Python：Day28 同步锁

同步锁：

Python不是有一把锁了吗？为什么还要加锁？

Python解释器的GIL的作用是同一时刻只有一个线程被CPU执行，而同步锁的作用同一时刻只有一个线程对锁定代码块操作

如果不加锁，当多个线程对同一数据操作的时候就有可能出问题，代码如下：

import threading
import time

def minus_num():
    global num
    temp = num
    time.sleep(0.001)
    num = temp - 1

num = 100
threads = []
for i in range(100):
    t = threading.Thread(target=minus_num)
    threads.append(t)
for t in threads:
    t.start()
for k in threads:
    k.join()
print(num)

加上同步锁后代码：

import threading
import time

def minus_num():
    global num
    r.acquire()
    temp = num
    time.sleep(0.001)
    num = temp - 1
    r.release()

num = 100
threads = []
r = threading.Lock()    # 创建一把锁，这个锁在任意时刻都只能被一个线程获取。
for i in range(100):
    t = threading.Thread(target=minus_num)
    threads.append(t)
for t in threads:
    t.start()
for k in threads:
    k.join()
print(num)　　# 0

死锁、递规锁

import threading
import time

class MyThread(threading.Thread):
    def doA(self):
        lockA.acquire()
        print("%s get the lockA" % self.name)
        time.sleep(1)
        lockB.acquire()
        print("%s get the lockB" % self.name)
        lockA.release()
        lockB.release()

    def doB(self):
        lockB.acquire()
        print("%s get the lockB" % self.name)
        time.sleep(1)
        lockA.acquire()
        print("%s get the lockA" % self.name)
        lockA.release()
        lockB.release()

    def run(self):
        self.doA()
        self.doB()

lockA = threading.Lock()
lockB = threading.Lock()

threads = []
for i in range(5):
    threads.append(MyThread())

for i in threads:
    i.start()

for i in threads:
    i.join()

print("----------the end----------")

之所以出现死锁的现象是因为一个线程需要两把或两把以上的锁，且出现了锁套锁，并且其中一个锁被其它线程占用且不能释放，这样就造成了死其锁。