Reproducible builds does not mandate that a given piece of source code is turned into the same bytes in all situations. This would be unfeasible. The output of a compiler is likely to be different from one version to another as better optimizations are integrated all the time.

Instead, reproducible builds happen in the context of a build environment. It usually comprises the set of tools, required versions, and other assumptions about the operating system and its configuration. A description of this environment should typically be provided alongside any distributed binary package.

Requirements

What exactly makes up the build environment is going to be different for each project. There might even be several build environments for a single release to accommodate different target operating systems. But there are some important aspects common to all environments.

It should be easy to install a matching build environment on their system. Ideally it should only be made of free software available on public Internet sites. The best way to provide the environment is probably using a documented and easily understood script.

It should be auditable. It must be easy to understand what tools are part of the build environment. And ideally it must be easy to review and rebuild them.

Content

The scope of the build environment needs to be properly specified as this will determine how much of the build system needs to be deterministic.

At least the defined environment has a list of the tools used by the build process and their versions.

The rest can be different from one project to the next, as long as it can be reproduced by interested users. To give some examples:

  • specific operating system (if cross-compiling is not supported),
  • build system architecture (if cross-compiling is not supported),
  • directory where the build must happen,
  • name of the user running the build,
  • locale,
  • timezone,
  • specific environment variables (like SOURCE_DATE_EPOCH).

Using virtual machines or containers as the recommended build environment can make it easier to ensure a specific operating system or user configuration. But they might also hide some assumptions on the environment, like specific optimizations enabled because of the system CPU type.