Last Updated: 2025-10-29

• The program belows spins up one process for each file argument passed in via ARGV. Yet it also shows constantly updated UI on progress (via repaint). How?
• It uses a pids hash to store the execution state of each process (working or done)
• In order to get a foothold for updating that pids hash, it traps the CHLD signal, which is the signal that occurs when a child process finishes. The code also has to deal with the complication that kernel batches child signals
  • so more than one process may have finished. This complication is dealt with by looping a call to waitpid(-1).
• Then finally the process id is updated to :done in the pids hash
• the WHOHANG argument to waitpid is used to get waitpid to return nil immediately if no processes finished, instead of waiting as is the default.
• Further down in the code (in another process) it loops until all the pids have status of done.

pids = {}

trap("CHLD") do
  while pids.values.include?(:working) &&
        pid = Process.waitpid(-1, Process::WNOHANG)
    pids[pid] = :done
  end
end

start_time = Time.now
FileUtils.mkpath "previews"
ARGV.each do |input_file|
  command = avconv_preview_command(input_file)
  pid = Process.spawn(command)
  pids[pid] = :working
end

until pids.values.all?{|v| v == :done}
  # repaint not shown
  repaint(pids, start_time)
  sleep 0.1
end

Resources

• https://www.rubytapas.com/2016/06/30/episode-423-subprocesses-part-5-sigchld/