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OS Tutorial

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S. H. [email protected]

Room No. 6120 K (NAB)
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

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BITS Pilani, Pilani Campus

Interprocess communication (IPC)

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Syntax

int dup (int oldfd)

dup() finds the smallest free file descriptorentry and points it to the same file as oldfd.

dup() system call

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Syntax

int dup2(int oldfd, int newfd )

An existing file descriptor oldfdis duplicated as file

descriptor newfd. If the file corresponding to descriptor newfd is open,

then it is closed.

In both cases, the original and copied file descriptorsshare the same file pointer and access mode.

dup2() system call

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They both return the index of the new filedescriptor if successful, and 1 otherwise.

dup/dup2 duplicates an existing file

descriptor, giving a new file descriptor that isopen to the same file or pipe.

The call fails if the argument is bad (not open)

or if 20 file descriptors are already open.

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dup/dup2 system call

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By this call, we have the close operation, andthe actual descriptor duplication, wrapped upin one system call.

It is guaranteed to be atomic, which essentiallymeans that it will never be interrupted by anarriving signal. The entire operation will

transpire before returning control to thekernel for signal dispatching.

dup2() system call

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With dup() system call, programmers had toperform a close() operation before calling it,i.e., resulted in two system calls, with a small

degree of vulnerability in the brief amount oftime which elapsed between them.

If a signal arrived during that brief instance,

the descriptor duplication would fail. Of course, dup2() solves this problem for us.

dup2() system call

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In computing, redirection is a functioncommon to most command-line interpreters,including the various Unix shells that can

redirect standard streams to user-specifiedlocations.

Redirection

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Output Redirection: The output from a commandnormally intended for standard output can be easilydiverted to a file instead. This capability is known asoutput redirection:

date > today

It writes its output to the file today instead of terminal.

No output appears at the terminal.

Because the output has been redirected from thedefault standard output device (the terminal) into thespecified file today.

Redirection

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Input Redirection: The input of a command beredirected from a file.

The commands that normally take their input from

standard input can have their input redirected from afile.

Redirection

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To count the number of lines in the file uu we canexecute : (It produces output 2 lines)

$wc-luu

2 uu

We can count the number of lines in the file byredirecting the standard input ofwc from the file uu:

$ w c - l < u u

2

Redirection

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In the first case, uu (file name) is listed with theline count; in the second case, it is not.

In the first case, wc knows that it is reading its

input from uu. In the second case, it only knows that it is reading

its input from standard input so it does not

display file name.

Redirection

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Sort < infile > outfile sort reads from infile and writes to outfile.

date >> today

Append the output at the end of file today.

Redirection

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The UNIX shell uses following information toimplement redirection.

When a process forks, the child inherits a copy of its

parents file descriptors. When process execs, the standard input, output, and

error channels remain unaffected.

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Implementation of Redirection

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To perform redirection, the shell performs thefollowing series of actions:

The parent shell forks and then waits for the child shell

to terminate. The child shell opens the file test.txt, creating it or

truncating as necessary.

The child shell then duplicates the file descriptor of

test.txt to the standard output file descriptor,number 1, and then closes the original descriptor oftest.txt.

All standard output is therefore redirected to test.txt.

Implementation of Redirection

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#include #include #include void main (argc, argv)int argc;

char *argv[] ;{int fd ; /* file descriptor or pointer */fd = open (argv[1], O_CREAT | O_TRUNC | O_RDWR, 0777) ;

/* open test.txt in argv[1] */dup2 (fd, 1) ; /* and assign it to fd */close (fd) ; /* duplicate fd with 1 which is standard output */execvp (argv[2], &argv[2]) ;

/* the output is not printed on screen but is redirected totest.txt file */

printf ("End\n") ; /* should never execute */}

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execvp()

#include #include main (int argc, char *argv[] ){execvp (ls", /* program to load - PATH searched */&argv[0] ) ;printf ("EXEC Failed\n") ;/* This above line will be printed only on error and not otherwise */

}

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To implement "ls|wc'' the shell will havecreated a pipe and then forked.

The parent will exec to be replaced by "ls'', and

the child will exec to be replaced by "wc.

The write end of the pipe may be descriptor 3and the read end may be descriptor 4.

"ls'' normally writes to descriptor 1 and "wc''normally reads from descriptor 0.

Example

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execlp()

#include #include void main ( ){

execlp ("ls", /* program to run - PATH Searched */"ls", /* name of program sent to argv[0] */"-l", /* first parameter (argv[1])*/"-a", /* second parameter (argv[2]) */NULL) ; /* terminate arg list */printf ("EXEC Failed\n") ;/* This above line will be printed only on error and not otherwise */

}

$ ls l a$ ls a Displays all files.$ls l Displays the long format listing.

