System V Shared Memory

Since a shared memory segment becomes part of a process’s user-space memory, no kernel intervention is required for IPC

48.1 Overview

In order to use a shared memory segment, we typically perform the following steps:

  1. Call shmget() to create a new shared memory segment or obtain the identifier of an existing segment (i.e., one created by another process). This call returns a shared memory identifier for use in later calls.
  2. Use shmat() to attach the shared memory segment; that is, make the segment part of the virtual memory of the calling process.
  3. At this point, the shared memory segment can be treated just like any other memory available to the program. In order to refer to the shared memory, the program uses the addr value returned by the shmat() call, which is a pointer to the start of the shared memory segment in the process’s virtual address space.
  4. Call shmdt() to detach the shared memory segment. After this call, the process can no longer refer to the shared memory. This step is optional, and happens automatically on process termination.

  5. Call shmctl() to delete the shared memory segment. The segment will be destroyed only after all currently attached processes have detached it. Only one process needs to perform this step

  6. http://man7.org/linux/man-pages/man2/shmget.2.html

48.2 Creating or Opening a Shared Memory Segment

shmget 和 semget 的参数列表很类似

48.3 Using Shared Memory

The shmat() system call attaches the shared memory segment identified by shmid to the calling process’s virtual address space.

48.4 Example: Transferring Data via Shared Memory

使用 reader writer 作为一个例子。 使用方法: 打开两个终端 运行 writer 运行 reader 在 writer 中输出 reader 中可以回显 but the shmem key is hardcoded !

48.5 Location of Shared Memory in Virtual Memory

In Section 6.3, we considered the layout of the various parts of a process in virtual memory. It is useful to revisit this topic in the context of attaching System V shared memory segments.

/*
 * This decides where the kernel will search for a free chunk of vm
 * space during mmap's.
 */
#define TASK_UNMAPPED_BASE	(PAGE_ALIGN(TASK_SIZE / 3))

64 位 内核上的定义

48.6 Storing Pointers in Shared Memory

skip

48.7 Storing Pointers in Shared Memory

The shmctl() system call performs a range of control operations on the shared memory segment identified by shmid.

A shared memory segment can be locked into RAM, so that it is never swapped out.

48.8 Shared Memory Associated Data Structure

内核中间 对于 shmid_ds 的定义　比如下的用户态的定义要麻烦的多。

struct shmid_ds {
  struct ipc_perm shm_perm; /* Ownership and permissions */
  size_t shm_segsz; /* Size of segment in bytes */
  time_t shm_atime; /* Time of last shmat() */
  time_t shm_dtime; /* Time of last shmdt() */
  time_t shm_ctime; /* Time of last change */
  pid_t shm_cpid; /* PID of creator */
  pid_t shm_lpid; /* PID of last shmat() / shmdt() */
  shmatt_t shm_nattch; /* Number of currently attached processes */
};

48.9 Shared Memory Limits

Most UNIX implementations impose various limits on System V shared memory. Below is a list of the Linux shared memory limits. The system call affected by the limit and the error that results if the limit is reached are noted in p

48.10 Summary

The recommended approach when attaching a shared memory segment is to allow the kernel to choose the address at which the segment is attached in the process’s virtual address space. This means that the segment may reside at different virtual addresses in different processes. For this reason, any references to addresses within the segment should be maintained as relative offsets, rather than as absolute pointers.

不同进程中间　shared memory 的起始位置不同，采用相对偏移。

问题

  1. 如果 shared memory 的 id 被泄露出去，那么岂不是通信内容也就被泄露了，还是其中都是加密的
  2. binder 对于 shared memory 的改进是什么

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