unix inter process communication

8/2/2019 Unix Inter Process Communication

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Unix

Inter Process Communication


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Types of IPC Pipes

Named Pipes

Messages

Shared Memory

Semaphores

Sockets


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Pipes Allow data transfer in a FIFO manner

Allow synchronization of process execution

Use file system for data storage

Can be created using pipe() system call

Pipe manipulation is done using regular

system calls for files e.g. read(), write() Only related processes can access a pipe


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Pipes

Syntax

pipe (file_descriptors)

where file_descriptors is a pointer to an array of 2 integers

which are the file descriptors for reading and writing the

pipe


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Named Pipes Similar to pipes

Have directory entries

open system call used to open named pipes

mknod system call to create named pipes

Unrelated processes can also communicate

using named pipes


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MESSAGES Allow processes to send data streams to

arbitrary processes

Four system calls cover all the operations msgget

msgsend

msgrec

msgctl


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msgget

Returns the message queue identifier assocaited with the key

msgget(key, flag)

key is the name of the message queue

flag is the modes which can be passed


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msgsnd

msgsnd (id, message, size, flag)

Sends message to the queue associated with the message queue identifier specified

by id

message is of the form

struct msgbuf

{

long mtype;

char mtext[];

}

Flags can be

IPC_NOWAIT- Return -1 if the message cant be stored

MSG_NOERROR- Message bigger than size is shortened with no error returned


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msgrcvmsgsnd (id, message, size, type, flag)

Reads message from the queue associated with the message

queue identifier specified by id

message is of the form

struct msgbuf

{

long mtype;

char mtext[];

}


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msgctlMsgctl ( id, command, buffer )

Provides a variety of message control functions as specified bycommand

Commands can be

IPC_STAT store information about queue in the buffer

IPC_SET Change the information in the buffer

IPC_RMID remove the message queue identifier specified

by id


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SHARED MEMORY Allows processes to share part of their virtual

address space

System calls similar to the ones in messages


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shmgetint shmget ( key, size, flag )

key is the name of the shared memorysize is the size of shared memory segment in bytes

flag is a combination of

IPC_CREAT

IPC_EXCL

mode


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shmatInserts a shared memory segment into a process address space

char *shmat ( id, at_address, flag )

id is the shared memory segment identifier

size is the address in process address space where the shared

memory segment has to be attached


SHM_RDONLY read only

0 read,write


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shmdtRemoves a shared memory segment into a process address space

int shmdt ( dt_address )

dt_address is the address in the process address space of a

shared memory segment to be detached


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shmctlshmctl ( id, cmd, buf)

Provides shared memory control operations as specified by thecmd

cmd IPC_STAT

IPC_SET

IPC_RMID

SHM_LOCK

SHM_UNLOCK


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SEMAPHORE

Non-negative integer count used tocoordinate access to the resources

Initial semaphore count set to the number offree resources

Count decremented or incremented by thethreads or processes as and when theyacquire or free resources.

Threads block at count zero till it becomespositive


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semget

semget ( key, num, flag )

Returns or creates the semaphore identifier associated with the

keykey is the name of the semaphore set

number defines the number of semaphores in the set


IPC_CREAT

IPC_EXCL


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semop

semop ( id, operations, number )

Performs operations on the semaphore

id is the semaphore identifier

operations is the array of semaphore operation structure

operation consists of

- semaphore number

-operation

-flags

number is the number of entries in operations


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semctl

semctl ( id, semnum, cmd, ... )

Performs operations on the semaphore as specified by the cmd

id is the semaphore identifier

Fourth argument is optional, depending upon the operation

requested.

If required, then its a buffer.

cmd GETVAL, SETVAL, GETPID, GETNCNT, GETZCNT


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SOCKET

Creates an endpoint for communication and returns a descriptor

int socket ( domain, type, protocol )

domain specifies communication domain

type type of communication over socket (virtual circuit or

datagram)

protocol protocol to control the communication


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Other Socket System Calls


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Close

Closes the communication endpoint

close ( sd )

sd is the socket descriptor


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Bind

Assigns a name to an unnamed socket

int bind ( sd, sockname, namelength)


sockname is the name of the socket

namelength is the length of the name


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Connect

Makes a connection to an existing socket

int connect ( sd, sockaddr, namelength)


sockaddr is the name of the target socket


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Accept

Receives incoming requests for a connection

accept ( fd, addr, addrlen )

addr user data array which kernel fills with the address of the

connecting client


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send

Transmits data over a connected socket

send ( sd, msg, length, flag)

msg pointer to data being sent

length length of the data


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recv

Receives data over a connected socket

recv ( sd, buf, length, flag)

bufpointer to data being sent

length length of the data


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shutdown

Closes a socket connection

shutdown ( sd, mode )

mode specifies which side no longer permits data transmission

unix inter process communication

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