System Programming Assignment 3 Solution

I. Problem Description

The goal of this assignment is to be familiar with the concepts of signal and inter-process communication covered in Chapter 10 and 14 in the textbook. In this assignment, you will handle the communication between parent processes and child processes faultlessly between a web server and CGI processes.

In this assignment, we will provide you a sample web server, which you can download from ceiba. First you should write a simple program called file_reader, which does a simple job: read a filename from STDIN and write the content of the file to STDOUT if the file is accessible. If the file is not accessible, the program should output appropriate error message to STDOUT and exit with a nonzero return code.

In the following texts, CGI program just refer to some simple program that reads from STDIN and writes its result to STDOUT. Then, your task is to revise the web server to assure that the web server can handle the CGI requests faultlessly, including invoking CGI program and communicating with the CGI program, and write the proper information to the output. The use scenario of the application is described in the following:

The web server will be started first. After the web server starts, the user can send the request to the web server via a URL like the following:

http://your_ip:port/cgi_program?filename=filename


where cgi_program specify the name of CGI program to be invoked. This name should only contain 'a'~'z', 'A'~'Z', '0'~'9', and '_', or your web server should inform the client of an error. The first and only query parameter is the filename. The filename should only contain 'a'~'z', 'A'~'Z', '0'~'9', and '_'too, or your web server should inform the client an error.

The web server interacts with CGI process by passing the filename to STDIN of CGI program. If everything works correctly in the above example, with cgi_program being file_reader, the server process will output the content of the file filename.

On the other hand, if the CGI program crashes or encounters any error, the web server should handle it properly and notify the client.

Your server should properly write HTTP header to the client. You can refer to the comment in line 274 ~ 284 in sample code as an example of how to write a valid (although simple) HTTP header. You may need to change "200 OK" to some other status, and the argument of "ContentLength: " to real value of the output.
In this assignment, you should complete the following tasks:

    1. Use your cgi_program program to read the data form filename and show on web browser.

    2. If cgi_program or filename doesn't match the condition above(invalid name), you should notify the client by HTTP status code "400 Bad Request", with some appropriate error message.

    3. Server executes cgi_program by fork and communicates with child processes by 
 pipes. After the CGI program writes output to its STDOUT, your child process should return the content back to parent process using only pipes, then the parent would add HTTP header to it, and output the result to the client.

But if the CGI program exits with a nonzero exit code, the server should change HTTP status code to "404 Not Found", and output the error message of the CGI program.

    4. When a user requests a CGI program or filename which does NOT exist, you should notify the client by HTTP status code "404 Not Found", with some appropriate error message, e.g. "CGI Program not found".

    5. If the URL is: 
 http://your_ip:port/info


Your server should NOT try to execute a CGI program named info. Instead, your server forks a child process and the child process sends SIGUSR1 signal to the server process.

After the server process receives SIGUSR1, it shows the child processes’ information, including the CGI processes that are still running (with pids each) and the CGIs that are terminated after the server process starts.


You may write a CGI program and make it sleep(5) so that you can show the running processes’ pid. Of course, other methods will be fine if you can demo this situation.

    6. Use mmap() to record the exit time of your cgi program and the filename. Your server should read the info from the share memory and show the last exit time and the filename on http://your_ip:port/info.

Your job is to write your own server, i.e., parent process, and CGI process, i.e., child process.

II. Sample execution

$ ./server 12345 tmp.log


The sample code has 2 arguments: [port]and [logname](not used in the sample
code). You can change the arguments to whatever you want. The sample code doesn't do much things, but if you've finished your server, you should observe the following behavior:

If the content of file example_file is "abcdef", and when you type the url in the browser: http://yourip:12345/file_reader?filename=example_file

where yourip is the ip where you're running the server, you should see "abcdef" in your browser. If you type in the url:

http://yourip:12345/info


you should see something like the following in the browser:

2 processes died previously.


PIDs of Running Processes: 1034, 4871, 5327


Last Exit CGI: Sat Dec 3 22:08:44 2016, Filename:[opened filename]

III. Tasks and scoring

There are 7 subtasks in this assignment. By finishing all subtasks you earn the full 14 points.

    1. Successfully read and show data on web server (1 point)


Your program should be able to read a file on web browser(200 OK).
 Hint: Use the sample code

        2. I/O multiplexing (1 points)


Your program should be able to handle multiple connections at once. You can demo this by using some slowresponding CGI programs, with multiple connections to the server. Hint: As in Programming Assignment 1, use select().

        3. Multiprocess (1 points)


Your program should be able to handle multiple running CGI programs. Hint: fork() & exec()

        4. Show process info (4 points)

1. Your program should return the right HTTP status code "400 Bad Request”, if cgi_program or filename has invalid name.

2. Your program should return the right HTTP status code "404 Not Found”, if filename or cgi_program doesn’t exist. (CGI program should exit with a non-zero exit code if the filename dose not exist)

3. Show the correct info on http://your_ip:port/info, including number of died processes and running processes.

Hint: Handle signal SIGUSR1, SIGCHLD.


(you must use signal to demonstrate these tasks)

    4. Show the exit time and the filename of your last exit CGI program on 
 http://your_ip:port/info. Hint: Use ctime() to formate the time info.

IV.

Submission


For this assignment, each student would have 5 minute time to demo his/her server and each functionality. You should prepare some CGI programs that can be used for
the demo of each task above and convince the TA that you do really conquer that subtask.

The score would be only based on the demo, NOT on any automatic testing.

Your assignment should be submitted to the course website by the due. Or you will receive penalty. At least four files should be included:

1.server.c 


2.file_reader.c

3.Makefile (as well as other *.c)


4.REAMDE.txt


Put your student ID (lowercase) and name in a comment at the first line of your source

code, with the following format

/* STUDENT_ID NAME */


In README.txt, please briefly state how to compile your program, how to run your server,

what functionality had you done, and anything you have done besides the basic functionality. These files should be put inside a folder named with your student ID and you should

compress the folder into a .zip before submission. The commands below will do the trick. Suppose your student ID is b05902000, and your name is 林聆凌:

And the first line of *.c files would be /*b05902000林聆凌 */

Please do NOT add executable files to the compressed file. Error in the submission file (such as files not in a directory named with your student ID, or so on) may cause deduction of your credits. (Really expensive, isn’t it? Please follow them carefully!)

V. Notes
All output / error messages above do not need to be strictly as the same as described above. For example, you can add some html tags around them. However, when you demo, it should be easy to identify which message corresponds to which case. Feel free to add more functionality or make the output look better.

Also, you should not change the name of "file_reader", the url format given, and the server should behave correctly when given these urls.

It would be beneficial to output some debug messages to console, so you can know what the server is doing better. It may also be useful in demo.

To demo that you returned the correct HTTP status code, you can use inspector in Chrome or Firefox (ctrl+shift+i, Network) to show the status code.

You can assume that TA would not try to use some edge case / extreme case to challenge your server. (e.g. super long query string, weird characters in url, ...). But it's encouraged to handle as many exception cases as you can.

VI. Punishments

    • Plagiarism


Plagiarism is strictly prohibited.

    • Late punishment


Your credits of this assignment will be deducted 20% for each day of delay submission.

(That is, you will lose all your credits on this assignment after 5 days delaying!)

Even though your credits will be deducted for delaying, a late submission will still be much

better than absence. Just remember one thing: there are System Programming courses to attend next morning.

If you have any question, refer to the slide to see how to contact us.