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http://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdf

http://www.cs.cf.ac.uk/Dave/C/node23.html

http://www.tldp.org/LDP/lpg/node10.html

http://ocamlunix.forge.ocamlcore.org/pipes.html http://beej.us/guide/bgipc/output/html/multipage/pipes.ht

ml

http://cm.bell-labs.com/who/dmr/ipcpaper.html

http://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.html

http://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.html

References

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http://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://cboard.cprogramming.com/c-programming/112536-implement-redirection-simple-shell.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://academic.udayton.edu/SaverioPerugini/courses/cps346/lecture_notes/pipes.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://cm.bell-labs.com/who/dmr/ipcpaper.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://beej.us/guide/bgipc/output/html/multipage/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://ocamlunix.forge.ocamlcore.org/pipes.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.tldp.org/LDP/lpg/node10.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.cf.ac.uk/Dave/C/node23.htmlhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdfhttp://www.cs.princeton.edu/courses/archive/spr02/cs217/lectures/communication.pdf

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Interprocess communication (IPC)(shared memory)

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How the processes can communicate amongthemselves using the Shared Memory.

Creating a Shared Memory Segment.

Controlling a Shared Memory Segment. Attaching and Detaching a Shared Memory Segment.

Objectives

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Shared Memory is an efficient means of passing databetween programs.

One program will create a memory portion, which other

processes (if permitted) can access.

A shared segment can be attached multiple times by the

same process.

A shared memory segment is described by a control

structure with a unique ID that points to an area ofphysical memory.

Shared Memory

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Shared Memory

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A shared memory is an extra piece of memory that isattached to some address spaces for their owners to

use.

As a result, all of these processes share the same

memory segment and have access to it.

Shared Memory

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Shared memory is a feature supported by UNIX SystemV, including Linux, SunOS and Solaris.

One process must explicitly ask for an area, using a key,

to be shared by other processes. This process will be

called the server. All other processes, the clients that

know the shared area can access it.

Shared Memory

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However, there is no protection to a shared memoryand any process that knows it can access it freely.

To protect a shared memory from being accessed at the

same time by several processes, a synchronization

protocol must be setup.

Shared Memory

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The shared memory itself is described by a structure oftype shmid_ds in header file sys/shm.h.

To use this file, following files must be included:

Shared Memory

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For a server, it should be started before any client.

The server should perform the following tasks:

Ask for a shared memory with a memory key and memorize

the returned shared memory ID. This is performed by system

call shmget().

Attach this shared memory to the server's address space with

system call shmat().

Initialize the shared memory, if necessary.

Do something and wait for all clients' completion.

Detach the shared memory with system call shmdt().

Remove the shared memory with system call shmctl().

Shared Memory

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For the client part, the procedure is almost the same: Ask for a shared memory with the same memory key and

memorize the returned shared memory ID.

Attach this shared memory to the client's address space.

Use the memory.

Detach all shared memory segments, if necessary.

Exit.

Shared Memory

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This system call requests a shared memory segment.

It is defined as follows:

shm_id = shmget (

key_t k, /* the key for the segment */int size, /* the size of the segment */

int flag /* create/use flag */

); If it is successful, it returns a non-negative integer, the

shared memory ID; otherwise, the function value is

negative.

shmget()

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k is of type key_t or IPC_PRIVATE. It is the numeric keyto be assigned to the returned shared memory segment.

size is the size of the requested shared memory.

flag is used to specify the way that the shared memorywill be used.

IPC_CREAT | 0666 for a server (i.e., creating and granting

read and write access to the server).

0666 for any client (i.e., granting read and write access tothe client)

shmget()

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shmget()

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UNIX requires a key of type key_t defined in filesys/types.h for requesting shared memory segments.

There are three different ways of using keys, namely:

A specific integer value (e.g., 123456)

A key generated with function ftok()

A uniquely generated key using IPC_PRIVATE (a private key).

key

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The first way is the easiest one; however, its use may bevery risky since a process can access your resource as

long as it uses the same key value to request that

resource.

The following example assigns 1234 to a key:

key_t SomeKey;

SomeKey = 1234;

The ftok() function has the following prototype:

key

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ftok() takes a character string that identifies a path andan integer (usually a character) value, and generates an

integer of type key_t based on the first argument with

the value of id in the most significant position.

Example: if the generated integer is 35028A5D16 and

the value of id is 'a' (ASCII value = 61), then ftok()

returns 61028A5D16. That is, 61 replaces the first byte

of 35028A5D16,generating 61028A5D16.

key

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If a processes use the same arguments to call ftok(), thereturned key value will always be the same.

The most commonly used value for the first argument is

".", the current directory.

If all related processes are stored in the same directory,

the following call to ftok() will generate the same key

value.

key

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If a resource is requested with IPC_PRIVATE in a placewhere a key is required, that process will receive a

unique key for that resource.

Since that resource is identified with a unique key unknown to

the outsiders, other processes will not be able to share that

resource and, as a result, the requesting process is guaranteed

that it owns and accesses that resource exclusively.

key

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