vxworks: add `cfg` to definition of `off64_t` and `off_t` by dybucc · Pull Request #5129 · rust-lang/libc

dybucc · 2026-05-31T17:18:51Z

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 · 2026-06-09T07:32:47Z

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 · 2026-06-19T08:31:52Z

@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 · 2026-06-19T08:32:56Z

(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.

dybucc · 2026-06-21T15:48:56Z

@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.

@rustbot ready review

tgross35 · 2026-06-22T23:36:30Z

-pub type off_t = c_longlong;
-pub type off64_t = off_t;
+cfg_if! {
+    if #[cfg(vxworks_rtp)] {
+        pub type off_t = c_longlong;
+    } else {
+        pub type off_t = c_long;
+        pub type off64_t = c_longlong;
+    }
+}


Given the existing definitions seem to match rtp, could there be an assumption that vxworks builds are always targeting RTPs rather than kernel? I don't see it documented here https://doc.rust-lang.org/rustc/platform-support/vxworks.html but that's usually a reasonable assumption for most targets.

Cc maintainers @biabbas @hax0kartik

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hax0kartik · 2026-06-23T03:01:06Z

hi, thanks for the PR but we do not support rust in VxWorks kernel mode. I’m going to reject this change as the platform maintainer.

tgross35 · 2026-06-23T03:11:47Z

Ack, thanks for taking a look!

@dybucc this can probably be repurposed as a docs PR to help others in the future. We don't really have anything like this yet but I think the new module would be a reasonable place to keep platform-specific docs, https://github.com/rust-lang/libc/blob/main/src/new/vxworks/mod.rs.

@rustbot author
@SnoozeThis /cancel

rustbot · 2026-06-23T03:11:51Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

SnoozeThis · 2026-06-23T03:27:42Z

Snooze cancelled.

@rustbot release-assignment.

rustbot · 2026-06-23T13:30:41Z

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.

VxWorks compiles to RTPs and kernels. RTPs do not expose `off64_t`. Rust can not be used with VxWorks kernels. `off64_t` should not be part of the crate. Two polyfill routines were removed. They were stubs for errors. They did not set up `errno` either.

dybucc · 2026-06-23T13:36:04Z

Ack, thanks for taking a look!

@dybucc this can probably be repurposed as a docs PR to help others in the future. We don't really have anything like this yet but I think the new module would be a reasonable place to keep platform-specific docs, https://github.com/rust-lang/libc/blob/main/src/new/vxworks/mod.rs.

@rustbot author @SnoozeThis /cancel

That's done now. The PR documents that. Kernel symbols are deprecated. This includes off64_t. This is not exposed in RTPs.

@rustbot ready

tgross35 · 2026-06-23T17:59:05Z

+    note = "Use `off_t` instead. This type does not exist. It's only present in VxWorks kernels. \
+            Those are unsupported in Rust."


Simplify to something like "Kernel-mode definitions are not supported in VxWorks. Use off_t instead.". Otherwise LGTM.

View changes since the review

tgross35 · 2026-06-23T18:02:47Z

+//! VxWorks allows compiling different types of programs. Exposed bindings are
+//! with respect to RTPs. VxWorks kernel interfaces are not exposed. `off_t` is
+//! an example of this. It's a `c_long` in kernels. It's a `c_longlong` in RTPs.


//! VxWorks allows compiling different types of programs but `libc` only supports //! RTPs (real-time processes), not kernel interfaces. Bindings should match RTP //! definitions when there is a discrepancy.

Dropping the concrete example

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rustbot added the S-waiting-on-review label May 31, 2026

dybucc force-pushed the vxworks-bit-width-transition branch 2 times, most recently from fb9aeb7 to 675bad7 Compare June 4, 2026 07:05

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rustbot added the stable-nominated This PR should be considered for cherry-pick to libc's stable release branch label Jun 9, 2026

dybucc force-pushed the vxworks-bit-width-transition branch from 3d55d66 to 04a0d17 Compare June 15, 2026 15:16

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dybucc force-pushed the vxworks-bit-width-transition branch from 1d99a12 to 0b3f97d Compare June 21, 2026 15:33

dybucc changed the title ~~refactor: deprecate off64_t in VxWorks~~ vxworks: add cfg to definition of off64_t and off_t Jun 21, 2026

rustbot added S-waiting-on-review and removed S-blocked labels Jun 22, 2026

tgross35 reviewed Jun 22, 2026

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rustbot added S-waiting-on-author and removed S-waiting-on-review labels Jun 23, 2026

dybucc force-pushed the vxworks-bit-width-transition branch from 0b3f97d to ef3abc5 Compare June 23, 2026 13:30

dybucc force-pushed the vxworks-bit-width-transition branch from ef3abc5 to df963e7 Compare June 23, 2026 13:35

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tgross35 reviewed Jun 23, 2026

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		note = "Use `off_t` instead. This type does not exist. It's only present in VxWorks kernels. \
		Those are unsupported in Rust."

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hax0kartik commented Jun 23, 2026

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