agenda terminal handling in unix file descriptors opening/assigning & closing sockets types of...

Agenda Terminal Handling in Unix

File Descriptors

Opening/Assigning & Closing

Sockets

Types of Sockets – Internal(Local) vs. Network(Internet) Programming for Local Sockets Programming for Network (Internet) Sockets Issues involving Server/Client models on different

computer architecture

Communication Among Devices One of the major features of the UNIX / Linux operating

system is that every element of the OS is interpreted as files:

Regular files Directory files Linked files Hardware (special character) files

Devices are files that are normally contained in the /dev directory.

Unix / Linux have mechanisms to work with files – thus work among terminals: pipes and redirection involvingfile descriptors.

Communication Among Devices File Descriptors

Each running program, called a process has file descriptors associated with it:

0 stdin 1 stdout 2 stderr

File descriptors 0, 1 & 2 are reserved by the OS, but other file descriptors from 3 onwards can be associated with processes by opening these descriptors and using them to redirect stdin, stdout and stderr.

View samples file_descriptor1, file_descriptor2, and mycp in ~msaul/pro511/sockets directory

Communication Among Devices

mycp script:#!/bin/bash

case $# in

0) exec 3<&0 4>&1 ;;

1) exec 3<$1 4>&1 ;;

2) exec 3<$1 4>$2 ;;

*) printf "\nUsage: mycp [source][destination]\n\n" >&2

exit 1;;

esac

cat <&3 >&4

exec 3<&- 4<&-

exit 0

Set up file descriptors (fd)

redirect stdin to fd3, fd4 to stdout

redirect stdin of arg1 to fd3, fd4 to stdout

redirect stdin of arg1 to fd3, fd4 to arg2

redirect stdin from fd3 and redirect stdout to fd4

Close file descriptors 3 & 4


Examples:

mycp <- redirect stdin from terminal to terminal

mycp file1 <- redirect stdin from terminal to “file1” (copy console)

mycp file1 file2 <- redirect stdin from file1 and redirect stdout to file2

mycp /dev/pts/1 /dev/pts/0 (try to send msg to user)

Q: will this work?A: Depends on owner of terminals logged in.


A neat little “work-around”

owner of /dev/pts/0 issues the commands:

ln –s /dev/pts/0 /tmp/back_door <- symlink to terminal

chmod 777 /tmp/back_door <- allow permissions

owner of /dev/pts/1 issues the command(s):

mycp /dev/pts/1 /tmp/back_door

or

Mycp /dev/pts/1 /dev/pts/0Solution works, but is “clumsy” and prone to security issues…In this case it is better to use sockets…

Sockets The Berkeley versions of UNIX (BSD) created the

socket interface for UNIX.

The socket interface was considered to be an extension of the “pipe” concept already used.

The socket communication mechanism allows client/server programs to work locally on a single machine (Internal or “Local” sockets) or across networks or the Internet (Network sockets).

Local Sockets Local sockets uses a named “file” stored on the server

(eg. in the /tmp directory) to allow communication between server and client processes.

Advantages: Simple Useful for inter-process communication for server & client

within the same system.

Disadvantages: Cannot provide inter-process communication if server &

client on different servers (machines) Not practical or possible for server handling multiple

clients

Local Sockets Server Program – Local Sockets:

Remove old named sockets (eg. unlink (“/tmp/unx511a01”);)

Create a socket using socket system call to assign resource to sever “process” (type of socket & Internet Protocol specified)

Name a socket using bind system call (associates the server’s file descriptor with the address of the un-named socket, upon successful operation, bind assigns the socket a name)eg. for local sockets, a pathname – eg. “/tmp/unx511a01”

Wait for clients to connect using listen system call (creates a queue for incoming connections)

TIP: use you own sigma account name for naming sockets!!

Sockets Server Program – Local Sockets:

Accept connection from client using accept system call (creates a separate socket for process communication between server & client, hangs up original socket to accept connect to client & re-enters listen mode)

During connection use read, write, ioctl & close system call(s) to communicate among server & client

Never use the open system call when writing C programs involving sockets (sockets use other special system calls)

Local Sockets Client Program – Local Sockets:

Create a socket using socket system call to assign resource to sever “process” (type of socket & Internet Protocol specified)

Connect to server using connect system call using the server’s named socket

Once connected, local sockets are used like low-level file descriptors, redirecting data between server’s process and client’s process. During connection use read, write, ioctl & close system call(s) to communicate among server & client

Local SocketsTIP: try to setup a consistent “back & forth” communication involving read and write commands to prevent lockups(eg. both reading or writing at same time!)

