Interprocess Communication in Operating System

What is a process?

• A process is in main memory, ready for execution(waiting for CPU).
• A process is not a program, but each program may have multiple processes, running concurrently or one after another. 
• Same program can be run as multiple processes concurrently(at the same time).

A process has

• A body of code to execute
• A reserved piece of memory
• It's own set of file descriptors
• A unique process id

How do processes communicate with each other?

1. Shared Memory (common areas in ram - main memory) - Shared memory allows processes to share parts of their virtual address space. Shared memory regions allow processes to communicate by reading and writing  to and from same region of memory. 
2. Signals (interupts like SIGINT, SIGKILL,..)
3. Message Queues (data + server process) - Messages allow processes to  send formatted data streams to arbitrary processes. Thus messages allow  processes to  cooperatively function by exchanging messages.
4. Semaphores (mainly for OS resource control)- Allow processes to synchronize execution using flags - This is used for things like OS telling if one has read access, write access. Semaphores allow two or more processes to coordinate access to shared resources and other behaviours.
5. Pipes(The '|' character from the command line)
6. FIFO (A named Pipe, made with mknod)
7. Sockets (point-to-point, 2 way communication)

Shared Memory

Shared memory is about two processes sharing a common segment of memory that they can both read to and write from to communicate with one another. Just a chunk of memory.

Because it’s just memory, shared memory is the fastest IPC mechanism of them all. 

/dev/shm is an implementation of shared memory.  It is an efficient means of passing data between programs. One program will create a memory portion, which other processes (if permitted) can access. This will result into speeding up things on Linux. Recent Linux distributions based on the 2.6 kernel have started to offer /dev/shm as shared memory in the form of a RAM disk, more specifically as a world-writable directory (a directory in which every user of the system can create files) that is stored in memory. Both the RedHat and Debian based distributions include it by default. Support for this type of RAM disk is completely optional within the kernel configuration file.

The main problem with shared memory is the race-condition. One very simple way of solving is semaphores.

Semaphores

They allow different processes to synchronize their access to certain resources.

In Computer Science, the most common and simplest kind of semaphore is called a binary semaphore because they have two states locked or unlocked. These act much like traffic lights.

When a process wants exclusive access to a resource, shared memory being an example, they attempt to lock the semaphore associated with that resource. If the semaphore they are attempting to lock is already locked, the caller is suspended, otherwise they are granted to lock. When you’ve finished doing whatever you wanted to do, you unlock the resource and any processes that have attempted to lock that semaphore in the meantime are woken up again to attempt the lock again. This way only one process can have access to the resource at once.

Pipes

• Pipes are one way stream to send data between two programs. Mainly, pipes can be defined as one way byte streams between related processes(parent/child interaction).
• The system assures that the order of output is same as the order of input(FIFO) and that no data is lost in communications.
• Programmed with the C function pipe() from <sys/unistd.h> or at the command line with "|"
• Modern implementations use sockets as the basis for pipes.

Note : stdin  is more like a pipe

FIFO

It is a Named Pipe which works on /dev devices. Using FIFO we can access /dev/urandom, cdrom device when mounted. Using mknod we can create named pipes.

Socket

Called BSD sockets, since the idea of sockets was developed by BSD.

• Allow point-to-point, two way communication between processes.
• They serve as endpoints in communication. The end points have IP addresess or filenames.
• All sockets have a type(selected when created) and options(chosen on fly - say like how long a socket should be open till they get timed out)
• They exist in "domains" which dictate how they are addressed and their communication protocols.
• There are 23 different socket domains. The most commonly used are the UNIX and INET domains.
• Created without names and must be bound to one.
• Communication between sockets is not symmetrical, but follows a client/server model.

There are 4 major types of BSD sockets, which dictates how data is sent and received and which protocols can be used to govern this exchange of information.

BSD socket types

full-duplex means data can be send back and forth at same time.

1. Stream - full duplex, sequential stream of data.
2. Datagram - full-duplex, non-sequential data transmission. Used for intermittent transmission.
3. Sequential Packet - full duplex, reliable, sequential connection for datagrams of fixed size.
4. Raw - An unregualted socket for constructing new  socket types and protocols.

Sockets will only connect with others of the same type and protocol.

Let us see about two important socket domains - Unix socket and Inet socket

Unix  sockets

1. used for local communication between processes
2. They are bound to pathnames, which do not already exist in the filesystem (/tmp directory)

How to find them?

