Semaphore(信號量) Back

• 信號量主要提供對進程間共享資源訪問的控制機制.
• 可用於訪問控制和進程間同步
• 類型:
  • 二值信號量: Boolean類型
  • 計數信號量: 非負值

0. Header Files

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

1. Create

• 调用成功時, 初始化相應的控制塊信息

method

int semget(key_t key, int nsems, int flags)

parameters

• key: 鍵值, 由ftok()獲取
  • IPC_PRIVATE: key由系統分配
• nsems: 信號量數
• flags: 操作標誌, 決定了信號量組的創建方式和權限

return value

• 0: 信號量組標識符

• -1: failure

2. Operation

method

int semop(int sid, struct sembuf** ops, unsigned nops)

/* sembuf */
struct sembuf
{
    short sem_num;  /* 信号量编号，從0開始 */
    short sem_op;       /* 要執行的操作 */
    short sem_flg;      /* 操作標誌 */
};

parameters

• sid: semget()返回的信號量組標識符
• ops: 操作的數據結構
• nops: 操作的個數

return value

• 0: success
• -1: failure

3. Control

method

int semctl(int sid, int snum, int cmd, union semun arg)

/* semun */
union semun
{
    int val;
    struct semid_ds* buf;    /* 指針信號量組控制塊的指針 */
    ushort* array;
};

parameters

• sid: semget()返回的信號量組標識符
• snum: 信號燈編號
• cmd: 命令

return value

• 0: success(與cmd相關)

• -1: failure

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