CSE 489/589 Spring 2021
Programming Assignment 1
Due Date: March 12, 2021
Text Chat Application
1. Objectives
Develop the client and server components of a text chat application, consisting of one chat
server and multiple chat clients over TCP connections.
2. Getting Started
2.1 Socket Programming
Refer to Beej Socket Guide for a tutorial on network socket programming
2.2 Login the servers at CSE
We have configured the following CSE servers for you to test and run your code.
You can access any of the above servers, using your ubit name through ssh,
Once logged in, your directory is created under /local/Spring_2021/your-ubit-name, please
keep your code and tests there. Also, limit the access permission so that others cannot
access your code.
chmod 600 -R your_directory
If you are outside the campus networks of UB, please ssh-login the servers through UB
VPNs following this guide:
http://www.buffalo.edu/ubit/service-guides/connecting/vpn/computer.html.
2.3 Install the PA1 template
Read the template document in full and install the template.
You should complete this step before reading further.
It is mandatory to use this template.
3. Implementation
3.1 Programming environment
1 stones.cse.buffalo.edu
2 embankment.cse.buffalo.edu
3 highgate.cse.buffalo.edu
4 euston.cse.buffalo.edu
You will write C (or C++) code that compiles under the GCC (GNU Compiler Collection)
environment. Furthermore, you should ensure that your code compiles and operates
correctly on 5 dedicated hosts/machines, which will be provided to you by the instructor.
Your code should successfully compile using the version of gcc (for C code) or g++ (for
C++ code) found on the 5 dedicated hosts and should function correctly when executed.
NOTE: You are NOT allowed to use any external (not present by default on the dedicated
hosts) libraries for the socket programming part. Bundling of code (or part of it) from
external libraries with your source will not be accepted either. You can however use
external modules for other parts of the assignment (like maintaining a linked list). If you are
not sure whether you are allowed to use an external library or not, consult with the course
staff. Further, your implementation should NOT invoke any external binaries (e.g., ifconfig,
nslookup, etc.) and should NOT involve any disk I/O unless explicitly mentioned in the PA
description.
3.2 Sockets
● Use TCP Sockets only for your implementation.
● Use the select() system call only for handling multiple socket connections. Do not
use multi-threading or fork-exec.
3.3 Running your program
Your program will take 2 command line parameters:
1. The first parameter (s/c) indicates whether your program instance should run as a
server or a client.
2. The second parameter (number) is the port number on which your process will
listen for incoming connections. In the rest of the document, this port is referred to as the
listening port.
E.g., if your executable is named chat_app:
● To run as a server listening on port 4322
./chat_app s 4322
● To run as a client listening on port 4322
./chat_app c 4322
3.4 Dedicated Hosts
For the purpose of this assignment, you should only use (for development and/or testing)
the directory created for you on each of the 5 dedicated hosts. Change the access
permission to this directory so that only you are allowed to access the contents of the
directory. This is to prevent others from getting access to your code.
4. Output Format
We will use automated tests to grade this assignment. The grader, among other things,
will also look at the output generated by your program. Towards this end, ALL the
required output (as described in section 5) generated by your program needs to be
written to BOTH stdout and to a specific logfile. Later sections provide the exact format
strings to be used for output, which need to be strictly followed.
4.1 Print and LOG
We have already provided a convenience function for this purpose in the template (see
src/logger.c and include/logger.h), which writes both to stdout and to the logfile. You
should use ONLY this function, for all output described in this assignment. On the
other hand, if you want to output something more to stdout (for debugging etc.) than what
is described, do NOT use this function and rather use native C/C++ function calls. Any
extra output in the log file will cause the test cases to fail.
To use the function, you will need to have the following statement at the top of your .c/.cpp
source file(s) where you want to use this function:
#include "../include/logger.h"
The function is designed to behave almost exactly as printf. You can use the function as:
cse4589_print_and_log(char* format, ...)
Read the comments above the function definition contained in the src/logger.c file, for
more information on the arguments and return value.
5. Detailed Description
The chat application follows a typical client-server model, whereby we will have one server
instance and two or more client instances. Given that we will be testing on the five
dedicated hosts listed before, you can assume that at most four clients will be online at
any given time.
The clients, when launched, log in to the server, identify themselves, and obtain the list of
other clients that are connected to the server. Clients can either send a unicast message
to any one of the other clients or a broadcast a message to all the other clients.
Note that the clients maintain an active connection only with the server and not with any
other clients. Consequently, all messages exchanged between the clients must flow
through the server. Clients never exchange messages directly with each other.
The server exists to facilitate the exchange of messages between the clients. The server
can exchange control messages with the clients. Among other things, it maintains a list of
all clients that are connected to it, and their related information (IP address, port number,
etc.). Further, the server stores/buffers any messages destined to clients that are not
logged-in at the time of the receipt of the message at the server from the sender, to
be delivered at a later time when the client logs in to the server. You do NOT need to
buffer messages for EXITed clients or from BLOCKed clients (see section 5.6). You can
assume that the total number of buffered messages will not exceed 100.
