Crate rucc_headers 

Derives a multi-version libc header tree from real installed header sets.

Design: spec/cross-compile/08-sysroots.md section 8.3, which calls the headers the hard half of a sysroot and says why. A glibc .so is a few hundred kilobytes of generated stubs and nothing equivalent exists for /usr/include, because headers are program text that has to be the same text the target’s libc was built against. Shipping a full copy per architecture per supported release is several hundred megabytes against document 13’s budget, so the per version differences go inside the files as conditionals on __GLIBC_MINOR__ and one tree serves every release. Zig’s generic-glibc is that artifact and ziglang/universal-headers is the technique; document 01.2 records both, and records that the second of them has been unfinished for years.

What this is and is not

It is the derivation, and nothing else. Given the headers that glibc installs for each release in the support table, it writes one tree in which every release’s text can still be read out, and it checks that claim for every file before it writes anything. It does not fetch glibc, build it, install its headers or decide which releases matter. Those are the producer’s job and the producer is tamnd/rucc-cross, for the reason section 8.3 gives: installing a libc means fetching a libc, and a compiler checkout should not download one to run its tests.

It is a build tool rather than a crate of the compiler because nothing in a compilation reads it. The compiler’s half of this is two lines in rucc-sysroot, the directory search order and bundled_glibc_minor, and they are there already.

The four pieces

• norm cuts a header into the pieces a conditional can go between, and says what makes two copies of one header the same. They are the same when their code is the same, because glibc moves a copyright year in every file every January and that is not a version difference.
• diff lines two releases of one header up.
• merge writes one file out of all of them, and reads it back to check it.
• tree does that for every file in every release and counts what happened.

How to run it

cargo run -q -p rucc-headers -- --release 2.28=<dir> --release 2.44=<dir> --out <dir>

Each directory is the one a release’s headers are under, so the one usr/include is in an install ends at. The releases have to be given oldest first, and there have to be at least two, because merging one release is copying it.

Modules

cond

The conditionals the merge writes, and reading them back.

diff

Lining two releases of the same header up, so the merge can see what moved and what did not.

merge

One header, merged across every release that has it.

norm

What counts as a change to a header, and what a header is cut into before it is merged.

tree

Every file of every release, merged into one tree, with the numbers that come out of doing it.