SYSTEM V Semaphores

真的是完全不知道内核的这一部分在说什么，首先阅读对应的用户态是一个不错的选择 One common use of a semaphore is to synchronize access to a block of shared memory, in order to prevent one process from accessing the shared memory at the same time as another process is updating it.

47.1 Overview

The general steps for using a System V semaphore are the following:

  1. semget
  2. semctl SETVAL SETALL
  3. semop
  4. semctl IPC_RMID

However, System V semaphores are rendered unusually complex by the fact that they are allocated in groups called semaphore sets.

好端端的 semaphore 为什么非要搞一个 set 出来，需要完成什么特殊功能　是　一个 semaphore 完成不了的 ?

The number of semaphores in a set is specified when the set is created using the semget() system call. While it is common to operate on a single semaphore at a time, the semop() system call allows us to atomically perform a group of operations on multiple semaphores in the same set.

为什么 semaphore 需要 id

接下来分析了一个例子 svsem/svsem_demo.c，简要的利用以上的三个函数。

47.2

The semget() system call creates a new semaphore set or obtains the identifier of an existing set.

47.3 Semaphore Control Operations

The semctl() system call performs a variety of control operations on a semaphore set or on an individual semaphore within a set

  1. Generic control operations
  2. Retrieving and initializing semaphore values
  3. Retrieving per-semaphore information
  4. Monitoring a semaphore set

47.4 Semaphore Associated Data Structure

Each semaphore set has an associated semid_ds data structure of the following form:

struct semid_ds {
  struct ipc_perm sem_perm; /* Ownership and permissions */
  time_t sem_otime; /* Time of last semop() */
  time_t sem_ctime; /* Time of last change */
  unsigned long sem_nsems; /* Number of semaphores in set */
};

47.5 Semaphore Initialization

由于创建和初始化不是原子操作，所以 semaphore 的初始化　需要被注意。

47.6 Semaphore Operations

The semop() system call performs one or more operations on the semaphores in the semaphore set identified by semid.

semop 实现　semaphore set 中间的 semaphore 的数值加减等操作

47.7 Handling of Multiple Blocked Semaphore Operations

描述如何处理　多个被阻塞的进程同时获取　semaphore 时的问题

47.8 Semaphore Undo Values

47.8 Semaphore Undo Values

此处以及后面都没有看

47.9 Implementing a Binary Semaphores Protocol

TO READ，综合训练，应该是很有意思的东西

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