vxworks: add cfg to definition of off64_t and off_t

[dybucc]

Description

This PR improves the definition of off_t. This applies to targets whose operating system is VxWorks.

The libc crate defines off_t as a c_longlong. This is only correct in programs compiled as RTPs. Programs compiled as VxWorks kernels are different. They expose off_t as a c_long.

The off64_ type is not present in RTP processes.

Automatic detection of this does not seem possible in Rust. A specific flag is issued to gcc to expose the feature test macro. This patch adds an equivalent cfg. The crate user must manually set it.

Two redundant routines were removed. These are explicitly out of the shipped SDK. It seems like the libc crate had polyfills set up. These would always return an error value.

Sources

• Lines 23-33 of file sysroot/usr/h/public/base/b_off_t.h. This was found in the VxWorks downloadable SDK for QEMU x86_64.
• File usr/3pp/develop/usr/include/python3.9/pyconfig.h. This was found in the same as SDK as quoted above. This defines certain included features. pread and pwrite are not defined.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget); especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated

[dybucc]

CI actually passes. There seems to be an issue with a glob import that is not used, but this has not
been changed in the patch (it's not even part of it, for that matter.) For some reason, rebasing
onto main with dependabot updates has ended up with a warning across all of my open PRs due to
that one (now apparently unused) import.

[tgross35]

@rustbot blocked
@SnoozeThis wait 2 months -> remove label S-blocked, add label S-waiting-on-review

Holding off until there is a good alternative to off64_t

[SnoozeThis]

(https://snoozeth.is/Pf0pTEeZDMs) I will wait until Wed, 19 Aug 2026 08:31:52 UTC and then add label S-waiting-on-review and remove label S-blocked.

@rustbot claim.

[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.

[dybucc]

@tgross35 The PR has been updated. I don't think I can find a better solution. Both off_t and off64_t are defined in VxWorks. The best I can do is to gate them.

off_t will always be defined. Its bit width may be 64-bits in RTPs. Its bit width may be 32-bits or 64-bits in kernel programs. off64_t will only be defined in kernel programs. off64_t will always be 64-bits wide.