netstat -an --unix

Proto RefCnt Flags       Type       State         I-Node Path
unix  2      [ ACC ]     STREAM     LISTENING     14270  /tmp/orbit-gdm/linc-8db-0-41bf49d5a8a9b
unix  2      [ ACC ]     STREAM     LISTENING     14041  @/tmp/gdm-greeter-NhVFwyLS
unix  2      [ ACC ]     STREAM     LISTENING     14342  /tmp/orbit-gdm/linc-8d4-0-28534edab128c
unix  2      [ ACC ]     STREAM     LISTENING     12082  /var/run/acpid.socket
unix  2      [ ACC ]     STREAM     LISTENING     7945   @/com/ubuntu/upstart
unix  2      [ ACC ]     STREAM     LISTENING     14477  /tmp/orbit-gdm/linc-8ee-0-1aab978fd6906
unix  2      [ ACC ]     STREAM     LISTENING     14481  /tmp/orbit-gdm/linc-8f0-0-52db2ebee2075
unix  2      [ ACC ]     STREAM     LISTENING     14485  /tmp/orbit-gdm/linc-8ea-0-6c2623f0e22da
unix  2      [ ACC ]     STREAM     LISTENING     10925  /var/run/rpcbind.sock
unix  2      [ ACC ]     STREAM     LISTENING     14602  /tmp/orbit-gdm/linc-8fa-0-4933106a2345e
unix  2      [ ACC ]     STREAM     LISTENING     13129  public/cleanup
unix  2      [ ACC ]     STREAM     LISTENING     14184  @/tmp/gdm-session-sYKCyvao
unix  2      [ ACC ]     STREAM     LISTENING     13969  @/tmp/.X11-unix/X0
unix  2      [ ACC ]     STREAM     LISTENING     14162  @/tmp/dbus-2sHX0FJO2L
unix  2      [ ACC ]     STREAM     LISTENING     11167  /var/run/dbus/system_bus_socket
unix  2      [ ACC ]     STREAM     LISTENING     11387  /var/run/avahi-daemon/socket
unix  2      [ ACC ]     STREAM     LISTENING     14803  /tmp/orbit-gdm/linc-8fb-0-6c6f4de337f7f
unix  2      [ ACC ]     STREAM     LISTENING     14831  /tmp/orbit-gdm/linc-8fc-0-4e60774e3fdc5
unix  2      [ ]         DGRAM                    8114   @/org/kernel/udev/udevd
unix  2      [ ACC ]     STREAM     LISTENING     15045  /var/lib/gdm/.pulse/2d622416adabbaa7f7ffa3c10000000f-runtime/native
unix  23     [ ]         DGRAM                    10630  /dev/log
unix  2      [ ]         DGRAM                    11130  /var/run/fcm/fcm_clif
unix  2      [ ]         DGRAM                    12269  @/org/freedesktop/hal/udev_event
unix  2      [ ACC ]     STREAM     LISTENING     11886  /var/run/cups/cups.sock
unix  2      [ ACC ]     STREAM     LISTENING     12644  /var/run/pcscd.comm
unix  2      [ ACC ]     STREAM     LISTENING     14205  @/tmp/.ICE-unix/2260
unix  2      [ ACC ]     STREAM     LISTENING     13136  private/tlsmgr
unix  2      [ ]         DGRAM                    11128  @0001b
unix  2      [ ACC ]     STREAM     LISTENING     13140  private/rewrite
unix  2      [ ACC ]     STREAM     LISTENING     13144  private/bounce
unix  2      [ ACC ]     STREAM     LISTENING     13148  private/defer
unix  2      [ ACC ]     STREAM     LISTENING     13152  private/trace
unix  2      [ ACC ]     STREAM     LISTENING     13156  private/verify
unix  2      [ ACC ]     STREAM     LISTENING     13160  public/flush
unix  2      [ ACC ]     STREAM     LISTENING     13164  private/proxymap
unix  2      [ ACC ]     STREAM     LISTENING     13168  private/proxywrite
unix  2      [ ACC ]     STREAM     LISTENING     13172


INET domain sockets

1. used for remote connection between processes
2. They are bound to an IP address(dotted quads) and a port number(65,356).

netstat -an --inet

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
tcp        0      0 127.0.0.1:199               0.0.0.0:*                   LISTEN
tcp        0      0 0.0.0.0:111                 0.0.0.0:*                   LISTEN
tcp        0      0 0.0.0.0:52053               0.0.0.0:*                   LISTEN
tcp        0      0 0.0.0.0:22                  0.0.0.0:*                   LISTEN
tcp        0      0 127.0.0.1:631               0.0.0.0:*                   LISTEN
tcp        0      0 127.0.0.1:25                0.0.0.0:*                   LISTEN
tcp        0     52 192.168.1.7:22              192.168.1.2:52471           ESTABLISHED
udp        0      0 0.0.0.0:5353                0.0.0.0:*
udp        0      0 0.0.0.0:111                 0.0.0.0:*
udp        0      0 0.0.0.0:631                 0.0.0.0:*
udp        0      0 0.0.0.0:768                 0.0.0.0:*
udp        0      0 0.0.0.0:901                 0.0.0.0:*
udp        0      0 0.0.0.0:51085               0.0.0.0:*
udp        0      0 0.0.0.0:43032               0.0.0.0:*
udp        0      0 0.0.0.0:161                 0.0.0.0:*
udp        0      0 0.0.0.0:68                  0.0.0.0:*

Socket Protocols

There are two major protocols, one for stream sockets(TCP) and the other for datagram sockets(UDP)