Online Exercises Week 11: Sockets
Due: Electronically, by 6 p.m. on Friday April 1, 2016
Introduction
The purpose of this exercise is to practice using the socket-related system calls. The buffering part of this exercise will be particularly useful in assignment 4.
The Unix programming concepts you will be using in this exercise are:
socket
Server functions: bind, listen, accept Client functions: connect
Getting started
Log into MarkUs to create your repository and then download the following files:
makefile readserver.c randclient.c bufserver.c
Choosing a Port
The port on which the server listens is defined on the second line of the makefile. I have it set to 30001 by default; before continuing I’d like you to change this.
To avoid port conflicts, I’m asking you to use the following number as the port on which your server will listen: take the last four digits of your student number, and add a 5 in front. For example, if your student number is 998123456, your port would be 53456. Using this base port, you may add 1 to it as necessary in order to use new ports (for example, the fictitious student here could also use 53457, 53458, 53459, 53460). Sometimes, when you shutdown your server (e.g. to compile and run it again), the OS will not release the old port immediately, so you may have to cycle through ports a bit.
The -D flag to gcc is something we haven’t used so far. It lets you define a constant at the commandline.
Sending and Receiving Messages
Take a look at randclient.c. It is a client that sends multiple copies of the same message to a server. Each message ends with a network newline “\r\n”.
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You might expect that a server using read will receive one complete line of text with each read call. However, this isn’t guaranteed to happen. For example, in a busy network or with a slow client, one line of text might be split across multiple TCP segments. A server cannot do a single read and expect to get any sensible unit of communication. To simulate this, randclient.c artificially splits up its copies of the messages over multiple write calls. This will cause trouble for any server that expects to read entire messages at a time. In fact, readserver.c is one such server that makes this unwarranted assumption. Run readserver & and then run randclient 127.0.0.1. Notice that the server thinks that every small piece of a message is a complete message, which of course is incorrect. Your task is to add buffering to your server so that it waits for a complete message (ending with the network newline) before printing the message. |
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Task 1: A Buffering Server |
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In bufserver.c, we have given you the skeleton for a buffering server. The Step comments in the code indicate what you should do to complete the server. The main idea is that you have a buffer (an array) where you store pieces of a message until you have a network newline (“\r\n”). Each message piece goes after the data that you’ve already placed in the buffer. When you find a network newline, you know that you have a complete message, so you can print that message and then shift any remaining bytes to the front of the buffer as the start of the next copy of the message. Once you finish the steps, try running bufserver and then run randclient to connect to the server. The server should have one line of output per message. If there is extra garbage printed or other errors, carefully go over the steps again — it’s very easy to get off-by-one mistakes here. |
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Task 2: Using the Debugger |
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For this part, you are asked to run the client and the server in the debugger to help you understand how the socket calls work. To carry out this exercise, you will need to have two terminal windows open on your machine. One will be used to run the server and the other will be used to run the client. If you’re working remotely, a simple option is to open two ssh connections (but make sure they are both on the same machine, like greywolf). Run bufserver in the server window and randclient 127.0.0.1 in the client window. Debugging Exercise 1 In the client window, run script client.1, and in the server window run script server.1. This will cause transcripts of your sessions to be written to files that you will submit. |
In the server window, run gdb bufserver and set a breakpoint at main (break main). Step through the code using step (or s) and next (or n) until you see the Bind call. (Remember that gdb shows you the line just before it is to be executed. You want to stop before the bind call happens.) Try running the client (randclient 127.0.0.1) in the client window and notice that the connection is refused.
In the server window, step once (just before Listen), and try running the client again. Still the connection is refused because the socket is not fully set up on the server.
In the server window, step until you see the Accept call, and then step one more time. Gdb does not give you the next prompt because the Accept call has not returned yet. The Accept call is blocked waiting for a connection from a client.
In the client window, run the client again. Note that the Accept call returns in the server. Keep stepping in the server until the read call. This is where the server starts reading the message from the client. Keep stepping in the server until all of the bytes are received.
In both the client and server windows, type exit to complete the scripts. Submit your script files before you move on.
Debugging Exercise 2
In the client window, run script client.2 and in the server window, run script server.2.
Now run gdb bufserver in the server window, and again break in main. Run gdb randclient in the client window. Break in main and then type run 127.0.01 to run the client.
Try stepping through the two programs in different orders so that you can see how the messages are transferred. Look at the values of variables (e.g. soc, buf) at different times to see their values.
This is your chance to really think about how the client and server talk to each other, so take advantage of the opportunity to learn a bit more gdb. Pay attention to how the data is being sent. Reading and writing on a socket has to be done carefully. Time you spend understanding what is happening with this example code will help you get started on Assignment 4.
After you have tried several variations, exit from gdb and type exit in both windows. Submit your script files.
Submitting
Submit bufserver.c, client.1, client.2, server.1, and server.2.