=over 4
-=item Long running opcodes
+=item Long-running opcodes
+
+As the Perl interpreter only looks at the signal flags when it is about
+to execute a new opcode, a signal that arrives during a long-running
+opcode (e.g. a regular expression operation on a very large string) will
+not be seen until the current opcode completes.
+
+N.B. If a signal of any given type fires multiple times during an opcode
+(such as from a fine-grained timer), the handler for that signal will
+only be called once after the opcode completes, and all the other
+instances will be discarded. Furthermore, if your system's signal queue
+gets flooded to the point that there are signals that have been raised
+but not yet caught (and thus not deferred) at the time an opcode
+completes, those signals may well be caught and deferred during
+subsequent opcodes, with sometimes surprising results. For example, you
+may see alarms delivered even after calling C<alarm(0)> as the latter
+stops the raising of alarms but does not cancel the delivery of alarms
+raised but not yet caught. Do not depend on the behaviors described in
+this paragraph as they are side effects of the current implementation and
+may change in future versions of Perl.
-As Perl interpreter only looks at the signal flags when it about to
-execute a new opcode if a signal arrives during a long running opcode
-(e.g. a regular expression operation on a very large string) then
-signal will not be seen until operation completes.
=item Interrupting IO
POSIX::SigAction->new(sub { die "alarm" }))
or die "Error setting SIGALRM handler: $!\n";
+Another way to disable the safe signal behavior locally is to use
+the C<Perl::Unsafe::Signals> module from CPAN (which will affect
+all signals).
+
=item Restartable system calls
On systems that supported it, older versions of Perl used the
=item Signals as "faults"
-Certain signals e.g. SEGV, ILL, BUS are generated as a result of
-virtual memory or other "faults". These are normally fatal and there
-is little a Perl-level handler can do with them. (In particular the
-old signal scheme was particularly unsafe in such cases.) However if
-a %SIG handler is set the new scheme simply sets a flag and returns as
-described above. This may cause the operating system to try the
-offending machine instruction again and - as nothing has changed - it
-will generate the signal again. The result of this is a rather odd
-"loop". In future Perl's signal mechanism may be changed to avoid this
-- perhaps by simply disallowing %SIG handlers on signals of that
-type. Until then the work-round is not to set a %SIG handler on those
-signals. (Which signals they are is operating system dependent.)
+Certain signals, e.g. SEGV, ILL, and BUS, are generated as a result of
+virtual memory or other "faults". These are normally fatal and there is
+little a Perl-level handler can do with them, so Perl now delivers them
+immediately rather than attempting to defer them.
=item Signals triggered by operating system state
The fork() has to come before the setsid() to ensure that you aren't a
process group leader (the setsid() will fail if you are). If your
system doesn't have the setsid() function, open F</dev/tty> and use the
-C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details.
+C<TIOCNOTTY> ioctl() on it instead. See tty(4) for details.
Non-Unix users should check their Your_OS::Process module for other
solutions.
projects / p5sagit/p5-mst-13.2.git / blobdiff