apple: change BPF_ALIGNMENT to usize

[sbogomolov]

Description

The header defines BPF_ALIGNMENT as sizeof(int32_t), which is usize, not c_int.

Breaking change for anyone matching on the current type, so no stable-nomination.

Sources

Apple BPF_ALIGNMENT (net/bpf.h):

• https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/net/bpf.h#L126

Checklist

• Relevant tests in libc-test/semver have been updated (no new symbols)
• No placeholder or unstable values like *LAST or *MAX are included
• Tested locally (cd libc-test && cargo test --target mytarget)

[xtqqczze]

I think stable nomination is appropriate here since this is a fix for an incorrect type definition rather than an intentional API change.

[sbogomolov]

I think stable nomination is appropriate here since this is a fix for an incorrect type definition rather than an intentional API change.

@rustbot label +stable-nominated

[xtqqczze]

LGTM

View changes since this review

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tgross35]

LGTM. I don't love backporting here - I'd call the fact that they use sizeof in the source more of an implementation detail than confirmation that the type should be size_t. But this will harmonize with FreeBSD and I doubt there are many users not already casting to usize to do something with alignment, so let's do it.

Cc @xingxue-ibm, AIX may want to change BPF_ALIGNMENT to size_t as well.

View changes since this review

[xtqqczze]

I'd call the fact that they use sizeof in the source more of an implementation detail than confirmation that the type should be size_t.

bindgen actually generates this as a u32:

$ bindgen $(xcrun --show-sdk-path)/usr/include/net/bpf.h --clang-macro-fallback | grep BPF_ALIGNMENT
pub const BPF_ALIGNMENT: u32 = 4;

[sbogomolov]

bindgen actually generates this as a u32:

$ bindgen $(xcrun --show-sdk-path)/usr/include/net/bpf.h --clang-macro-fallback | grep BPF_ALIGNMENT
pub const BPF_ALIGNMENT: u32 = 4;

As far as I understand, u32 here is a value heuristic, not type information. For integer macro constants bindgen evaluates the value (which is 4) and then picks a Rust type by magnitude using its default rule, which is the smallest of u32/u64 that fits. In C the expression sizeof(...) will have type size_t.

[xingxue-ibm]

Cc @xingxue-ibm, AIX may want to change BPF_ALIGNMENT to size_t as well.

Thanks, @tgross35! Posted PR #5373 for AIX.