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(1)是;因为两个进程不能同时对同一个缓存区进行写入操作

(2)

semaphore empty = 1;
semaphore full = 0;
P1
{
  wait(empty);
  C1();
  signal(full);
}
P2
{
  wait(full);
  C2();
  signal(empty);
}

(3)

semaphore turn1 = 1;
semaphore turn2 = 0;
P1
{
  wait(turn1);
  C3();
  signal(turn2);
}
P2
{
  wait(turn2);
  C3();
  signal(turn1);
}