Project 3

- SEM_UNDO - Emergency rollback of binary semaphore if process holding it crashes
  * Tested: SEM_UNDO works on acad & CATS. Assume it works on all machines. Verdict: Use it.
    > If any issues with semaphores, change SEM_UNDO to 0 and if that fixes it, POST!!!!!!!!

select() - permits handling multiple inlets to a program, e.g. waiting for data on both stdin and from a socket
- The prototype of select() is:
   int select(int nfds, fd_set *readset, fd_set *writeset, fd_set errorset,  timeval *timeout )

  * ndfs tells select() the highest file descriptor value will be considered
  * readset, writeset, and errorset are bit maps (binary words) in which each bit represents a particular file descriptor.
  * timeout tells select() whether to block and wait and if waiting is required timeout explicitly specifies how long

To attain concurrency with incoming data from user & socket, best way is to read incoming data from the socket in a child process
  and route it to the parent via a pipe.

         if(FD_ISSET(ReadEnd, &fdvar))
          {
		// From user
		Input from user
          }
          
          if(FD_ISSET(UserRead, &fdvar))
          {   
          	// From pipe or directly from the socket  
		Packet from server
          }


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Conditional Critical Regions

- region X when <cond> do
  * Allows entry to the following code if the condition is true.
  * The region X is guaranteed to be mutually exclusive, 
      i.e. only one waiting process may access X and test the condition
  * If there isn't a 'when' part, then a process gaining mutex to X enters immediately.

============================================================================
Monitors
- Separate entity that guarantees mututal exclusion within
  * Only one process may be active within a monitor at any one time.
    > Many processes may be blocked.

- Condition variables
  * Two member operations, wai & signal (no parents)
  * A process that issues a wait suspends until signaled
  * signal wakens the next process waiting on the same condition
    > If no process is waiting it has no effect.

============================================================================
Deadlock

- Deadlock can arise if four conditions hold simultaneously: 
  * Mutual Exclusion:  only one process at a time can use a resource.
  * Hold and Wait:  a process holding at least one resource is waiting to acquire additional resources held by other processes.
  * No Preemption:  a resource can be released only voluntarily by the process holding it, after that process has completed its task.
  * Circular Wait:  there exists a set {P0, P1, …, Pn} of waiting processes such that:
	P0 is waiting for a resource held by P1
	P1 is waiting for a resource held by P2
	…
	Pn is waiting for a resource held by P0

These conditions are necessary but may not be sufficient.