c - 使用二进制信号量实现计数信号量

Question

我已经实现了某种用户级线程系统。我需要一些帮助来使用二进制信号量来实现计数信号量实现(向上和向下功能如下所述)。这是我实现的二进制信号量的接口(interface):

typedef enum BinSemStatus{
   locked,
   unlocked
} BinSemStatus;


struct semaphore {
BinSemStatus status;
int bid;
};

int bsem_alloc();//allocate a new binary semaphore,return its descriptor
void bsem_free(int id);
void bsem_down(int id);
void bsem_up(int id);

这是计数信号量接口(interface)的接口(interface):

struct counting_semaphore* counting_alloc(uint value);
counting_free(struct counting_semaphore* sem);

// If the value representing the count of
// the semaphore variable is not negative, decrement it by 1. If the 
// semaphore variable is now
// negative, the thread executing acquire is blocked until the value is 
// greater or equal to 1.
// Otherwise, the thread continues execution.
void up(struct counting_semaphore* sem);
// Increments the value of semaphore
// variable by 1.
void down(struct counting_semaphore* sem);

我尝试做的是 void up(structcounting_semaphore* sem)锁定值。但是正如您在下面看到的那样，这还不够。我已经在有问题的情况下添加了评论。

struct counting_semaphore {
int binary_descriptor;
int value;
}; 

void down(struct counting_semaphore *sem){
  bsem_down(sem->binary_descriptor);
  if (sem->value > 0){
    sem->value--;
  }
  else{
     //not sure what to do here, maybe use anather semaphore in some way?
  } 
  bsem_up(sem->binary_descriptor);
 }
void up(struct counting_semaphore *sem){
  bsem_down(sem->binary_descriptor);
  sem->value++;
  bsem_up(sem->binary_descriptor);
}

Answer 1

当sem->value达到0时，线程阻塞，需要重新调度。你没有显示你的调度代码，所以我无法给出具体的建议。调度程序可能应该代表线程调用bsem_up(sem->binary_descriptor);。