Bug #14009


macOS High Sierra and “fork” compatibility

Added by ticky (Jessica Stokes) almost 6 years ago. Updated about 5 years ago.

Target version:
ruby -v:
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-darwin17]


This was originally discussed on the issue tracker for Puma (, however, it is possible that it would make more sense for inclusion in the Ruby implementation itself.

macOS High Sierra has changed the behaviour of the fork syscall such that initialising Objective-C APIs in forked processes are treated as errors. (see for more details)

This means that many applications which use forking to process concurrently will forcibly crash if the forked process calls out to any Objective-C library when Objective-C was not already initialised in the host process. This includes Puma, Unicorn, iodine and Passenger.

A workaround I proposed for Puma was to implicitly load the Objective-C runtime before performing any forks ( This causes forked processes using other Objective-C APIs to not crash.

The workaround (specific to Puma’s DSL) was:

# Work around macOS 10.13 and later being very picky about
# `fork` usage and interactions with Objective-C code
# see: <>
if /darwin/ =~ RUBY_PLATFORM
  before_fork do
    require 'fiddle'
    # Dynamically load Foundation.framework, ~implicitly~ initialising
    # the Objective-C runtime before any forking happens in Puma
    Fiddle.dlopen '/System/Library/Frameworks/Foundation.framework/Foundation'

A similar fix has now been included in Passenger (

It was, however, proposed that it might make more sense for Ruby on macOS High Sierra and onward to implicitly initialise the Objective-C framework itself, so that forked processes work roughly as expected even if they intend to use Objective-C APIs.

I understand that this is a heavy-handed move, but it seems to me that this relatively common technique will remain broken in Ruby unless everyone deploys a workaround (iodine has already expressed disinterest in doing so) or Ruby adopts one at the higher level.

This issue is also applicable to all Ruby versions which support fork and run on macOS High Sierra.

Thank you for your time. :)

Related issues 1 (0 open1 closed)

Related to Ruby master - Feature #5446: at_fork callback APIClosedkosaki (Motohiro KOSAKI)Actions

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