Project 2 
- Deadline: 12 Noon Oct 28
- Issue: Input can come from one of two sources
  * User - Ordinary read
  * Message queue - Read in signal handler
   
- Commands can update your data file. Keep a pristine backup handy!

- Packets should be predictable in size
  * Can the server know how many bytes to pull off the message queue? 
    > Consider: Can any process have pid < 10? This would allow encoding commands in msgtyp.
      + A union might be useful for the varying packets

  * Note: Be sure to use a cast for the msgp, as it is a void pointer

- Script commands should be of form

p2cli < <name of script file>
  * Script file should contain all keystrokes to run completely.

Log file:
- May be binary, but you must include a way to display it in plain text.
- Does the log file persist? In final version, no. You may want it to persist for testing the log file command.

IMPORTANT: Q)uit command should use Q.

Key client-server rule: Do as much error checking as possible in the client.

Signals

- Signals are generated in the kernel and sent to a process:
  * All signals have a default action. Most can be set to a different action
    > Actions: Block, Catch, Ignore, Default
      + A blocked signal will eventually be unblocked and at that time can be caught, ignored, or allowed to act by default
    > Signals are handled by default in the following manners: Terminate, ignore

List: https://dsa.cs.tsinghua.edu.cn/oj/static/unix_signal.html

- signal() sets up a signal handler for a particular signal
  * Example    signal(SIGINT, removeQ)   sets up the program to catch the signal and call removeQ, which is a handler.
    > A signal handler can have only one parameter, and that can only be an int for the signal #.

- A signal mask is a bit string with each element representing one signal. 
  * sigprocmask is used to set selected signals to block or unblock the signals whose corresponding bits are set in the sigmask, which is of type sigset_t. 

 sigemptyset(&sigset); 			// Clear the sigset_t ; all signals unset
 sigfillset(&sigmask);			// Set all bits in the sigset_t
 sigaddset(&sigset,signalNum);  	// Set the bit for signalNum
 sigdelset(&sigset,signalNum);  	// Unset the bit for signalNum
 sigprocmask(SIG_BLOCK,&sigset,&oldset);// For all signals with bit set, the action on that signal will be to block it.
 sigprocmask(SIG_UNBLOCK,&sigset,&oldset);// For all signals with bit set, the action on that signal will be to unblock it.
 sigpending(&pendingset);		// Get all signals that are currently blocked and waiting to be active
						pending set is an export parameter - has sigset_t when it returns
 sigismember(&pendingset,i)		// Returns 0 or 1 depending on whether signal i is in pendingset

Notes:
- 19 is SIGSTOP
- 18 is SIGCONT
- In  sigprocmask(SIG_BLOCK,&sigset,&oldset), sigset is the new_mask, and oldset is old_mask (relative to sigprocmask specs).