GoalsGoalsThe ProgramWrite a program called 'hftp', implementing a stateless file server. The command-line usage for running the server is:
hftp <port-number>
Do not write a client program that interacts with the server; instead, you should write a library that lets others write a client program that suites them best. The library should implement the header file hftp.h, and be called 'libhftp.a' when compiled. You should also submit a Makefile, which, when executed, creates exactly two files named 'hftp' and 'libhftp.a' from the source files in the current directory.
As the client header file suggests, a client starts by calling hftp_open() and should end its connection with the server by calling hftp_close(). Over an open connection, it is possible to send and receive files, rename a remote file or move it between directories, change the remote directory we are in and get a short or long listing of the files in the current remote directory. A client process can only have one open connection to the server at any time, but it can close an open connection and reopen a different one if necessary. The connection between the client and the server should be implemented using TCP/IP sockets.
The server must be able to handle multiple clients at once; this should be done by forking a new process to handle every new connection. However, in order to avoid an overload, the server should never agree to have more than 10 open clients at once. Since the server is multi-process, calls to wait() and handle of SIGCHLD must be taken care of. This has at least two consequences on the program. First, the accept() and other system calls can be interrupted by a signal, in which case the system call will return -1 and set errno to EINTR. Remember that this case is not an error. Second, you might need to block signals (SIGCHLD and SIGPIPE) in some parts of the program, in order to avoid critical sections. This is not required in all implementations, but make sure if and where your code needs it.
When a client connects, it connects to a default directory on the server - the directory the server was started from. This is considered the current remote working directory for this connection, which means the file names the client refers to will be relative to that directory. The client can also change this directory using the hftp_cwd() function. Note that the server can have different "current directories" for the different connections it currently holds open.
In case of errors, the server program should print detailed error messages to the standard error, and return to accepting new connections. The server should never terminate due to an error; it can only be terminated by explicitly killing it from the shell that started it (see the man pages of the "jobs", "ps" and "kill" shell commands).
The client library must never write anything to the screen, including error messages. All the library functions return an integer, which should be zero on success and -1 on error. The specific error code of the last library function call is saved in the variable hftp_errno. This variable must be updated after every library function call, whether it was successful or not. The code 0 (zero) is saved for success, and you may define other error codes as you wish. The function hftp_error() works just like perror(), but for library errors; it also resets the hftp_errno variable to zero. Remember to write to the standard error and not the standard output.
Some of the errors will be caused by improper use of the library - getting or putting a file on a closed connection, giving NULL as a file name parameter, refusal of the server to open a connection due to overload, and so forth. But many errors will be file errors - file not found, access denied, file of unsupported type and others you recall from ex9. For this kind of errors, the easiest implementation of hftp_error() will be to call perror(). For the previous kind of errors, you need to write your own detailed error messages.
BonusThere is one ten points bonus for this exercise: Extend the server to support the transfer of entire directories as well. If the file type of the arguments passed to hftp_putfile() or hftp_getfile() is a directory, then the entire directory and everything inside it - recursively, as in ex9 - is transferred. Symbolic links are still handled differently that in ex9 - we don't send or receive the links themselves, but the files the link point to.
The bonus is not hard to write - when having to transfer a directory tree, pack it using your ex9 code on one side, send it as one regular file, and unpack it again on the other side. Assuming your packing and unpacking code works correctly, all you need to do is change the treatment of symbolic links and then cut&paste.
A wise way to do that will be to insert your ex9 code into a library as well - a library that can pack a file, a symbolic link or a directory tree into a new regular file, and then unpack it. It's the right thing to do from a software engineering point of view, and it will make the code easier to reuse if you will need it for next year's project.
MiscellaneousYour submission must include a README, a Makefile, and the source code of the programs. The Makefile should create exactly one executable called 'hftp' and one library called 'libhftp.a'. You may include as many source files as you wish. To submit, send an email to the bodek, containing the full names, id numbers and logins of the submitting pair, and attach all the relevant files. Do not attach 'hftp.h' - we will check the exercise using the published version, not any modification you wrote. Remember that the files you send will be checked (not the ones in your account), and the time of submission (for late submission penalties) will be the time of sending the mail.
Your README file should include a vivid description of how your program does what it's supposed to. If you choose to implement the bonus, include an exact description of how you did so in the README. You might lose some of the bonus points for not being able to describe your accomplishment well.
Start working from the example client.c and server.c files from class. Use your code from ex9 for reading and writing files (although you'll need to change it since this time we treat symbolic links differently). A simple way to write the hftp_ls() and hftp_dir() functions is to call the real "ls" program using fork, exec and wait.
Good luck!
