Fork and open files

Prelude: Writing a file in ruby

File 05_fork_and_open_files/write.rb:

#!/usr/bin/env ruby

File.open "write.out", "w" do |file|
  file.puts "I am process #{Process::pid}."
end

What do you expect?

will be the content of write_conflict.out after this:

File 05_fork_and_open_files/write_conflict.rb:

#!/usr/bin/env ruby

File.open "write_conflict.out", "w" do |file|
  file.puts "I'm the parent, #{Process.pid}."
  child_pid = fork
  if child_pid.nil?
    file.puts "I'm the child, #{Process.pid}."
  else
    Process::wait(child_pid)
    if $?.success?
      file.puts "Child ok."
    else
      file.puts "Child unhappy."
    end
  end
end

It is:

I'm the parent, 24778.
I'm the child, 24780.
I'm the parent, 24778.
Child ok.

WTF?

The call to file.puts does not actually write, but buffers. The buffer gets copied by the fork and so gets written later.

To fix, flush the buffer.

If you absolutely want to, use syswrite (which does not buffer).