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add a warning about using safe on extern c-variadic functions #1839

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merged 2 commits into from
Jun 4, 2025
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add a warning about using safe on extern c-variadic functions #1839

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f02d746
Add warning about the safety of extern c-variadic functions
folkertdev Jun 3, 2025
c13d2d2
Revise language of warning about safe variadics
traviscross Jun 4, 2025
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6 changes: 6 additions & 0 deletions src/items/external-blocks.md
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Original file line number Diff line number Diff line change
Expand Up @@ -179,9 +179,15 @@ unsafe extern "C" {
unsafe fn foo(...);
unsafe fn bar(x: i32, ...);
unsafe fn with_name(format: *const u8, args: ...);
// SAFETY: This function guarantees it will not access
// variadic arguments.
safe fn ignores_variadic_arguments(x: i32, ...);
}
```

> [!WARNING]
> The `safe` qualifier should not be used on a function in an `extern` block unless that function guarantees that it will not access the variadic arguments at all. Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].

r[items.extern.attributes]
## Attributes on extern blocks

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