多线程-共享全局变量问题

多线程开发可能遇到的问题

假设两个线程t1和t2都要对全局变量g_num(默认是0)进行加1运算，t1和t2都各对g_num加10次，g_num的最终的结果应该为20。

但是由于是多线程同时操作，有可能出现下面情况：

1. 在g_num=0时，t1取得g_num=0。此时系统把t1调度为”sleeping”状态，把t2转换为”running”状态，t2也获得g_num=0
2. 然后t2对得到的值进行加1并赋给g_num，使得g_num=1
3. 然后系统又把t2调度为”sleeping”，把t1转为”running”。线程t1又把它之前得到的0加1后赋值给g_num。
4. 这样导致虽然t1和t2都对g_num加1，但结果仍然是g_num=1

测试1

import threading
import time

g_num = 0

def work1(num): global g_num for i in range(num): g_num += 1 print("----in work1, g_num is %d---"%g_num) def work2(num): global g_num for i in range(num): g_num += 1 print("----in work2, g_num is %d---"%g_num) print("---线程创建之前g_num is %d---"%g_num) t1 = threading.Thread(target=work1, args=(100,)) t1.start() t2 = threading.Thread(target=work2, args=(100,)) t2.start() while len(threading.enumerate()) != 1: time.sleep(1) print("2个线程对同一个全局变量操作之后的最终结果是:%s" % g_num) 

运行结果：

---线程创建之前g_num is 0---
----in work1, g_num is 100--- ----in work2, g_num is 200--- 2个线程对同一个全局变量操作之后的最终结果是:200 

测试2

import threading
import time

g_num = 0

def work1(num): global g_num for i in range(num): g_num += 1 print("----in work1, g_num is %d---"%g_num) def work2(num): global g_num for i in range(num): g_num += 1 print("----in work2, g_num is %d---"%g_num) print("---线程创建之前g_num is %d---"%g_num) t1 = threading.Thread(target=work1, args=(1000000,)) t1.start() t2 = threading.Thread(target=work2, args=(1000000,)) t2.start() while len(threading.enumerate()) != 1: time.sleep(1) print("2个线程对同一个全局变量操作之后的最终结果是:%s" % g_num) 

运行结果：

---线程创建之前g_num is 0---
----in work1, g_num is 1088005--- ----in work2, g_num is 1286202--- 2个线程对同一个全局变量操作之后的最终结果是:1286202 

结论

• 如果多个线程同时对同一个全局变量操作，会出现资源竞争问题，从而数据结果会不正确