think about your possible case about process synchronization, and give out a solution for the problem.

 

say a = 10

process a and process b take a and do a -= 5 simultaneously :

p1 = a

p2 = a

p1 = p1 - 5

p2 = p2 - 5

a = p1

a = p2

so variable a is equal to 5 instead of 0 that we want.

solution:

using lock