Stdout redirection with dup2 + tempfile works in main.rs but fails in #[test]

Stdout redirection with dup2 + tempfile works in main.rs but fails in #[test] — why?

⚓ Rust 📅 2025-05-29 👤 surdeus 👁️ 10

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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Stdout redirection with dup2 + tempfile works in main.rs but fails in #[test] — why?

I'm working on a logging system in Rust where I want to capture the output of a macro that writes to stdout (via println!) and validate it in tests.

To capture output, I'm using a dup2 + tempfile + libc::STDOUT_FILENO strategy that redirects stdout to a temporary file. It works perfectly when invoked from main.rs, but fails when the exact same code is run inside a #[test] function.

I’m not using io::set_output_capture

Even with --nocapture, stdout seems not to flow to the redirected output

My macro prints correctly, but the capture shows ""

Has anyone else run into this? Is the test harness doing some internal redirection or buffering of stdout before my dup2 has a chance to take effect?

Here's a minimal example (with main.rs and tests/test-main.rs) to reproduce.

Rust version: rustc 1.86.0 (05f9846f8 2025-03-31)

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Appreciate any pointers. Thanks!

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