String Mangler Client/Server Program

Due: Sunday, Feb. 5 before 11:30pm

You must work with one partner on this assignment, and I'd like at least one partner in each group to be someone who has taken CS45.

%  nslookup curry
  ...
  Address  130.58.68.36

% client 130.58.68.36      # start the client and try to connect to server
			    # running on curry, 

# client program should print a prompt for user to enter text
Enter the next message:  hello there

# then send the "hello there" string to the server that will mangle the
# received string and send the mangled string back to the client.  As a 
# debugging step, have the server print out the received string and 
# the returned mangled string. 

# the client should then print out the result
HELLO THERE

#the client repeats printing the prompt, reading input, sending ...
Do you want to continue (yes/no): yes
Enter the next message: 

% cp /home/newhall/public/cs85/hw1_startingpt/* .
% ls
Makefile  README  client.c  mangler.h  server.c

• Steven's Unix Network Programming (2nd Edition Vol. 1). Chapters 3 and 4 are most helpful. A copy of this book is in the CS Lab.
• Beej's Guide to Network Programming
• read the man pages for socket, connect, accept, listen, ...
• My Unix Help Pages contain links to Unix help and C programming help.

Read through my C Style Guide and make sure that your code conforms to it (good comments, good modular design, robust, etc.).

Start by getting the TCP connection protocol correct: implement a simple client and server program that create sockets and connect and pass a simple message from client to server (send an int value for example), and then try sending a simple response message from server to client.

Once that works, implement the mangling server so that it handles just a single client connection at a time. Once you get that to work, then add support to the server for handling multiple, simultaneous client connetions, and making sure to clean up dead child server processes when they exit (get rid of zombies). Finally, stress test your solution to ensure it is both correct and robust.

Some problems you will need to think about: how the server knows when it has read all of the string sent to it by a client in a single request; how the server knows when a client has terminated its connection; and how to ensure that two clients requests don't interfere with each other.

help with specific detais of project:

supporting multiple client connections

int  pid;

// create a new child process
pid = fork();

if(pid == -1) { 
   // fork returned an error, handle it

} else if (pid == 0) {   // this is the child process:
   // child code to handle this specific connection
   // until the child exits

} else { // this is the parent process:
   // parent process to get ready to accept the next connection
  
}

cleaning up server threads after client's exit

In the server set up a sighandler routine that will be invoked when the parent receives a SIGCHILD from an exiting child process:

void my_sigchild_handler(int x)
{
        while(waitpid(-1, NULL, WNOHANG) > 0) {  }
}

struct sigaction myhandler;

...

//set the handler to my_sigchild_handler
myhandler.sa_handler = my_sigchild_handler; 
// initialize the set of signals:
sigemptyset(&myhandler.sa_mask);
// set the flags...I'm not sure what the best setting is, 
//   but this works pretty well (it is a flag that say to 
//   restart a system call if the signal occurred during a syscall)
myhandler.sa_flags = SA_RESTART;
// call sigaction to register my handler with SIGCHILD signals:
if (sigaction(SIGCHLD, &myhandler, NULL) == -1) {
    // something bad happend, lets exit
    perror("sigaction");
    exit(1);
}

setting sockopts

	struct linger linger_val;

	linger_val.l_onoff = 0;
	linger_val.l_linger = 0;

	setsockopt(sock_fd, , SO_LINGER, (void *)linger_val, 
		(socklen_t) (sizeof(struct linger));

getting socket and IP info

# info for tcp sockets on  machine	

netstat -tcp -v		

# proc files with socket info:

/proc/net/dev           # device information
/proc/net/raw           # raw socket information
/proc/net/tcp           # TCP socket information
/proc/net/udp           # UDP socket information
/proc/net/ipx           # IPX socket information
/proc/net/snmp           # statistics

# get my IP addr

ifconfig                # in the eth0 entry it is "inet addr:" entry

getting the state of a process

# one way to find state of server processes:
ps -A | grep myserver
27318 pts/2    00:00:00 myserver
27321 pts/2    00:00:00 myserver

# if you see a listing like this, then you are not correctly handling SIGCHILDS
27321 pts/2    00:00:00 myserver <defunct>   

# /proc/pid contains info about process with matching pid:
cat /proc/27321/status     # this contains the state of the process