ClientServer

read

read

read

read

write

write

write

write

Tip:

Setup logic within a loop to have server and client do specific actions based on messages sent to each other

Useful Techniques / Commands:

Send message as first character in string (strcpy, strcmp, sprintf, sscanf)

Refer to examples in Sigma:~msaul/unx511/sockets

Sockets The server/client method of using sockets is similar to a

company switchboard (server) accepting incoming calls from customers (clients)

Customer A(client A)

Company Server

switchboard

CompanyDepartment 1

CompanyDepartment 2

Switchboard waits (listen)for a call from a client




Company Server

switchboard

CompanyDepartment 1

Incoming call

CompanyDepartment 2

Switchboard waits (listen)for a call from a client




Company Server

switchboard

CompanyDepartment 1

Connectionestablished

Switchboard answers andaccepts call.

CompanyDepartment 2




Company Server

switchboard

CompanyDepartment 1


Switchboard determines departmentrequested & creates direct line to department

CompanyDepartment 2

Direct line (connection)




Company Server

switchboard

CompanyDepartment 1

Switchboard disconnects initialconnection & listens for incoming calls

CompanyDepartment 2





Company Server

switchboard

CompanyDepartment 1

Switchboard listens for incoming calls

CompanyDepartment 2


Customer B(client B)

Incoming call




Company Server

switchboard

CompanyDepartment 1

CompanyDepartment 2




Switchboard answers andaccepts call.




Company Server

switchboard

CompanyDepartment 1

CompanyDepartment 2





Switchboard determines departmentrequested & creates direct line to department




Company Server

switchboard

CompanyDepartment 1

CompanyDepartment 2




Switchboard disconnects initialconnection & listens for incoming calls

Sockets LOCAL SOCKETS - Used to create server/client applications

when run local to a specific machine (i.e. not across different servers or across the Internet). Refer to ~msaul/pro511/sockets in Sigma for examples:

local_server1.c / local_client1.c Sends character ‘a’ from client to server to client.

local_server2.c / local_client2.c Prompts user for characters to send in a loop.

local_server3.c / local_client3.c Allows user within client to enter “q” to shutdown server, but

will try to read data from shutdown server causing the dreaded “broken pipe” stderr message.

Local_server4.c / local_client4.c Allows user within client program to enter “q” to shutdown

server, without the “broken pipe” stderr message.

Sockets Network Sockets

Network sockets are specified by the “domain” parameter AF_INET as opposed to AF_UNIX for local sockets.Network sockets specify a static IP address and a port number (as opposed to just a filename for local sockets)

Various port numbers are used to perform networking services in UNIX. Port numbers less than 1024 are reserved for services such as telnet, ftp, http, etc. port numbers >= 1024 can be used for own programs. (refer to /etc/services for predefined port services)

For network sockets, we need to possibly convert the different byte ordering sequence of port and address numbers for different computers. These functions are: htonl() – host to network long, and htons – host to network short. Refer to examples discussed in next slide.

Sockets Network Sockets

network_server1.c / network_client1.c Same is local_server4.c/local_client4.c, but using network socket

over port 7400 and loop-back address 127.0.0.1 (loop-back port on same machine) instead of a local socket file. This is good for trouble-shooting in case of network problems.

network_server2.c / network _client2.c Same as network_server1.c / network_client1.c, but client can

connect from any server (in this case to Sigma – 142.204.57.76)

network _server3.c / network _client3.c Sending strings between client and server on different machines

via the Internet network _server3.c / network _client3.c

Converting an integer as string (and vise versa) on different machines via the Internet

Sockets Sockets Issues

A simple server cannot accept more than 1 client at a time (i.e. until the previous client disconnects).

Solutions: use fork system call to create a new process for each client (not good for databases)

use select system call to read from any of several file descriptors (useful to update databases and read from keyboard as well…)

Discussed in next class/lab

Sockets Sockets Issues

Parallel requests which access the same data can cause conflicts. For example, client A is writing the same database record as client B is reading.

Solutions: design server program to prevent this type of conflict from occurring.

use semaphores to allow only one process at a time to execute critical sections of code.

Covered later in course

agenda terminal handling in unix file descriptors opening/assigning & closing sockets types of...

Documents

c network

sockets sockets

internet network

case of network problems

local sockets programming

devices file descriptors

stderr file descriptors

internet slide