IPC 1

1
Chapter 6. Interprocess Communications

• Introduction
• Universal IPC facilities
• System V IPC
• Mach IPC

2
Introduction

• Purposes of IPC
• data transfer
• sharing data
• event notification
• resource sharing
• process control

3
Universal IPC Facilities

• Signals
• expensive, limited bandwidth
• Pipes
• unidirectional, first-in first-out, unstructured
  data stream
• simple flow control mechanism
• pipe( ) system call
• return two file descriptor, one for reading an
  one for writing

4
Universal IPC Facilities (cont)

• I/O is performed through read and write system
  calls
• implementation of pipe
• SVR2 inode
• BSD socket
• SVR4 STREAM (bi-directional pipe)
• Process tracing ptrace( )
• used by debuggers such as dbx
• control the execution of a child process

5
System V IPC

• Semaphores, Message queues, Shared memory
• Common elements
• attributes
• key, creator, owner, permission
• acquires a resource
• shmget( ), semget( ), msgget( )
• IPC_CREAT, IPC_EXCL flags
• control system call
• shmctl( ), semctl( ), msgctl( )
• IPC_STAT, IPC_SET, IPC_RMID commands

6
Semaphores

• Creates and obtains an array of semaphores
• semid semget(key, count, flag)
• Operations on individual semaphore
• status semop(semid, sops, nsops)
• sops is a pointer to an nsops-element array of
  sembuf structures
• struct sembuf unsigned short sem-num short se
  m_op short sem_flag

7
Semaphores (cont)

• sem_op gt 0
• add sem_op to the semaphores current value. This
  may result in waking up processes
• sem_op 0
• block until the semaphores value becomes zero
• sem_op lt 0
• block until the semaphores value becomes greater
  than or equal to the absolute value of sem_op,
  then subtract sem_op from that value
• Race condition and deadlock

8
Message Queues

• Creates or obtains a message queue
• msgqid msgget(key, flag)
• Places messages on the queue
• msgsnd(msgqid, msgp, count, flag)
• Removes messages from the queue
• count msgrcv(msgqid, msgp, maxcnt, msgtype,
  flag)

9
Message Queues (cont)

• Using a message queue

receivers
senders
struct msgqid_ds
P
P
msg
new messages added at tail
messages read from head
msg
msg
P
P
10
Message Queues (cont)

• Discussion
• message type can be used to associate priorities
  with messages
• effective for transferring small amounts of data
• expensive for large transfers
• each message transfer involves two data copy
  operations

11
Shared Memory

• A portion of physical memory shard by multiple
  processes
• Creates and obtains a shared memory
• shmid shmget(key, size, flag)
• Attaches the region to a virtual address
• addr shmat(shmid, shmaddr, shmflag)
• Implementation depends heavily on the virtual
  memory architecture
• Does not provide synchronization
• Modern UNIX (SVR4) provides mmap( )
• maps a file into the address space of callers

12
Shared Memory (cont)
memory map of process B
memory map of process A
0
0
0x30000
0x50000
0x50000
0x70000
Attaching a shared memory region
13
Mach IPC

• Based on message passing
• Supports secure communications
• Provides communications btw user tasks, also btw
  user and kernel
• extensible to a distributed environment
• Basic concepts
• messages
• a collection of typed data
• port
• a protected queue of messages
• send rights, receiver rights

14
Mach IPC (cont)

• task_self port, thread_self port
• task_notify port, reply port
• exception port

senders
port
T
receiver
msg
msg
T
T
T
Communicating via a Mach port
15
Messages

• Three basic types of data
• ordinary data
• not interpreted by kernel and is passed on to the
  receiver by physically copying it
• out-of-line memory
• used to transfer large chunks of data using
  copy-on-write techniques
• send or receive rights to ports

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Mach Message Format
type
size
Message header
Local port
Destination port
Message id
name
size
Type descriptor
number
flags
data
Type descriptor
name
size
number
flags
data
. . .
17
Ports

• Port name space
• simple integer local to each task
• Port data structure
• kern_port_t data structure for each port
• Port translation
• translation entry lttask, port, local_name, typegt
• TP_chain hash chain based on lttask, portgt
• TL_chain hash chain based on lttask,
  local_namegt
• task_chain list of all translations owned by the
  same task
• obj_chain list of all translations for this port

18
Port Translation
struct kernel_port_t
struct task
Task
Port
a
b
c
d
f
g
TP hash chain
e
TL hash chain
Translation entries (port_hash_t)
19
Message Passing
sender
receiver
kernel
Task
Task
outgoing message
kern_msg_t (internal message)
received message
copy
copy
in-line data
copy of data
copy of data
pointers to data copied into address space
pointers to out-of-line memory
holding map
copy mappings
copy mappings
port right (local name)
pointer to port object
port right (local name)
translate
translate
20
Out-of-Line Memory

• Copy-on-write
• a page must be copied only if and when one of the
  two tasks attempts to modify it
• Message copied to holding map

address map
sender
holding map
T
COW
COW
COW
COW
COW copy-on-write
pages in memory
21
Out-of-Line Memory (cont)

• Message copied to receiver task

address map
address map
sender
receiver
T
T
COW
COW
COW
COW
pages in memory
22
Out-of-Line Memory (cont)

• Receiver modifies a page

address map
address map
sender
receiver
T
T
COW
COW
COW
pages in memory
23
Extensibility of Mach IPC

• User-level program called netmsgserver extends
  Mack IPC across a network

node A
node B
RP
SP
client
server
reply port
reply port
netmsgserver
netmsgserver
proxy for SP
proxy for RP
SP
RP