stdbuf: fix stdio getters in libstdbuf by Ecordonnier · Pull Request #15480 · uutils/coreutils

Ecordonnier · 2026-10-18T15:22:19Z

On Cygwin, __stdbuf_get_stderr returned _stdin, so stdbuf -e changed stdin's buffering ( Introduced in 54ba74b ("Add more Cygwin support", Add more Cygwin support #9686). )
On Apple and FreeBSD, __std{in,out,err}p were declared as functions, but they are variables, so calling them crashed (SIGBUS on macOS).

The new unit test checks that each getter returns its own stream (fd 0, 1, 2).

__stdbuf_get_stderr read _stdin from the reent struct on cygwin, so stdbuf -e changed the buffering of stdin instead of stderr. Introduced in 54ba74b ("Add more Cygwin support", uutils#9686).

Ecordonnier · 2026-10-18T15:23:43Z

@500-internal-server-error FYI

Copilot

🟡 Changes recommended

The bug fix lacks the regression test required by repository guidelines.

1 open issue

Add Cygwin regression test for stderr file descriptor getter · New

What changed in this PR

Fixes Cygwin’s stderr getter so stdbuf -e changes stderr buffering rather than stdin buffering.

Changes:

Returns _stderr instead of _stdin from the Cygwin reentrancy structure.

File	Description
src/uu/stdbuf/src/libstdbuf/src/libstdbuf.rs	Corrects the Cygwin stderr stream pointer.

_{Review effort: Balanced}

500-internal-server-error · 2026-10-18T15:50:58Z

__stdbuf_get_stderr read _stdin from the reent struct on cygwin, so stdbuf -e changed the buffering of stdin instead of stderr.

Whoops, it looks like I forgot to replace this one when copy pasting... apologies :(

Check that the stdin, stdout and stderr getters give file descriptors 0, 1 and 2. This would have caught the cygwin stderr getter returning stdin. Enable the unit tests of libstdbuf for this.

Ecordonnier · 2026-10-18T15:56:53Z

__stdbuf_get_stderr read _stdin from the reent struct on cygwin, so stdbuf -e changed the buffering of stdin instead of stderr.

Whoops, it looks like I forgot to replace this one when copy pasting... apologies :(

No problem. I just thought you may want to cherry-pick this if you have a use-case where you're using stdbuf on cygwin.

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2 open issues

Removing test = false enables unit tests, but the crate is still configured as `crate-type =… · New
Add Cygwin regression test for stderr file descriptor getter

_{Review effort: Lite}

sylvestre · 2026-10-18T21:17:18Z

Seems that the mac CI is sad

__stdinp, __stdoutp and __stderrp are data variables, not functions. Calling them jumped into data and crashed with SIGBUS on macOS.

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5 open issues

With test = false removed, cargo test will now attempt to build this crate’s unit tests. A… · New
Removing test = false enables unit tests, but the crate is still configured as `crate-type =…
The PR description focuses on the Cygwin _stdin vs _stderr bug, but this PR also changes the… · New
This test hard-codes that fileno(stdin/stdout/stderr) must be exactly 0/1/2. While that’s… · New
Add Cygwin regression test for stderr file descriptor getter

_{Review effort: Lite}

xtqqczze · 2026-10-19T12:19:27Z

+            assert_eq!(fileno(__stdbuf_get_stdin()), 0);
+            assert_eq!(fileno(__stdbuf_get_stdout()), 1);
+            assert_eq!(fileno(__stdbuf_get_stderr()), 2);


could also use constants like: libc::STDIN_FILENO

OK. I added a commit using the libc constants.

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5 open issues

With test = false removed, cargo test will now attempt to build this crate’s unit tests. A…
Removing test = false enables unit tests, but the crate is still configured as `crate-type =…
This test hard-codes that fileno(stdin/stdout/stderr) must be exactly 0/1/2. While that’s…
The PR description focuses on the Cygwin _stdin vs _stderr bug, but this PR also changes the…
Add Cygwin regression test for stderr file descriptor getter

_{Review effort: Lite}

Copilot

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Copilot couldn't run its full agentic review because it didn't start before the timeout. Make sure your repository has a runner available, or add a copilot-code-review.yml file specifying one with the runs-on attribute. See the docs for more details.

