奇怪的python线程输出 [英] Strange python threading output

问题描述

为我提供了一个简单的python程序进行分析.它工作正常，并随机输出13、14和15(当然).我可以看到为什么打印13和14，但是我不知道15的来源.

I was given a simple python program to analyze. It works fine, and outputs 13, 14, and 15 randomly (of course). I can see why 13 and 14 where printed, but I don't understand where 15 came from.

请解释.

from threading import Thread
import random
import time
import sys

def rwait():
    amt = random.uniform(0.01,0.1)
    time.sleep(amt)

x = 0

key = True

def lockx():
    global key
    while not key:
        time.sleep(0)
    rwait()
    key = False

def unlockx():
    global key
    key = True

def A():
    global x
    rwait()
    lockx()
    reg = x
    reg = reg+1
    rwait()
    x = reg
    unlockx()

def B():
    global x
    rwait()
    lockx()
    reg = x
    reg = reg+2
    rwait()
    x = reg
    unlockx()

def main():
    global x
    x = 12
    p1 = Thread(target=B)
    p1.start()
    A()
    p1.join()
    print("x=",x)


for k in range(20):
    main()

推荐答案

可能发生三种不同的情况:

Three different things can happen:

• 线程A和B在更改之前先读取x ，然后

• 线程A写入其结果(13)，并且

• thread A writes its result (13), and

线程B写入其结果(14)，

thread B writes its result (14),

和第二个写胜利的线程.

and the second thread to write wins.

线程A或B首先读取x，在另一个线程读取之前，和写入.结果:15，因为A读12，加1并写13，然后B读13并写15，反之亦然.

thread A or B reads x first, and writes before the other thread reads. Result: 15, as either A reads 12, adds one and writes 13, then B reads 13 and writes 15, or vice-versa.

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