librelist archives

« back to archive

Unix signals - primitive interprocess communication

Unix signals - primitive interprocess communication

Eric Wong
2012-03-20 @ 20:56
Signals are the most basic way for processes to communicate with each
other on a Unix system.  Each process has signal handlers configured
by default, so some actions (e.g. killing a process) are always

There are two distinct classes of signals defined by POSIX:

	standard and real-time

Currently, Ruby implementations (that I'm aware of) only deal with
standard signals.  Ruby implementations also control the signal
mask and will temporarily block/unblock signals to perform critical
operations in the interest of (portable) thread/async-safety.

Operating systems have a pre-defined list of signals.  This list of
signals is available for all processes to send and receive.  Each signal
has a human-readable name (e.g.  "KILL" or "SIGKILL") and non-negative
number (e.g. 9) associated with it.  To see which ones are supported by
both Ruby and your OS, the "Signal.list" method returns a Hash mapping
their names to respective signal numbers.

Some signals (e.g. SEGV, VTALRM) may be reserved by the Ruby
implementation for internal use.  Other signals (STOP, KILL) may
not be blocked, ignored, or handled at all by any userspace process.

Signals may be assigned (and reassigned) handler functions (aka
"callbacks") which execute upon delivery of a signal.  Those of you
familiar with event-driven programming models will realize signals are
another type of event (not much unlike I/O) which you may define
callbacks for.

However, writing signal handlers requires more care and attention than
"normal" code in your application.  The universal wisdom is to keep
signal handlers simple and short, regardless of the programming language
you're writing in.

While Ruby helps users avoid many of the low-level details and caveats
of implementing signal handling, there are still pitfalls to be
aware of and avoid.

Methods for working with signals in Ruby

"Process.kill" maps to the kill(2)/killpg(2) system calls for sending
signals to processes or process groups.  "Signal.trap" (Kernel#trap)
is analogous (but not directly mapped to sigaction(2) (or signal(2)).

Read more about them via ri or rdoc.

License: GPLv3 (or later, at the discretion of Eric Wong)
Eric Wong