Your code, when compiled, will produce a single executable file. Depending on what
arguments are passed to this executable (see section 3.3), a client or a server instance
should be started.
5.1 Network and SHELL Dual Functionality
When launched (either as server or client), your application should work like a UNIX shell
accepting specific commands (described below), in addition to performing network
operations required for the chat application to work. You will need to use the select()
system call which will allow you to provide a user interface and perform network functions
at the same time (simultaneously).
5.2 SHELL Commands
Your application should accept commands only when they are inputted:
● In UPPER CASE.
● Having exactly the same syntax as described below.
5.3. SHELL Command Output [IMPORTANT]
The first line of output of all the commands should declare whether it was successfully
executed or it failed.
● If the command was successful, use the following format string:
("[%s:SUCCESS]\n", command_str) //where command_str is the command inputted
without its arguments
● If the command failed with error, use the following format string:
("[%s:ERROR]\n", command_str) //where command_str is the command inputted
without its arguments
For all such required output, you should only use the special print/log function described in
section 4. See section 5.4 (IP command) for an example code snippet.
If the command is successful, it should immediately be followed by its real output (if any)
as described in the sections below. Extra output lines in the log file between the
success message and output will cause test cases to fail.
The last line of the output of all the commands (whether success or failure) should use
the following format string:
("[%s:END]\n", command_str) //where command_str is the command inputted
without its arguments
For events, printing format/requirements will be the same as for commands. Each of the
event’s description tells the value of the command_str you should use for it.
5.4 Server/Client SHELL Command Description
This set of commands should work irrespective of whether the application is started as a
server or a client.
● AUTHOR
Print a statement using the following format string:
("I, %s, have read and understood the course academic integrity policy.\n",
your_ubit_name)
Your submission will not be graded if the AUTHOR command fails to work.
● IP
Print the IP address of this process. Note that this should not be the localhost address
(127.0.0.1), but the external IP address. Use the following format string:
("IP:%s\n", ip_addr) //where ip_addr is a null-terminated char array storing IP
Example Code Snippet
To generate the required output for this command, you would need the following lines in
your code:
//Successful
cse4589_print_and_log("[%s:SUCCESS]\n", command_str);
cse4589_print_and_log("IP:%s\n", ip_addr);
cse4589_print_and_log("[%s:END]\n", command_str);
//Error
cse4589_print_and_log("[%s:ERROR]\n", command_str);
cse4589_print_and_log("[%s:END]\n", command_str);
Here, command_str and ip_str are char arrays containing "IP" and some valid IP address
like "xxx.xx.xx.xx", respectively. Any extra output in between the SUCCESS/ERROR and
END output will cause the test cases to fail.
● PORT
Print the port number this process is listening on. Use the following format string:
("PORT:%d\n", port)
● LIST
Display a numbered list of all the currently logged-in clients. The output should display the
hostname, IP address, and the listening port numbers, sorted by their listening port
numbers, in increasing order. E.g.,
1 stones.cse.buffalo.ed
Use the following format string:
/*The following printf will print out one host. Repeat this printf statement to
* print all hosts
* list_id: integer item number
* hostname: null-terminated char array containing fully qualified hostname
* ip_addr: null-terminated char array storing IP
* port_num: integer storing listening port num */
("%-5d%-35s%-20s%-8d\n", list_id, hostname, ip_addr, port_num)
Notes:
➔ LIST output should contain all the currently logged-in clients, including the client
that executed the command.
➔ The server should NOT be included in the output.
If you do not implement the LIST command correctly, most automated tests for other
commands will fail.
5.5 Server SHELL Command/Event Description
This set of commands should work only when the application is started as a server.
● STATISTICS
Display a numbered list of all the clients that have ever logged-in to the server (but have
never executed the EXIT command) and statistics about each one. The output should
display the hostname, #messages-sent, #messages-received, and the current status:
logged-in/logged-out depending on whether the client is currently logged-in or not, sorted
by their listening port numbers, in increasing order. E.g.,
Use the following format string:
/*The following printf will print out one host. Repeat this printf statement to
* print all hosts
9
4 euston.cse.buffalo.ed
* list_id: integer item number
* hostname: null-terminated char array containing fully qualified hostname
* num_msg_sent: integer number of messages sent by the client
* num_msg_rcv: integer number of messages received by the client */
* status: null-terminated char array containing logged-in or logged-out
("%-5d%-35s%-8d%-8d%-8s\n", list_id, hostname, num_msg_sent, num_msg_rcv,
status)
● BLOCKED
Display a numbered list of all the clients (see BLOCK command in section 5.6) blocked by
the client with ip address: . The output should display the hostname, IP
address, and the listening port numbers, sorted by their listening port numbers, in
increasing order. The output format should be identical to that of the LIST command.
