、LIBUV相关函数
三个相关函数原型：
int uv_barrier_init(uv_barrier_t* barrier, unsigned int count)
void uv_barrier_destroy(uv_barrier_t* barrier)
int uv_barrier_wait(uv_barrier_t* barrier)
使用方法和pthread一样，下面用LIBUV里面的一个例子解释一下：
1 #include <stdio.h> 2 #include <uv.h> 3 4 uv_barrier_t blocker; 5 uv_rwlock_t numlock; 6 int shared_num; 7 8 void reader(void *n) 9 { 10 int num = *(int *)n; 11 int i; 12 for (i = 0; i < 20; i++) { 13 uv_rwlock_rdlock(&numlock);//读锁 14 printf("Reader %d: acquired lock\n", num); 15 printf("Reader %d: shared num = %d\n", num, shared_num); 16 uv_rwlock_rdunlock(&numlock); 17 printf("Reader %d: released lock\n", num); 18 } 19 uv_barrier_wait(&blocker); 20 } 21 22 void writer(void *n) 23 { 24 int num = *(int *)n; 25 int i; 26 for (i = 0; i < 20; i++) { 27 uv_rwlock_wrlock(&numlock);//写锁 28 printf("Writer %d: acquired lock\n", num); 29 shared_num++; 30 printf("Writer %d: incremented shared num = %d\n", num, shared_num); 31 uv_rwlock_wrunlock(&numlock); 32 printf("Writer %d: released lock\n", num); 33 } 34 uv_barrier_wait(&blocker); 35 } 36 37 int main() 38 { 39 uv_barrier_init(&blocker, 4); 40 41 shared_num = 0; 42 uv_rwlock_init(&numlock); 43 44 uv_thread_t threads[3]; 45 46 int thread_nums[] = {1, 2, 1}; 47 uv_thread_create(&threads[0], reader, &thread_nums[0]); 48 uv_thread_create(&threads[1], reader, &thread_nums[1]); 49 50 uv_thread_create(&threads[2], writer, &thread_nums[2]); 51 52 uv_barrier_wait(&blocker);//等待子线程全部调用uv_barrier_wait 53 uv_barrier_destroy(&blocker); 54 55 uv_rwlock_destroy(&numlock); 56 return 0; 57 }
一个可能的输出：
Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 1: acquired lock Reader 1: shared num = 0 Reader 1: released lock Reader 2: acquired lock Reader 2: shared num = 0 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 1 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 2 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 3 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 4 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 5 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 6 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 7 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 8 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 9 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 10 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 11 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 12 Writer 1: released lock Writer 1: acquired lock Writer 1: incremented shared num = 13 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 13 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 13 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 13 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 13 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 13 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 14 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 14 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 15 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 15 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 16 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 16 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 17 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 17 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 18 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 18 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 19 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 19 Reader 2: released lock Writer 1: acquired lock Writer 1: incremented shared num = 20 Writer 1: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock Reader 2: acquired lock Reader 2: shared num = 20 Reader 2: released lock
参考：https://blog.csdn.net/jackailson/article/details/51052609
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