最简单来说,假如有3个线程同时访问一个全局变量,那么很可能会操作互斥错误的情况,代码如下:
#!/usr/bin/python #encoding=utf-8 import time import threading index = 0 def printIndex(): global index global lock while index < 100: print index, index += 1 time.sleep(1) t1 = threading.Thread(target = printIndex, args=()) t1.setDaemon(True) t1.start() t2 = threading.Thread(target = printIndex, args=()) t2.setDaemon(True) t2.start() t3 = threading.Thread(target = printIndex, args=()) t3.setDaemon(True) t3.start() time.sleep(10)
由此可见,输出的完全是乱码,因为这个地方除了全局变量是临界资源外控制台也是一个临界资源。所以需要互斥操作。将程序改为:
#!/usr/bin/python #encoding=utf-8 import time import threading index = 0 lock = threading.RLock() def printIndex(): global index global lock while index < 100: lock.acquire() print(index), index += 1 lock.release() time.sleep(1) t1 = threading.Thread(target = printIndex, args=()) t1.setDaemon(True) t1.start() t2 = threading.Thread(target = printIndex, args=()) t2.setDaemon(True) t2.start() t3 = threading.Thread(target = printIndex, args=()) t3.setDaemon(True) t3.start() time.sleep(100)最后输出结果:
由此可见,输出时完全按顺序的。在代码中先用RLock()创建一个锁,当需要访问临界资源时进行加锁lock.require(),等退出时再释放锁lock.release()。
对于临界资源来说,用锁可以很方便实现互斥,而如果要实现同步,则需要用event。
假如程序中需要进行复杂的计算,现在将这计算在另一线程中运行,但是主程序怎么才能知道计算完成了呢?实例代码如下:
import time import threading def getSum(n, sum): for i in range(0, n + 1): sum[0] += i time.sleep(0.1) def main(): sum = [0] t = threading.Thread(target = getSum, args=(100, sum,)) t.setDaemon(True) t.start() time.sleep(2) print('sum = ', sum[0]) if __name__ == '__main__': main()现在需要求1 + 2 + 3 + ... + N 的和,计算放在单独线程中进行,但是我们不知道计算什么时候结束,只能假设2s,最后结果如下:
最后结果sum = 153,这显然是不正确的,即使是正确,也是巧合。这时就需要一个类似于信号的东西,让子进程结束后能够通过这个信号告诉主进程,event就可以配上用
场了。event和lock的用法差不多,下面是代码:
import time import threading def getSum(n, sum, event): for i in range(0, n + 1): sum[0] += i time.sleep(0.1) event .set() def main(): sum = [0] event = threading.Event() t = threading.Thread(target = getSum, args=(100, sum, event)) t.setDaemon(True) t.start() event.wait() print('sum = ', sum[0]) if __name__ == '__main__': main()程序中用了event作为一个信号量,初始是没信号的,当子进程完成计算后将其设置为有信号,主进程wait返回,最后结果:
最后,写个生产者和消费者的模型吧:
import time import random import threading def producter(buffer, event): while True: event.wait() event.clear() if len(buffer) > 4: print("the buffer is full! producter wait...") else: data = random.randint(1, 10) print('the producter has put data: ', data) buffer.append(data) event.set() time.sleep(0.8) pass def consumer(buffer, event): while True: event.wait() event.clear() if len(buffer) == 0: print('he buffer is empty! consumer wait...') else: data = buffer[0] del buffer[0] print('the consumer has get data:', data) event.set() time.sleep(0.4) def main(): buffer = [] event = threading.Event() event.set() p = threading.Thread(target = producter, args=(buffer, event)) p.setDaemon(True) p.start() c = threading.Thread(target = consumer, args=(buffer, event)) c.setDaemon(True) c.start() if __name__ == '__main__': main() time.sleep(100)
版权声明:本文为博主原创文章,未经博主允许不得转载。