System V IPC Identifiers and Keys

1
System V IPCIdentifiers and Keys

• IPC structures are identified by a key
• usually defined as long integer
• keys can be arbitrarily defined, system defined
  via a get call, or defined using the ftok()
  function
• arbitrary definition gives rise to conflicts
• programs must use the same key to access the same
  resource
• typically passed from parent to child or shared
  in a header in client/server suites

2
Permissions

• IPC structures have permissions, similar to files
• to change permissions, process must either be the
  creator of the structure or the superuser
• write permissions for semaphores are termed
  alter
• there are no permissions corresponding to
  execute

3
Pros and Cons of SV-IPC

• Pros
• flow controlled
• record oriented
• not first-in, first-out limited
• Cons
• have to be deliberately removed
• arent addressable with simple file I/O commands
  (open, read, write, etc.)
• are overly complex

4
Picking Keys

• keys may be any key_t type value
• keys may be chosen arbitrarily
• can result in conflicts
• a key can be generated by the system using the
  IPC_PRIVATE keyword
• has to somehow be made available to other
  processes wishing to use the resource
• a key can be generated using the ftok() function
• doesnt require shared headers or file reading

5
Message Queues

• linked list of messages stored in the kernel
• identified by a queue ID
• msgget() function creates a new queue or opens an
  existing one
• msgsnd() places a message on a queue
• msgrcv() reads a message from a queue
• msgctl() controls the properties of a queue

6
Semaphores

• used for resource allocation, not data transfer
• essentially, an advisory resource locking
  mechanism
• simple counter, which starts with a number of
  resources available, is decremented when a
  resource is used, and incremented when its
  returned
• semaphores are manipulated in sets of one or more

7
Semaphore Functions

• semget() creates or fetches the ID of a semaphore
  set
• semctl() controls the properties of a semaphore
  set
• uses the semun union
• semop() performs operations on semaphores, such
  as decrementing or incrementing them
• uses the sembuf structure

8
Shared Memory

• allows multiple processes to share regions of
  memory
• fastest possible form of IPC
• shmget() creates or fetches ID of existing shared
  memory segment
• shmctl() controls the properties of a shared
  memory segment
• shmat() attaches to a shared memory segment
• shmdt() detaches from a shared memory segment
• this does NOT remove it, call to shmctl()
  required for that

9
Shell Tools

• two shell tools are available for examining and
  removing IPC structures
• ipcs will list structures the user has read
  access to
• always check for structures you may have
  inadvertently left behind when testing
• ipcrm will remove structures the user has
  created or owns, or will remove any for the
  superuser