We have a soccer stadium with three entrance doors and need to keep track of the number of spectators currently inside. We use a shared mutable variable count for this purpose. Whenever a spectator enters, the following steps are executed, with the intent of incrementing the shared count:

public void inc() { final int local = count; count = local + 1; }

 
a. 0.5 Suppose the stadium is empty, count is zero, and three spectators enter at the same time through different doors. Whatis the conceptually correct value of count once the three spectators are inside?
 

b. 0.5 Under the same scenario, and given our implementation of inc, what are the possible resulting values for count (assumingmultiple threads of execution)? Which one(s) are conceptually correct? Be careful of the possible resulting values!

 
Still under the same scenario, one possible ordering of the four steps corresponding to the two invocations of inc is

local1 = count;     // f1
count = local1 + 1; // s1 local2 = count;     // f2 count = local2 + 1; // s2 local3 = count;     // f3 count = local3 + 1; // s3

where local1, local2, and local3 are distinct local variables associated with the three doors.

c. 0.5 (TRUE or FALSE) Is the following ordering possible? f1 s1 s2 f2 f3 s3

d. 1.5 List all possible orderings of these steps, using abbreviations f i for fetch i and s i for set i as shown in the comments.(I.e., you would list the ordering from part b as "f1 s1 f2 s2 f3 s3".)

Answer rating: 100% (QA)

a The conceptually correct value of count once the three spectators are inside should be 3 because ... View the full answer