99 lines
4.4 KiB
Markdown
99 lines
4.4 KiB
Markdown
#### 概述
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SYS V信号量。
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SYS V信号量是System V进程间通信的一部分。System V进程间通信的方式包括：消息队列、信号量和共享内存。
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`semid_ds`描述了一组信号量，它定义在`bits/sem.h`：
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```c
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/* Data structure describing a set of semaphores. */
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struct semid_ds {
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struct ipc_perm sem_perm; /* Ownership and permissions */
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time_t sem_otime; /* Last semop time */
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time_t sem_ctime; /* Last change time */
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unsigned long sem_nsems; /* No. of semaphores in set */
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};
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struct ipc_perm {
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key_t __key; /* Key supplied to semget(2) */
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uid_t uid; /* Effective UID of owner */
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gid_t gid; /* Effective GID of owner */
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uid_t cuid; /* Effective UID of creator */
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gid_t cgid; /* Effective GID of creator */
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unsigned short mode; /* Permissions */
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unsigned short __seq; /* Sequence number */
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};
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```
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#### 相关函数
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```c
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/* System V信号量集合里的每个信号量都有如下相关变量(由内核维护，仅内核可见) */
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unsigned short semval; /* semaphore value */
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unsigned short semzcnt; /* # waiting for zero */
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unsigned short semncnt; /* # waiting for increase */
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pid_t sempid; /* PID of process that last */
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/* sembuf定义了对单个信号量的操作 */
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struct sembuf{
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unsigned short int sem_num; /* 要操作的信号量在信号量集合中的编号 */
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short int sem_op; /* 信号量操作，正数是V操作，负数是P操作 */
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short int sem_flg; /* 操作标志符 */
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// 可接受的sem_flg:IPC_NOWAIT and SEM_UNDO
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// SEM_UNDO - 由进程自动释放信号量
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// IPC_NOWAIT - 不阻塞
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};
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/* semctl()第四个参数的数据类型 */
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union semun {
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int val; /* Value for SETVAL */
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struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
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unsigned short *array; /* Array for GETALL, SETALL */
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struct seminfo *__buf; /* Buffer for IPC_INFO(Linux-specific) */
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};
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/* 功能：System V信号量集合的控制操作。
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* semid：要控制的信号量所在的信号量集合。
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* semnum：要控制的信号量在信号量集合中的序号。(从0开始编号。)
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* cmd：对信号量的控制操作。
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* IPC_RMID - 0, 删除标识符
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* IPC_SET - 1, 设置ipc_perm选项
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* IPC_STAT - 2, 获取ipc_perm选项
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* IPC_INFO - 3, See ipcs
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* GETPID - 11, get sempid
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* GETVAL - 12, get semval
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* GETALL - 13, get all semval's
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* GETNCNT - 14, get semncnt
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* GETZCNT - 15, get semzcnt
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* SETVAL - 16, set semval
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* SETALL - 17, set all semval's
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* 第四个参数的类型为union semun，此参数是否存在取决于第三个参数cmd。
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*/
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int semctl(int semid, int semnum, int cmd, ...);
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/* 功能：获取System V信号量集合的描述符。semop()和semctl()会使用它。
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* key：返回的信号量集合描述符是与本参数相关联的，一个key只对应一个固定的描述符。
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* IPC_PRIVATE：私有的key。
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* nsems：信号量集合里的元素数。
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* semflg：
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* IPC_CREATE：如果key不存在则创建之。(01000)
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* IPC_EXCL：如果key存在则报错。(02000)
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* IPC_NOWAIT：Return error on wait.(04000)
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* 返回值：成功则返回信号量集合的描述符(非负整数)；失败则返回-1。
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*/
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int semget(key_t key, int nsems, int semflg);
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/* 功能：System V信号量操作。即P/V操作。
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* semid：信号量集合的描述符。执行semctl()函数可得。
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* sops：是一个数组指针，这个数组的元素是结构体sembuf。
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* nsops：数组spos里的元素要操作的元素的数量。
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*/
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int semop(int semid, struct sembuf *sops, size_t nsops);
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```
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#### 参考文档
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- [linengier - Linux系统编程之信号量](https://www.cnblogs.com/linengier/p/9399880.html#:~:text=%E5%9C%A8%E5%AD%A6%E4%B9%A0%E4%BF%A1%E5%8F%B7%E9%87%8F%E4%B9%8B%E5%89%8D%EF%BC%8C%E6%88%91%E4%BB%AC%E5%BF%85%E9%A1%BB%E5%85%88%E7%9F%A5%E9%81%93%E2%80%94%E2%80%94Linux%E6%8F%90%E4%BE%9B%E4%B8%A4%E7%A7%8D%E4%BF%A1%E5%8F%B7%E9%87%8F%EF%BC%9A%20%EF%BC%881%EF%BC%89,%E5%86%85%E6%A0%B8%E4%BF%A1%E5%8F%B7%E9%87%8F%EF%BC%8C%E7%94%B1%E5%86%85%E6%A0%B8%E6%8E%A7%E5%88%B6%E8%B7%AF%E5%BE%84%E4%BD%BF%E7%94%A8%20%EF%BC%882%EF%BC%89%20%E7%94%A8%E6%88%B7%E6%80%81%E8%BF%9B%E7%A8%8B%E4%BD%BF%E7%94%A8%E7%9A%84%E4%BF%A1%E5%8F%B7%E9%87%8F%EF%BC%8C%E8%BF%99%E7%A7%8D%E4%BF%A1%E5%8F%B7%E9%87%8F%E5%8F%88%E5%88%86%E4%B8%BAPOSIX%E4%BF%A1%E5%8F%B7%E9%87%8F%E5%92%8CSYSTEMV%E4%BF%A1%E5%8F%B7%E9%87%8F%E3%80%82) |