Bug #21995: mkmf generates a non-deterministic log file

• Status: Assigned
• Assignee: hsbt (Hiroshi SHIBATA)
• Target version: -
• ruby -v: ruby 4.1.0dev (2026-04-12T14:25:33Z master 6593cc52d6) +PRISM [x86_64-linux]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[gemmaro (Gemma Kosaka)]

Hello,

When compiling Ruby's C extensions with mkmf, it can generate a mkmf.log file with non-deterministic content, especially at the line below:

ld: /tmp/rubytest.h8pay9/cc3fbuGm.o: in function `t':
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This can be problematic on some distributions/package-managers, since the mkmf.log file is included in the files generated by gem install.

example flowexample flow

Note about an example flow where randomized filenames occur:

1. extconf.rb has calls to have_func
2. gem install is run by the user
3. It invokes mkmf to process the extconf.rb file
4. The checks fail and print intermediate randomized filenames, which are recorded in mkmf.log file
5. gem install includes mkmf.log in the packaged files
6. When the gem is built/packaged multiple times, a reproducibility problem occurs

backgroundbackground

As a background, the previously reported/related issues are:

• Bug #15304: Package build with extensions is not reproducible

  • This mentions temporary filenames in the generated Makefile, and they are not reproducible.

  • The necessity/value of including Makefile, gem_make.out, and mkmf.log files are questioned.

  • So the pull request was created to omit these files when installing gems.

  • However, deleting these files might break compatibility, so the pull request was closed.

  • Note: There is an entry about mkmf.log in the RubyGems' changelog:

    2.2.2 / 2014-02-05

    Bug fixes:

    • [...]
    • The mkmf.log is now placed next to gem_make.out when building extensions.

• Bug #19329: "gem install" installs logs and other "noisy", non-reproducible files

  • This points out that gem install installs mkmf.log file.
  • It was transferred at RubyGems GitHub repository.

• There are some issues related to this topic on the GNU Guix channel

  • For example, when I was packaging Ruby 4.0 for Guix, this reproducibility problem occured.

• There should be other workarounds in the Nix community, apparently.

To wrap up, I think the followings are desirable:

• Conventional file locations should be kept as is on the RubyGems side.
  • Just deleting these files breaks compatibilities.
  • These might be helpful when users report issues to gem owners with detailed information.
• It is helpful if generated file contents are as reproducible as possible.
  • In some contexts where reproducibility matters, this issue makes packaging Ruby gems a bit harder.
  • Luckily, workarounds are possible in distribution package recipes such as deleting these files.
• -> the component that generates mkmf.log is mkmf, so it might be resolvable by modifying mkmf itself.

Thank you,
gemmaro.

[gemmaro (Gemma Kosaka)]

I created a pull request about this.

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned
• Assignee set to hsbt (Hiroshi SHIBATA)

I will look this.

[gemmaro (Gemma Kosaka)]

Thanks for the reply! I'm sorry -- this pull request #note-1 currently fails CI, and I'm still investigating why. It's not ready for review yet, so it would be appreciated if you could revisit when it is.

[Hanmac (Hans Mackowiak)]

i think the main question is:

Why is mkmf.log file is included in the files generated by gem install?

Shouldn't it be okay to skip these files?
Especially for distributions/package-managers?