The first known correct software solution to the critical-section problem for two processes was developed by Dekker. The two processes, P0 and P1, share the following variables:

boolean flag[2]; / initially false /
int turn;

The structure of process Pi (i == 0 or 1) is shown in Figure 5.21. The other process is Pj (j == 1 or 0). Prove that the algorithm satisfies all three requirements for the critical-section problem.

FIGURE 5.21
do {
flag[i] = true;

while (flag[j]) {

if (turn == j) {
flag[i] = false;
while (turn == j) ; / do nothing /
flag[i] = true;
}
}

/ critical section /

turn = j;
flag[i] = false;

/ remainder section /

} while (true);

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