Class 17: Exceptional Control Flow II (ECF)

From last time:

* ECF occurs at all levels of a system:
  * Exceptions
  * Concurrent processes
  * Signals
  * Non-local jumps

Todays topics:
1. Process hierarchy
2. Shells
3. Signals
4. Non-local jumps

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1. Unix process hierarchy
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* Picture of process hierarchy
* Viewed using ps
* Unix startup sequence

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2. Putting it together: Shells
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* shell uses fork, exec, wait, signal to
run programs on behalf of the user.

* Show simple shell program and use it
to motivate the need for signals.

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3. Signals
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* Signal is a small message that notifies a process
of an event in the system.

* Signal terminology:
  * Sending a signal
  * Receiving a signal
  * Pending signals
  * Pending and blocked bit vectors
    * pending: set when delivered and cleared when received
    * blocked: set by application (sigprocmask), set on entry
      to signal handler, cleared on exit
    
* Sending signals
  * Process groups
  * Sending signals with kill program (fork16)
  * Sending signals from keyboard (fork17)
  * Sending signals with kill function (fork12)

* Receiving signals
 * Operational description of receiving signals
 * Default actions
 * Installing signal handlers
 * Example: simple signals (fork13)

* Signal handling funkiness
 * Pending signals are blocked
 * Pending signals not queued (fork15)
 * Syscalls can be interrupted

* Signal Examples
 * external.c: Program that reacts to externally generated events (ctrl-c)
 * internal.c: Program that reacts to internall generated events (SIGALARM)

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4. Nonlocal jumps
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* setjmp
* longjmp
* limitations
* restart.c: A program that restarts itself