Recipe

Declare and call a C function from Rust

Use when: a Rust program needs a function from a C library, and you have its prototype from a header or its documentation

Goal: a correct declaration, called from one small safe Rust function

Taught in Calling C from Rust

1. 1

   Find the function's prototype in the C header, such as size_t strlen(const char *s);.

   Example

   double hypot(double x, double y);

   Why: the header is the only source of truth for the ABI, and Rust will believe whatever you write.

2. 2

   Replace every C type with its Rust match: c_int, c_double, usize for size_t, *const c_char for const char *.

   Example

   fn hypot(x: c_double, y: c_double) -> c_double; inside unsafe extern "C" { }.

   Why: only FFI-safe types have the size and layout C expects; the improper_ctypes lint catches the obvious misses.

3. 3

   Mark it safe fn only if every possible argument value is valid input; otherwise leave it unsafe.

   Example

   safe fn hypot(...), but plain fn strlen(...), because a pointer can be bad.

   Why: a safe fn can be called from safe code, so it must not be able to cause undefined behavior.

4. 4

   Write a safe Rust function whose parameter types guarantee the C function's requirements, with a // SAFETY: comment on the unsafe block.

   Example

   fn c_len(s: &CStr) -> usize { unsafe { strlen(s.as_ptr()) } }

   Why: the rest of the program then calls safe Rust, and the unsafe reasoning lives in one place.