Index :

This is an old unix syscall very useful to implement using signals. It's not for some kind of timeout, but rather trigger some code after a defined amount of time. With no need of a loop. This is basically an asynchronous timer provided by the linux kernel. So if it's in the kernel, why reimplementing it in userland ?

I couldn't find any references using the alarm syscall in ruby. Very few resources came up with these keywords. All solutions proposed on various forums are using sleep within a thread, like this :

trap "ALRM" do
    puts "Alarm received!"
end

def alarm
  Thread.new do
      sleep 5
      Process.kill "ALRM", $$
  end
end

But this does not provide the same behavior ! Take this example :

alarm(5)
sleep(4)
alarm(5)

With the above thread implementation, the alarm will be triggered twice, once at 5 sec, and another one at 9 sec. With the real alarm(), the signal would be trigerred only once, at 9 seconds. See man alarm for more details.

The solution

The syscall alarm(2) is part of the standard libc. There is way in ruby to direclty call some C function from dynamic libraries. Here's a good articles about it : https://aonemd.github.io/blog/making-system-calls-from-ruby

I did some test with Fiddle, unfortunately the documentation is not very abundant but after a bit of fiddling around with that lib, I succeeded to find a working setup. A direct call to the syscall function should work but very hardcore, and I could'nt find the syscall_number for alarm() on my ARM system. You can find it here for x86_64 system though : https://github.com/torvalds/linux/blob/v3.13/arch/x86/syscalls/syscall_64.tbl

But we can call directly the alarm syscall :

require 'fiddle'

libc = Fiddle.dlopen('/lib/arm-linux-gnueabihf/libc.so.6')
alarm = Fiddle::Function.new(libc['alarm'], [Fiddle::TYPE_INT], Fiddle::TYPE_INT)

trap 'ALRM' do
  puts "alarm received !"
end

alarm.call(2)

This solution is not ideal, as you need to locate the libc.6.so file but it does the job. In that example I'm using Rasbian on a Raspberry pi, hence the armhf architecture. You can locate yours with ldconfig -p | grep libc.so.