eval {
  local $SIG{ALRM} = sub { die "timeout" };
  alarm($timeout);
  connect(SOCKET, $paddr) || error();
  alarm(0);
};

[download]

What is happening is (most likely) this:  when there's nothing listening on the port in question, the connect() fails more or less immediately ("connection refused") and the (non-existent) routine error() is being called. This dies with "Undefined subroutine &main::error called at...", so the alarm(0) isn't getting executed.  As the latter is supposed to reset the timer set up with the initial alarm($timeout), the timer keeps running and then fires later, outside of the eval{} scope, where the localized $SIG{ALRM} handler is no longer in effect...

In other words, the uncaught ALRM exception (on Windows emulated via SetTimer()/Windows Messages) is being reported as "Terminating on signal SIGALRM(14)".

Simplified demo: 

#!perl

eval {
    local $SIG{ALRM} = sub { die "timeout" };
    alarm(1);
    error();  # not found -> dies
    alarm(0);
};
print "\$@: $@\n" if $@;

# do something that takes longer than a sec,
# without itself being implemented via SetTimer()
rand for 1..2e7;

__END__
H:\PM>perl 759131.pl
$@: Undefined subroutine &main::error called at 759131.pl line 6.

Terminating on signal SIGALRM(14)

[download]

Solution:  either move the alarm(0) outside of the eval{} (so it's always being executed), or set $SIG{ALRM} ='IGNORE' outside of the eval{} to ignore the timer 'signal'.

In reply to Re: Detecting a port "in use" on localhost:$port by almut
in thread Detecting a port "in use" on localhost:$port by freddo411

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