semaphore overA = 0;
semaphore overB = 0;
semaphore overC = 0;
semaphore overD = 0;
A
{
操作；
V(overA)
}
B
{
操作;
V(overB)
}
C
{
P(overA);
P(overB);
操作;
V(overC);
}
D
{
操作;
V(overD);
}
E
{
P(overC);
P(overD);
操作；
}