Unsafe External Functions

Functions in a foreign language may also be unsafe:

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Speaker Notes
  • Rust used to consider all extern functions unsafe, but this changed in Rust 1.82 with unsafe extern blocks.
  • abs must be explicitly marked as safe because it is an external function (FFI). Calling external functions is usually only a problem when those functions do things with pointers which which might violate Rust’s memory model, but in general any C function might have undefined behaviour under any arbitrary circumstances.
  • The "C" in this example is the ABI; other ABIs are available too.
  • Note that there is no verification that the Rust function signature matches that of the function definition – that’s up to you!