L10: Shared Memory

Question 1

Shared Memory

Overview

Answer 1

• Allows 2 or more processes to share a given region of memory
• Fastest form of IPC because there is no copying between a “client” and “server”
• The trick is synchronizing access
• Semaphores are used to synchronize shared memory access

Question 2

Shared Memory:

Important Functions (4)

Answer 2

• shmat()
  
  • how a process attaches to shared memory
• shmdt()
  
  • process detaches from shared memory
• shmctl()
  
  • attempt a command on the shared memory
• mmap()
  
  • memory mapped input/output
  • alternative to shared memory

Question 3

Shared Memory Functions:

shmat()

• Full declaration
• Summary

Answer 3

void * shmat( int shmid, void *addr, int flag);

Used by a process to attach to shared memory

Returns a pointer to the shared memory

Leaves memory when the process dies. Potential for memory leak!

Question 4

Shared Memory Functions:

shmctl()

• Full Declaration
• Summary

Answer 4

int shmctl( int shmid, int cmd,

struct shmid_ds *buf );

• Performs various operations on shared memory
• A command is specified with “cmd”
• Returns 0 if ok, -1 on error

Question 5

Shared Memory:

Commond commands for

shmctl()

Answer 5

• IPC_STAT
  
  • fills the “buf” data structure
• IPC_SET
  
  • change uid, gid and mode fo the shmid
• IPC_RMID
  
  • sets up the shared memory segment to be removed once all processes terminate or dettach from the segment

Question 6

Shared Memory Functions:

shmdt()

• Full Declaration
• Summary

Answer 6

void shmdt( void *addr)

• Used by a process to detach itself from a shared memory segment
• Sort of similar to using free() for heap memory

Question 7

Shared Memory Functions:

mmap()

Answer 7

• Memory Mapped Input/Output
• Alternative to shared memory
• Maps a file on DISK into buffer
• Contents are non-volatile

Question 8

Unix Commands

Dealing with

Shared Memory Segments

Answer 8

• ipcs
  
  • used for listing shared memory segments
• ipcrm
  
  • used to remove shared memory segments

Question 9

Shared Memory:

Steps for

Creating Shared Memory

Answer 9

1. Use shmget()
   
   • to obtain a segment of memory with an ID
2. Use shmat()
   
   • to get a pointer to the memory segment
3. Use shmctl()
   
   • to set up segment to be removed when processes are done with it
4. Use the pointer as a pointer to a memory block