Project 2 
- Deadline: 12 Noon Oct 28

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Signals - Review

- 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 #.

sigset_t - bitmap where each of the 32 bits corresponds to one signal.
- Used to create signal masks for blocking/unblocking desired signals.
- sigset below is a sigset_t

- 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. 

These are used for working with a signal mask for blocking/unblocking signals:
 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

sigaction() and signal() are used to designate a signal handler

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Sockets 

Two types:

Stream - TCP-IP
- TCP-IP stands for Transmission control protocol, internet protocol

Datagram - UDP
- UDP stands for Unconnected data protocol

TCP-IP Notes
- C/C++: The server services clients by forking child processes
- Java : The server services clients within threads, i.e. lightweight processes

Processes
- Server
  * Allocate socket file descriptor 	socket()
  * Claim port #			bind()
    > Only the server needs to allocate a specific port, because its port for handling service requests must be 'well-known'
  * Set up listening queue size		listen()
  * Wait for connect requests		accept() - fires when a connect request comes in from a client

- Client
  * Allocate socket file descriptor 	socket()
  * Make connection request   		connect()

Both client and server must populate a sockaddr struct:
- Server: Specify who can connect and port number. bind() executes 
- Client: Specify server IP address and port.      connect executes.

Connecting
- connect() in client causes accept() in server to fire.

Some machines are big-endian and some are little-endian. 
  To insure that data transferred over the internet is not corrupted, hton and ntoh are used to convert to and from network format. 
  They are suffixed with an s for short or an n for long.

Notes:
- write() and read() can be used if no flags need to be specified
  * send() and recv() provide an argument to specify a flag
    > See peekser for an example.
- See sersig for an example of catching SIGCHLD
  * EINTR is the signal received when a system call is interrupted.
  * In sersig, accept() is NOT re-entrant. This means we must restart it manually
- Datagram client must use sendto()
  * Otherwise, system has no way to route content
- Return address of client
  * In accept() for stream
  * In recvfrom() for dgram
    > Datagram server can use read or recv if no return message or ack will be sent.