Exceptions to be handled
➔ Invalid IP address
➔ Valid but incorrect/non-existent IP address
● [EVENT]: Message Relayed
All messages exchanged between clients pass through (are relayed by) the server. In the
event of relay of a message from a client with ip address:
addressed to another client with ip address: , print/log the message using the
following format string:
("msg from:%s, to:%s\n[msg]:%s\n", from-client-ip, to-client-ip, msg)
In case of a broadcast message, will be 255.255.255.255
For the purposes of printing/logging, use command_str: RELAYED
5.6 Client SHELL Command/Event Description
This set of commands should work only when the application is started as a client.
● LOGIN
This command is used by a client to login to the server located at ip address:
listening on port: . The LOGIN command takes 2 arguments. The first
argument is the IP address of the server and the second argument is the listening port of
the server.
On successful registration, the server responds with:
1. List of all currently logged-in clients. The client should store this list for later display
and use.
2. All the stored/buffered messages for this client in the order they were received at
the server. Each of these messages will trigger an [EVENT]: Message Received,
described at the end of this section.
A client should not accept any other command, except LOGIN, EXIT, IP, PORT, and
AUTHOR, or receive packets, unless it is successfully logged-in to the server.
Notes
➔ You should NOT print the list of clients received as part of the output of the LOGIN
command.
➔ The LOGIN SUCCESS/ERROR message should be printed after all the event
related output (triggered by the buffered messages).
Exceptions to be handled
➔ Invalid IP address/port number (e.g., 127.abc is an invalid IP; 43f is an invalid port).
You can assume that a valid IP/port will always be the actual IP/listening port of the server.
● REFRESH
Get an updated list of currently logged-in clients from the server.
● SEND
Send message: to client with ip address: . can have a maximum
length of 256 bytes and will consist of valid ASCII characters.
Exceptions to be handled
➔ Invalid IP address.
➔ Valid IP address which does not exist in the local copy of the list of logged-in
clients (This list may be outdated. Do not update it as a result of this check).
● BROADCAST
Send message: to all logged-in clients. can have a maximum length of 256
bytes and will consist of valid ASCII characters.
This should be a server-assisted broadcast. The sending client should send only one
message to the server, indicating it is a broadcast. The server then forwards/relays this
message to all the currently logged-in clients and stores/buffers the message for the
others.
Notes
➔ The client that executes BROADCAST should not receive the same message
back.
● BLOCK
Block all incoming messages (unicast and broadcast) from the client with IP address:
. The client implementation should notify the server about this blocking. The
server should not relay or store/buffer any messages from a blocked sender destined for
the blocking client. The blocked sender, however, will be unaware about this blocking and
should execute the SEND command without any error.
Exceptions to be handled
➔ Invalid IP address.
➔ Valid IP address which does not exist in the local copy of the list of logged-in
clients (This list may be outdated. Do not update it as a result of this check).
➔ Client with IP address: is already blocked.
● UNBLOCK
Unblock a previously blocked client with IP address: . The client implementation
should notify the server about the unblocking.
Exceptions to be handled
➔ Invalid IP address.
➔ Valid IP address which does not exist in the local copy of the list of logged-in
clients (This list may be outdated. Do not update it as a result of this check).
➔ Client with IP address: is not blocked.
● LOGOUT
Logout from the server. However, your application should not exit and continue to accept
LOGIN, EXIT, IP, PORT, and AUTHOR commands. In general, on LOGOUT all state
related to this client is maintained on both the client and the server.
Notes
➔ LOGOUT does NOT reset the statistic counters (see STATISTICS command in
section 5.5).
➔ LOGOUT does NOT unblock any clients blocked by this client.
➔ LOGOUT does NOT change the blocked/unblocked status of this client on the
server.
● EXIT
Logout from the server (if logged-in) and terminate the application with exit code 0. This
should delete all the state for this client on the server. You can assume that an EXITed
client will never start again.
● [EVENT]: Message Received
In the event of receipt of a message from a client with ip address: ,
print/log the message using the following format string:
("msg from:%s\n[msg]:%s\n", client-ip, msg)
Note that here is the IP address of the original sender, not of the relaying
server. For the purposes of printing/logging, use command_str: RECEIVED
5.7 BONUS: Peer-to-peer (P2P) file transfer
Implement additional functionality to allow clients to send/receive files. Here, however, the
transfer will take place directly between two clients and will not involve the server.
For this, your implementation should establish a TCP connection between the two clients
involved in a file transfer. The implementation does not need to handle any broadcast file
transfers. All transfers will take place between a pair of clients.
To send a file residing in the same folder as the executable to a client with ip
address: , a client would execute the following command:
SENDFILE
The receiving client should store the file in the same folder as the executable, with the
same name.
Your implementation should be able to transfer both text and binary files. You can assume
the maximum file size to be 10 MB.
6. Grading and Submission
The grading will be done using automated tests. Any deviation from the output
format/syntax described in previous sections will cause the tests to fail. For a detailed
breakup of points associated with each command/functions, see CSE4/589: PA1 Grading
Guidelines.
For packaging and submission, see the section Packaging and Submission in the
template document.