COS-461 Assignment 0: Socket Programming

Assignment 0: Introduction to Socket Programming

Due 11:59pm Thursday February 14

This assignment introduces you to basic socket programming as discussed in Precept 1. The goal is to get you comfortable with (a) Unix socket programming and (b) simple client-server interaction.

You are required to submit code, which compiles and runs on the FC 010 machines. Emphasis is on the correctness of the code (buffer management, handling return values), system calls and their arguments. Both programs should be written in C or C++. You should also include a makefile for compiling your programs, and submit a gzipped tar file (with the .tgz extension) including your client, server, and makefile, via the DropBox site for this assignment.

Grading: This assignment will count towards your participation grade.

We want you to write a server capable of receiving text messages from clients over TCP sockets. The server should print these text messages on the standard output. From the server perspective, the message corresponds to the data received from a particular client during a communication session with that client. The server should be listening for text messages on a port known to the clients. It should handle the client connections sequentially and accept connections from multiple clients. After servicing one client to completion, it should proceed to the next. If multiple clients try to simultaneously send text messages to the server, the server should handle them one at a time (in any order). Serving one client at a time is enough.

We also want you to write a client. The client should read from a file -- either one named "filein.txt" that can be hardcoded in your source code, or from stdin -- then transmit the message and exit.

You can assume that the client is run as

  $ ./client server-IP-address port-number
where "server-IP-address" is the IP address of the server, and "port-number" is the TCP port the server listens on. The server is run as "server port-number". If the server cannot bind on a port, print a message to standard error.

The FC Lab machines firewalls low ports, so you should only use a port greater than 10000 and less than 60000 while testing.

You should use fread and fwrite calls for reading/writing data to and from sockets and files. Do not use special "string" versions of these calls (e.g., fgets and fputs as they are not designed for binary data).

Make sure you handle the following correctly:

  1. Local and remote operation: Your program should be able to operate when connection over both localhost ( or to between machines. When testing, make sure you use an appropriate port to avoid firewall in the lab cluster. You can get the machine's routing IP address via ifconfig (in Linux/Mac) or your GUI Network Preferences. Note online websites tools like may not work because your machine may be behind a NAT and use a different IP address for wide-area communication than from communication between the client and server in the local cluster.

  2. Buffer management: Assume that the file contents can be arbitrarily large (assume a typical size of 20KB), but the buffers you use to read/write to the file or socket must be small and of fixed size (e.g., 4096 bytes).

  3. Handling return values: By default, sockets are blocking, and for this assignment we will use only blocking sockets. "Blocking" means that when we issue a socket call that cannot be done immediately (including not being able to read or write), our process is waits until it can perform the action. This includes the case when
However, if there is some data available to be read or some can be written, the call will return the number of bytes read or written respectively. NOTE: This returned value can be less than specified in the length argument to the call or indicate an error. You must handle this.

Last updated: Sun Feb 17 10:28:01 -0500 2013