Question

I am accessing a C function in Ruby through FFI. The function is called AllocTilts::summary.

I want the method to not print anything to STDOUT. However my temporary redirect of STDOUT is not working. Is there something else I can do?

puts 'test outside before' #prints successfully
File.open("/var/alloc_tilts/summary_dump", "w") do |out|
  stdout, $stdout = $stdout, out
  puts 'test inside' #doesn't print to STDOUT as expected
  AllocTilts.summary(2012, 2011) #prints undesired stuff to STDOUT
  $stdout = stdout
end
puts 'test outside after' #prints successfully

Answer 1

How do AllocTilts.summary write to stdout? If it uses printf and you do not have access to its source code there is nothing you can do (short of calling libc dup).

The way ruby writes to $stdout rather than the real stdout, is

# from io.c

rb_funcall2(rb_stdout, rb_intern("puts"), argc, argv);

You should use similar code to generate output inside AllocTilts.summary.