CS 3733 Operating Systems

Lecture 24 [3/9/98]: Signals

Reading: Sections 5.1-5.2 of PUP

Brief look at socket implementation of UICI
Look at how signals come in to the UICI implementation.

Basic terminology

Signal - software notification of an event.
Lifetime of a signal - time between generation and delivery
Delivery of a signal - takes place when process takes action on signal.
A pending signal has been generated but not yet delivered.
Signal handler - function executed when is delivered.
Signal mask - list of currently blocked signals for a process.
A blocked signal - is a signal that is prevented from being delivered to a process.
Signal set - a list of signals used in various signal operations.

Sending a signal

Signals are specified by integers. symbol.h contains their symbolic names.
kill sends specified signal to a process or a group of processes.
raise sends specified signal to self.
Certain keystrokes cause the keyboard device driver to generate a signal which is sent to the foreground process group:
- INTR (ctrl-c) generates SIGINT
- QUIT (ctrl-|) generates SIGQUIT
- SUSP (ctrl-z) generates SIGSTOP
- DSUSP (ctrl-y) generates SIGCONT
alarm sends a SIGALRM to caller.

Signal masks and signal sets:

Signal mask is associated with blocking of signals. These signals are not lost---their delivery is just postponed.
Signal set operations: sigemptyset, sigfillset, sigaddset, sigdelset, sigismember
Signal mask operations are performed by calling sigprocmask with SIG_BLOCK, SIG_UNBLOCK and SIG_SETMASK.

Skill: Understand the basics of signals

Revision Date: 3/9/98