A,B,C,D四个进程，A向buf里面写数据，B,C,D向buf里面读数据，

当A写完，且B，C，D都读一次后，A才能再写。用P，V操作实现。

semaphore empty = n

semaphore full;

semaphore mutex =1;

semaphore b = 1;

semaphore c = 1 ;

semaphore d = 1;

A (){
while(true){
     p(empty);
     p(b);
     p(c);
p(d);
p(mutex);
write();
v(mutex);
v(full);
}
}

B (){
while(true){
p(full);
p(mutex);
write();
v(mutex);
v(empty);
v(b);
}
}

C(){
while(true){
p(full);
p(mutex);
write();
v(mutex);
v(empty);
v(c);
}
}

D (){
while(true){
p(full);
p(mutex);
write();
v(mutex);
v(empty);
v(d);
}
}