Operating Systems

Recitation 9, May 19-20, 2002

Iterative server

• Handle one connection request at a time.

• Connection requests stored in queue associated with socket (avoid loosing them)

serverserver applicationthread

operating system socket forconnection requests

Q

socket forindividual

connection

Concurrent server

• Handle multiple connection requests at a time.

masterserver applicationthread

operating system socket forconnection requests

Q

sockets forindividual

connections

slave slaveslave

Using socket calls

client sidesocket

connectsendrecv

closesocket

server side

socket

bind

listen

accept

recv

send

closesocket

bind

• Specifies the local address for a socket; well-known port at which server will wait for connections.

int bind(int socket,

struct sockaddr* local_addr, int local_addr_len);

listen

• OS creates queue of connection requests: hold new connection requests that arrive while server is busy handling a previous request; separate queue (initially empty) for each socket.

• Inserts connection requests arriving from clients.

• When server asks to retrieve an incoming connection request from socket, the OS returns the next request from queue.

• If queue is full when connection request arrives then it’s rejected.

listen

• Server calls listen to place a socket in passive mode and make it ready to accept incoming connections. Tells the OS to enqueue connection requests for a socket

• Input:– socket descriptor– size of the queue

int listen(int socket, int queue_size);

accept

• Extracts the next incoming connection request.

• If connection request queue is not empty, accept returns immediately, otherwise system blocks the server until a client forms a connection.

• Before accept, a request ties up the socket that is bound to well-known port. Accept re-routes it to a new socket allowing original socket to receive additional clients.

accept

• All the different sockets created by accept share the same port on the server side.

client A sd=3

IP

client B sd=3

IP

host A

host B

port 1234

port 5678

serverprocess

sd=4

IP

well-known port

sd=5

sd=3

src=1234@A

src=5678@B

src=other

accept• Input:

– descriptor of a socket (that server has created and bound to a specific port with wildcard foreign destination) from which a connection should be accepted.

• Output (when connection request arrives):– address structure of client that formed the connection

and its length.– descriptor of new socket.

• Server uses each new socket to transfer data for each new connection, and original socket to accept additional connection requests.

int accept(int socket, struct sockaddr* client_addr, int* client_addr_len);

Exercise description

• Concurrent server, implemented by forking a child process to handle each connection request.

• HTTP server providing dynamic content: client requests that server run a program; server runs program and returns output to client.

Exercise description

socket

bind

listen

loop

accept

fork and child handles connection

child forks & it’s child executes program

Exercise description: server argument

• Initial (main) program argument:

port number that server binds to (above 1024 otherwise permission denied).

Exercise description: server’s reply

• Server runs a program as requested by a client and sends back:– status– output length– output type– output

• Sending the output length before the output itself requires the server to use a buffer to store the output.

Exercise description: server’s reply

• Format:HTTP/1.0 200 OKContent-Length: 29Content-Type: text/plaindata

• Status: 200 OK, if program run is successful; 300 ERROR, if program doesn’t exist or execv fails.

• Actual length of program output.• Each line ends with \r\n

Pipe

• Half-duplex• Used only between processes that have a

common ancestor.#include <unistd.h>int pipe(int filedes[2]);• Returns 0 if OK, -1 on error• filedes[0] is open for reading, filedes[1] for

writing: output of filedes[1] is the input of filedes[0].

Using a pipe between parent and child

• Pipe created by a process.

• Process calls fork.

• Pipe is used between parent and child in one direction.

parent

fd[0] fd[1]

pipekernel

childfork

fd[0] fd[1]

Exercise description: reading from standard output using a pipe & dup2

childpipefork

parent childclose(pipefd[1]) close(pipefd[0])

read(pipefd[0],buf,size) dup2(pipefd[1],1)execv standard

output

Duplicating an existing file descriptor

#include <unistd.h>

int dup(int filedes);

int dup2(int filedes, int filedes2);

• Returns new file descriptor if OK, -1 on error.

Exercise description

• Server replies only to GET requests from clients, receiving an entire GET request from client (message ends with \r\n\r\n)

• Parses client requests in the format:GET http://nova.math.tau.ac.il:2000/date+-u HTTP/1.0\r\n

• The server prints on screen all the HTTP requests it receives, from which you can tell the format of the browser request.

programto run

program argumentsdelimited with +

portmachine running the server

Exercise description: server tests

• Using a commercial web browser (IE), check your server, printout the results of two requests:

http://nova.math.tau.ac.il:2000/ls+-l+-a

and verify that the servers directory appears in the browser.

http://nova.math.tau.ac.il:2000/date+-u

and verify that you receive an updated date each time you refresh the browser.

• Copy the programs you run, such as date and ls, from /bin to the servers directory.

• Use the first four digits (after 0) of your ID number as the port.

Exercise description

You can incrementally implement the serverin several steps, testing each one:1. Iteratively handle each connection

request, ignoring the URL and sending a constant message to the client, with the proper header.

2. Concurrency handle each client connection request by a new process.

3. Parse the URL and execute the requested program, replying to the client.

Exercise submission

• Chapter 7.8 in Toledo’s book, pages 135-139.• Submission: Monday, June 17th.• Software

Directory: ~username/os02b/ex-serveFiles: ex-serve.cPermissions: chmod ugo+rx (to above)

• Hardcopyname, ID, login, CIDex-serve.cprintout result of the two requests from web browser.submit in 281, Nir Neumark, nirn@post.tau.ac.il

• Environment: Unix, Linux

Exercise notes

• You can use the functions recv/send or read/write (the latter receiving three arguments instead of four).

References

• Operating Systems, Sivan Toledo, Akademon, 2001.

• Internetworking with TCP/IP, Vol 3: client-server programming and applications, Douglas Comer & David Stevens, Prentice-Hall, 1997.

• Advanced programming in the Unix environment, Richard Stevens, Addison-Wesley, 1993.

• Example program on course webpage.