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Unsafe extern blocks

Summary

Details

Rust 1.82 added the ability in all editions to mark extern blocks with the unsafe keyword.1 Adding the unsafe keyword helps to emphasize that it is the responsibility of the author of the extern block to ensure that the signatures are correct. If the signatures are not correct, then it may result in undefined behavior.

The syntax for an unsafe extern block looks like this:

#![allow(unused)]
fn main() {
unsafe extern "C" {
    // sqrt (from libm) may be called with any `f64`
    pub safe fn sqrt(x: f64) -> f64;

    // strlen (from libc) requires a valid pointer,
    // so we mark it as being an unsafe fn
    pub unsafe fn strlen(p: *const std::ffi::c_char) -> usize;

    // this function doesn't say safe or unsafe, so it defaults to unsafe
    pub fn free(p: *mut core::ffi::c_void);

    pub safe static IMPORTANT_BYTES: [u8; 256];
}
}

In addition to being able to mark an extern block as unsafe, you can also specify if individual items in the extern block are safe or unsafe. Items marked as safe can be used without an unsafe block.

Starting with the 2024 Edition, it is now required to include the unsafe keyword on an extern block. This is intended to make it very clear that there are safety requirements that must be upheld by the extern definitions.

1

See RFC 3484 for the original proposal.

Migration

The missing_unsafe_on_extern lint can update extern blocks to add the unsafe keyword. The lint is part of the rust-2024-compatibility lint group which is included in the automatic edition migration. In order to migrate your code to be Rust 2024 Edition compatible, run:

cargo fix --edition

Just beware that this automatic migration will not be able to verify that the signatures in the extern block are correct. It is still your responsibility to manually review their definition.

Alternatively, you can manually enable the lint to find places where there are unsafe blocks that need to be updated.

#![allow(unused)]
fn main() {
// Add this to the root of your crate to do a manual migration.
#![warn(missing_unsafe_on_extern)]
}