CSE111 PWD Assignment 4 Solution

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URL: https://www2.ucsc.edu/courses/cse111-wm/:/Assignments/asg4-client-server/
1. Overview
This project will implement a client/server application using sockets. A daemon (cxid) listens on a socket for client connection requests. Each connection will cause the server to fork a child process to serve the client. The daemon runs in an infinite loop listening. The server exits when the client disconnects. A client (cxi) connects to a server and can send files, receive files, and get a summary listing of all files present.
A socket is a two-way means of communication between processes, not necessarily running on the same host. An IPv4 host is know by a 4-octet sequence such as 128.114.108.152, and a port is an unsigned 16-bit number (0 to 65535). Communication will be done via TCP/IP over IPv4 sockets.
2. Programs
In this project, two main programs are to be written for the three parts of the project : the daemon and server, and the client. There will also be several library files written to be used by the programs. The general function is similar to sftp(1).
cxid
Usage : cxid port
Creates a server socket and goes into an infinite loop: When it accepts a client socket, it uses fork(2) to create a child process, which functions as the server to communicate with the client. The daemon listens for connections on the given port.
The server is forked with an open socket communicating with the client. Its loop repeatedly reads commands and information from the client and acts on those commands, returning information back to the client. Its loop will be a receive followed by a send, responding to client requests. It exits when the client closes the socket. It does no terminal I/O except for debugging purposes.
cxi
Usage : cxi host port
The client interacts with the user. Commands are read from the terminal (or redirect), each of which is executed one at a time by communicating with the server. Results are then displayed at the terminal. The client connects to the given host and port.
3. Interactive Commands
The cxi client responds to commands read from the standard output and writes output to the standard output and error and accesses files. In the syntax below, Courier Bold are literal characters actually typed in, while Roman Italic stands for appropriate substitutions.
exit
Quit the program. An end of file marker or Control/D is equivalent.
get filename
Copy the file named filename on the remote server and create or overwrite a file of the same name in the current directory.
help
A summary of available commands is printed.
ls
Causes the remote server to execute the command ls -l and prints the output to the user’s terminal.
put filename
Copies a local file into the socket and causes the remote server to create that file in its directory.
rm filename
Causes the remote server to remove the file.
4. Protocol used by the cxi* programs
For the client and server to communicate, a protocol needs to be established. Each message must be framed in terms of a header and a payload. The header always consists of a struct of size 64 bytes. All messages between client and server consist of these 64 bytes, possibly followed by a payload. For alignment purposes, the nbytes field is first. Before filling in the fields, use memset(3) to clear the struct.
enum class cxi_command : uint8_t {
ERROR = 0, EXIT, GET, HELP, LS, PUT, RM, FILEOUT, LSOUT, ACK, NAK }; size_t constexpr FILENAME_SIZE = 59; struct cxi_header { uint32_t nbytes {0}; cxi_command command {cxi_command::ERROR}; char filename[FILENAME_SIZE] {};
};
The purposes of the fields are as follows:
uint32_t nbytes;
The number of bytes in the payload if there is any payload. Otherwise it must be zero (MBZ). This field is sent in network byte order and so must use the functions ntohl(3) and htonl(3) when loading and storing data.
cxi_command command;
A single byte containing one of the cxi_command constants. Note that the enum is specifically a uint8_t single byte type.
char filename[59];
Following are the meanings of each of the cxi_command values. Each is either client to server (C→S) or server to client (S→C), but never both.
cxi_command::ERROR
An error flag to indicate an invalid header. Used internally.
cxi_command::EXIT
Internal to cxi, not used in communication.
cxi_command::GET (C→S)
Request a file from the server. The filename is used both remotely and locally. The payload length is 0.
cxi_command::HELP
Internal to cxi, not used in communication.
cxi_command::LS (C→S)
Request file (ls) information. The payload length and filename are zeroed.
cxi_command::PUT (C→S)
cxi_command::RM (C→S)
Request to remove a file. The payload length is 0.
cxi_command::FILEOUT (S→C)
Response to a cxi_command::GET. The filename is the same as in the request and the payload length reflects the number of bytes in the file. The payload consists of the bytes of the file.
cxi_command::LSOUT (S→C)
Response to a cxi_command::LS. The filename is zeroed and the payload length is the number of bytes sent in the payload. The payload is the output of the command ls -l.
cxi_command::ACK (S→C)
Response to either a cxi_command::PUT or a cxi_command::RM indicating that the request was successfully completed.
cxi_command::NAK (S→C)
Response to any request that fails. There is no payload. The filename field is the same as was in the original request. The nbytes field is set to the value of errno in the server’s attempt to preform a task.
5. Procedures
Each of the above commands requires procedures for accessing files, including reading files from disk and writing files to disk, as well as accessing directories. When any of the system calls fails in the server, the server immediately terminates the operation and sends the value of errno back to the client in a cxi_command::NAK message.
(c) When receiving a file from the socket, Receive the header and determine the size of the file. Create an ofstream and use ofstream::write() to write the parts of the file as they are received from the socket. A C++ stream is closed when the variable goes out of scope, or you can call close.
(d) To delete a file for the cxi_command::RM command, use unlink(2) :
rc = unlink (filename);
(e) To execute the cxi_command::LS command use popen(2) and pclose(2) to create a pipe stream from the ls(1) command
FILE* pipe = popen ("ls -l", "r");
Then read the characters from the pipe in the easiest way, probably by using fgets(3). Finally, pclose(pipe). Then send the output back the client in a cxi_ command::LSOUT message.
6. Modules
There will need to be several modules in this suite of programs. Each of the programs, of course, will have its own source file with a main function in it.
The sockets module will be a useful inclusion into the program as its own module.
There should also be a cxi_protocol module to implement the protocols and contain code for accessing files and sockets, since these will be used by both the client and the server.
7. Use of sockets
-bash-71$ hostname unix2.lt.ucsc.edu
The name unix.ucsc.edu is not actually a real host. It is just an alias for one of the real hosts, which have name patterns like the one shown here.
8. Use of ports
If your daemon listens on a port that has been bound by another process, you will get the message ‘‘Address already in use’’ because only one process at any given time is allowed to listen on a particular process. To avoid this, choose a port number not being used by anyone else on the same server.
Port numbers are divided into three ranges:
(b) Port numbers from 1024 (0x400) through 49151 (0xBFFF) are registered.They are used for multiple purposes. Choose any of these ports for your server if not already in use.
(c) Dynamic and private ports are those from 49152 (0xC000) through 65535 (0xFFFF) and services should not be associated with them.
9. Runaway Processes
Be careful in using fork(2) so that you don’t accidentally create a fork-bomb. The command pkill(1) can be used to kill all processes matching a particular pattern.
So the command pkill cxi
will kill all of your processes whose executables contain the string ‘‘cxi’’. A really quick way to log out is to use kill(1) : kill -9 -1 kill -s KILL -1
will send SIGKILL to all of your processes, thus logging you out.
10. What to Submit
Submit Makefile which builds both programs, all necessary C++ header and implementation files. And if doing pair programming, the PARTNER file. When the grader uses the command make in the submit directory, the both binaries should be built.