nphw.rst

Remote Access System

A rsh-like access system

This is my homework of Network Programming (for graduate student) in 2014.

Spec

• The length of a single-line input will not exceed 10000 characters

• There may be huge number of commands in a single-line input

• Each command will not exceed 256 characters

• There must be one or more spaces between commands and symbols (or arguments.), but no spaces between pipe and numbers

  • cat hello.txt | number
  • cat hello.txt |1 number

• There will not be any '/' character in demo input

• Pipe ("|") will not come with "printenv" and "setenv"

• Use '% ' as the command line prompt

• welcome message

  ****************************************
  ** Welcome to the information server. **
  ****************************************
  

• Close the connection between the server and the client immediately when the server receive "exit"

• Note that the forked process of server MUST be killed when the connection to the client is closed. Otherwise, there may be lots zombie processes

• If there is command not found, print as follows:

  Unknown command: [command].
  

• |N means the stdout of last command should be piped to next Nth legal process, where 1 <= N <= 1000

• If there is any error in a input line, the line number still counts

  % ls |1
  % ctt               <= unknown command, process number is not counted
  Unknown command: [ctt].
  % number
  1 bin/
  2 test.html
  
  % ls |1 ctt               <= if find any process illegal, process will stop immediately
  Unknown command: [ctt].
  % cat               <= this command is first legal process after "ls |1"
  bin/
  test.html
  

• We will use pipe or numbered-pipe after unknown command like "ls |1 ctt | cat" or "ls |1 ctt |1". In this case, process will stop when running unknown command, so the command or numbered-pipe after unknown command will not execute and numbered-pipe will not be counted.

  % ls |1 ctt | cat   <= cat will not execute and numbered-pipe will not be counted.
  Unknown command: [ctt].
  % cat
  bin/
  test.html
  

• If mutiple commands output to the same pipe, the output should be ordered, not mixed.

  % ls |3 removetag test.html |2 ls |1 cat   <= cat will not execute and numbered-pipe will not be counted.
  bin/
  test.html
  
  Test
  This is a test program
  for ras.
  
  bin/
  test.html
  

• There must be "ls", "cat", "removetag", "removetag0", "number” in "bin/" of “ras/“.

• You have to execute the files in "bin/" with an "exec()"-based function.(eg. execvp() or execlp() ...)

Implementation

Environment

I use C++ map (called dict in some language) to save my environment variable, use C++ string as key and C++ vector as value

Write to socket

The write_socket function get a connection fd and a const char* data, then it will make sure that the data is truely send to the client

Other functions like welcome, prompt are just a wrapper of wirte_socket to send the data

printenv & setenv

printenv will get the data from "ENV" variable (which is a map) and send the value to client

setenv will modify the data in "ENV" variable (select by key)

queue for pipes

I use a priority queue to save the pipes that waiting for other command to receive, each element in the queue has a pair of data (pipe count + next n & pipe fd), the command will get the pipes (the pipes with the same pipe count) from priority queue, and make it as STDIN to run the command

generate argv

The gen_argv function will get a C++ string with single command (with arguments), then it will parse the command and return a vector with separated arguments

fork & exec

the fork_exec function will get the "argv" and find the command path, then fork to execute the command (with argv), child will do the exec, parent will read from the pipe or wirte to a file