Advanced UNIX IPC Facilities

1
Advanced UNIX IPC Facilities

• After Haviland, et al.s book

2
Introduction and Basic Concepts

• UNIX offers a variety of advanced IPC mechanisms
• This makes UNIX an extremely rich system in the
  IPC area
• Allows developer to use a variety of approaches
  when programming a system made up of cooperating
  tasks

3
Categories of advanced IPC

• Message passing
• Semaphores
• Shared memory

4
IPC facility keys

• UNIX has made the 3 categories of IPC as similar
  as possible for programming convenience
• The most (?) important similarity is the IPC key
• Keys are numbers used to identify an IPC object
  on a UNIX system
• Much like a file name being used to identify
  files
• Key allows IPC object to be shared by several
  processes

5
Difference between keys and file names

• Keys are not file names and carry less meaning
• Actual key type is determined by implementation,
  and defined by key_t in ltsys/types.hgt
• UNIX uses a simple library funciton that maps a
  files pathname to a key
• Routine is called ftok( )

6
IPC get operation

• With the get operation, a program uses a key to
• Create an IPC, or,
• Gain access to an existing one

Mqid msgget((key_t)0100, 0644IPC_CREATIPC_EXCL
)
For semaphores semget For shared mem shmget
Msg queue key
Non-neg id returned if successful
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Other IPC Operations

• Control operations
• Get status information
• Set control values
• Actual calls are msgctl, semctl, shmctl
• More specific operations to perform various
  functions
• Do interesting work
• Called IPC operations
• For example, for message queues we have
• Msgsend
• msgrcv

8
Status data structures

• When an IPC object is created, the system also
  creates an IPC facility status structure
• This contains any administrative info assoc with
  object
• There is one type of status structure for
  messages, semaphores and shared mem
• However all 3 contain a common permission
  structure, ipc_perm

9
Message passing

• A message is basically a sequence of characters
  of bytes, not necessarily null-terminated
• One process creates a message queue using msgget
• One a queue is established, a process with the
  right permissions can put messages into it with
  msgsnd
• Another process can then read this message with
  msgrcv

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Form of msgget
include ltsys/msg.hgt int msgget(key_t key, int
permflags)
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Permflags

• Determines the exact action perform by msgget
• Two constants are relevant here, defined in
  ltsys/ipc.hgt
• Can be used alone or ORed together
• IPC_CREAT
• Tells msgget to create a message queue for the
  value key if one doesn't exist
• Message queue will not be overwritten if it
  already exists
• If not set, then msgget returns id of existing
  msgqueue, if exists
• IPC_EXCL
• If this and IPC_CREAT are both set, then only
  intend to create new msgqueue
• Return -1 if already exists