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7 open issues

With [lib] crate-type = [\"cdylib\"], Rust unit tests often fail to link because the test harness… · New
With test = false removed, cargo test will now attempt to build this crate’s unit tests. A…
Removing test = false enables unit tests, but the crate is still configured as `crate-type =…
These FFI bindings are read-only in this code path, but they are declared as static mut, which… · New
This test hard-codes that fileno(stdin/stdout/stderr) must be exactly 0/1/2. While that’s…
The PR description focuses on the Cygwin _stdin vs _stderr bug, but this PR also changes the…
Add Cygwin regression test for stderr file descriptor getter

_{Review effort: Lite}

 [lib]
 name = "stdbuf"
 path = "src/libstdbuf.rs"
 crate-type = ["cdylib"]


+            static mut __stdinp: *mut FILE;
        }
-        unsafe { __stdinp() }
+        unsafe { __stdinp }


Copilot

🔵 Needs a closer look

No defects were identified, but affected-platform CI results remain unverified for these foreign-function interface fixes.

3 open issues

With [lib] crate-type = [\"cdylib\"], Rust unit tests often fail to link because the test harness…
These FFI bindings are read-only in this code path, but they are declared as static mut, which…
This test hard-codes that fileno(stdin/stdout/stderr) must be exactly 0/1/2. While that’s…

4 resolved since last review

With test = false removed, cargo test will now attempt to build this crate’s unit tests. A…
Removing test = false enables unit tests, but the crate is still configured as `crate-type =…
The PR description focuses on the Cygwin _stdin vs _stderr bug, but this PR also changes the…
Add Cygwin regression test for stderr file descriptor getter

_{Review effort: Balanced}

Ecordonnier · 2026-10-20T07:33:06Z

Seems that the mac CI is sad

The Mac CI is fixed now.

sylvestre · 2026-10-24T12:31:15Z

Thanks for your PR

stdbuf: return stderr from the cygwin stderr getter

42c182f

__stdbuf_get_stderr read _stdin from the reent struct on cygwin, so stdbuf -e changed the buffering of stdin instead of stderr. Introduced in 54ba74b ("Add more Cygwin support", uutils#9686).

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Comment thread src/uu/stdbuf/src/libstdbuf/src/libstdbuf.rs

stdbuf: test that the stream getters return their own streams

f08bb7a

Check that the stdin, stdout and stderr getters give file descriptors 0, 1 and 2. This would have caught the cygwin stderr getter returning stdin. Enable the unit tests of libstdbuf for this.

Copilot AI balanced review requested due to automatic review settings October 18, 2026 15:54

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Comment thread src/uu/stdbuf/src/libstdbuf/Cargo.toml

Copilot stopped reviewing on behalf of Ecordonnier due to an error October 18, 2026 16:25
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stdbuf: read Apple/FreeBSD stdio pointers as statics

62799f7

__stdinp, __stdoutp and __stderrp are data variables, not functions. Calling them jumped into data and crashed with SIGBUS on macOS.

Copilot AI balanced review requested due to automatic review settings October 19, 2026 10:59

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Ecordonnier changed the title ~~stdbuf: return stderr from the cygwin stderr getter~~ stdbuf: fix stdio getters in libstdbuf Oct 19, 2026

stdbuf: use libc constants for stdio fds in test

d0294ce

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Comment thread src/uu/stdbuf/src/libstdbuf/Cargo.toml

[lib]

name = "stdbuf"

path = "src/libstdbuf.rs"

crate-type = ["cdylib"]

Comment thread src/uu/stdbuf/src/libstdbuf/src/libstdbuf.rs

Comment on lines +27 to +29

static mut __stdinp: *mut FILE;

}

unsafe { __stdinp() }

unsafe { __stdinp }

Copilot stopped reviewing on behalf of Ecordonnier due to an error October 19, 2026 14:33
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Merge branch 'main' into eco/stdbuf-cygwin-stderr-getter

7837b7a

Copilot AI balanced review requested due to automatic review settings October 19, 2026 16:31

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sylvestre merged commit dbaeb4c into uutils:main Oct 24, 2026
97 checks passed

Ecordonnier deleted the eco/stdbuf-cygwin-stderr-getter branch October 24, 2026 20:19

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🟡 Changes recommended

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🔵 Needs a closer look

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Ecordonnier commented Oct 20, 2026

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Ecordonnier commented Oct 18, 2026 •

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