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我已经实现了某种用户级线程系统。我需要一些帮助来实现计数信号量，使用二进制信号量实现（如下所述的向上和向下函数）。这是我实现二进制信号量的接口：

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);

这里是计数信号量接口的接口：

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);

我试图做的是在 void up(structcounting_semaphore* sem) 处锁定值。但正如您在下面看到的那样，这还不够。我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);
}
4

2 回答 2

0

临界区由sem->binary_descriptor1每个操作保护down(S)并up(S)使用它来正确更新sem.value

更新两个操作后，sem->binary_descriptor2仅当值为正时才释放信号量。

S.value永远不会是负数，因为任何执行down(S)的进程都在sem->binary_descriptor2.

  struct counting_semaphore* counting_semaphore_alloc(int value) {

    struct counting_semaphore* sem = malloc(sizeof(struct counting_semaphore));
    sem->value = value;
    sem->binary_descriptor1= bsem_alloc();//locks the value
    sem->binary_descriptor2= bsem_alloc();//locks the sing of the value
    if (value <= 0) {
        bsem_down(sem->binary_descriptor2);
    }
    return sem;
    }

void down(struct counting_semaphore *sem){
    bsem_down(sem->binary_descriptor2);
    bsem_down(sem->binary_descriptor1);
    sem->value--;
    if (sem->value>0){
        bsem_up(sem->binary_descriptor2);
    }
    bsem_up(sem->binary_descriptor1);
}


void up(struct counting_semaphore* sem) {
    bsem_down(sem->binary_descriptor1);
    sem->value++;
    if (sem->value == 1) {
        bsem_up(sem->binary_descriptor2);
    }
    bsem_up(sem->binary_descriptor1);
}
于 2017-05-06T12:50:12.220 回答
0

当sem->value达到 0 时，线程阻塞，是时候重新调度了。你没有显示你的调度代码，所以我不能给出具体的建议。调度程序可能应该bsem_up(sem->binary_descriptor);代表线程调用。

于 2017-05-05T17:13:04.103 回答