Learn Python together: multithreading-sharing global variable issues

Multithreading-sharing global variable issues

Issues you may encounter with multithreaded development

Assuming that the two threads T1 and T2 are going to add 1 to the global variable g_num (0 by default), T1 and T2 each add 10 times to G_num, the final result of G_num should be 20.

However, due to the simultaneous operation of multiple threads, the following conditions may occur:

At the time of g_num=0, T1 obtained g_num=0. At this time the system T1 to "sleeping" state, the T2 into the "running" state, T2 also get g_num=0
T2 then adds 1 to the resulting value and assigns it to G_num, making G_num=1
Then the system T2 to "sleeping", the T1 to "running". The thread T1 and assigns it to G_num after 0 plus 1.
This led to T1 and T2 g_num plus 1, but the result is still g_num=1
Test 1

Import threadingImport Timeg_num =0DefWork1(num):Global G_numFor IIn range (num): G_num + =1 print ( "----in Work1, G_num is%d---"%g_num) def work2global g_num for i in Range (num): g_ num + = 1 print ( "----in Work2, G_num is%d---"%g_num) print ( "---thread was created before 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 threads the end result after the same global variable operation is:%s "% g_num)          

Operation Result:

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

Test 2

Import threadingImport Timeg_num =0DefWork1(num):Global G_numFor IIn 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 ("---thread was created before G_num is%d---"%g_num) T1 = Threading. Thread (Target=work1, args= (1000000,)) T1.start () t2 = Threading. Thread (TARGET=WORK2, args= (1000000,)) T2.start () whileLen (threading.enumerate ())! = 1:time.sleep ( 1) Print ("The end result of 2 threads after the same global variable operation is:%s"% g_num)          

Operation Result:

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

Conclusion

If multiple threads operate on the same global variable at the same time, there is a resource contention problem that results in incorrect data

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Learn Python together: multithreading-sharing global variable issues