windows: expose `cfg` for 64-bit `time_t` by dybucc · Pull Request #5062 · rust-lang/libc

dybucc · 2026-04-16T16:11:07Z

Description

Under Windows GNU x86, time_t doesn't have the right size and alignment. This is a backwards-incompatible change, so for folks to experiment on stable, a cfg has been exposed to allow exposing a 64-bit time_t instead. Tests don't fail because we still skip time_t and records containing some value of such type whenever we're under this target platform/environment.

In #5059, we address the issue with routines involving time_t values by making them link to their 32-bit symbols on that platform/environment.

This should allow not breaking existing code (even though incorrect,) while still allowing new code to experiment with correct size/alignment for time_t values. The cfg tries to follow the new practices introduced in #4977.

An additional job was not added to the test matrix in CI because it was instead introduced as part of the tests in ci/run.sh that run only under 32-bit machine word and GNU-hosted targets.

Sources

None, if any the same as #5032 as this stems from it.

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 · 2026-06-26T09:27:50Z

Seems done. We don't check for time_t on Windows x86 GNU if gnu_time_bits64 is not set.

@rustbot ready

Adds a test to ensure the changes in rust-lang#5062 are correct. The test ensures the wrong definition is kept on stable. It will use the right bit width in the following cases. - Under targets other than x86 with GNU. - If the `gnu_time_bits64` `cfg` is set.

rustbot · 2026-06-27T15:00:42Z

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 · 2026-06-27T16:50:52Z

I had to add a cfg in the libc-test build script. The Windows test added on #5059 needs it with the changes in this PR.

I'm not sure if this is the way to go.

tgross35 · 2026-06-27T23:16:22Z

+    // Corresponds to `_TIME_BITS=64` in glibc. Also used in Windows to fix
+    // faulty `time_t` bindings.
    "gnu_time_bits64",


Try to be more precise with wording like faulty/wrong/broken. In this case there should be nothing broken in Rust since the link name fix merged, it's just a mismatch to what users get on C. (Applies to the commit message and a few other PRs too).

Also 64-bit time_t wasn't even always an option until last year mingw-w64/mingw-w64@fe21b7e and uclibc a few years before that - libc's bindings just happen to predate that. (Even glibc and musl support didn't fully exist until ~2020.)

View changes since the review

tgross35 · 2026-06-27T23:17:52Z

        "ssize_t" if !gnu => true,
        // FIXME(windows): The size and alignment of this type are incorrect
-        "time_t" if gnu && i686 => true,
+        "time_t" if gnu && i686 && !redirect_time_t => true,


I think you may be able to delete the time_t exclusion entirely with:

if i686 && !redirect_time_t { cfg.define("_TIME_BITS", Some("32")); }

View changes since the review

Doesn't seem like it. It may be that the MinGW pre-defined macros target the x86_64 host even if the binary targets x86. That would make the conditional in ¹ evaluate false.

Footnotes

https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/crt/_mingw.h.in#L393 ↩

Use `gnu_time_bits64` on Windows x86 GNU to expose a 64-bit `time_t`. `time_t` is always 64-bits on Windows. The target environment matters not. There's a feature test macro to make it 32-bits wide. In `libc` we expose it as 32-bits in Windows x86 GNU. This is not the same default as in C. This `cfg` exposes the same default as in C.

tgross35

LGTM, thanks!

View changes since this review

Use `gnu_time_bits64` on Windows x86 GNU to expose a 64-bit `time_t`. `time_t` is always 64-bits on Windows. The target environment matters not. There's a feature test macro to make it 32-bits wide. In `libc` we expose it as 32-bits in Windows x86 GNU. This is not the same default as in C. This `cfg` exposes the same default as in C. (backport <rust-lang#5062>) (cherry picked from commit 89612a2)

rustbot added A-CI Area: CI-related items S-waiting-on-review stable-nominated This PR should be considered for cherry-pick to libc's stable release branch labels Apr 16, 2026

dybucc mentioned this pull request Apr 16, 2026

Deprecate windows time64_t #5032

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3 tasks

dybucc force-pushed the fix-windows-timet-cfg branch from 08f37e4 to 137efa8 Compare April 18, 2026 06:38