[rbanffy]

That's why I wrote "it doesn't introduce anything beyond the source code into the binary". Unfortunately, I forgot to emphasize the anything.

A build process that names things with timestamps or leaks your locale into the build configuration (or doesn't pin build-time dependency versions) will make the build depend on things other than the source code (both program and build settings) you made available.

It may even be desirable for it to be non-reproductible - if, for instance, you want to use optimizations targeted to your specific system, then your build system will have to introduce the architecture information into the build process and your build will result in a unique binary that targets your own machine.

[rfoo]

Unfortunately, if we take this definition of "anything" literally, it is impossible to build such a build system.

For example, depending on the input order, linker may produce different output. Surely you can sort the object files, but the sorted object files order is still effectively "stored" into the binary, and that's not source code.

You can only normalize such things (like in the example above, sorting), you can not eliminate them, they naturally exist.

[rbanffy]

> you can sort the object files, but the sorted object files order is still effectively "stored" into the binary, and that's not source code.

No, but the order should be explicitly defined in the build scripts or the result will not be deterministic.

If the order triggers, say, a linker bug that makes one in 50 builds crash, execution will not be deterministic and that's really, really bad.

[vishvananda]

This is actually an annoying challenge of reproducible builds. In many cases it is actually useful to have a build timestamp, git sha, or build number available for debug output from the program. I've often gone as far as embedding a sha and/or timestamp into a file on export into a tgz which allows it to be reproducible from the tarfile, although builds directly out of source control would not be.

[maccam94]

Git hashes can be inserted in reproducible builds, they are deterministic.