The C ABI

Level: 201 · working knowledge

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: C and Rust meet at the binary level, not the source level — a symbol name in an object file, and a calling convention both compilers follow — so an extern "C" declaration is a promise about a function the Rust compiler never sees, and a wrong signature is undefined behaviour rather than an error.

What it has to cover

• What an ABI fixes that an API does not: argument passing, return values, which registers survive a call, struct layout
• Calling C from Rust: an unsafe extern "C" block written by hand, and why edition 2024 put the unsafe on the block — the promise is made at the declaration
• Calling Rust from C: extern "C" fn plus #[unsafe(no_mangle)], and the mangled name you get without it, read with nm
• #[repr(C)]: when a struct needs it (whenever its layout crosses), and when it does not (a pointer to it is all C ever sees)
• The linker's part: an undefined symbol at link time against a mismatched symbol that links and then misbehaves
• A wrong signature, on purpose: declare a C function with one argument too few or an i32 where C returns a double, and record what happens — no compiler error, and output that depends on the machine
• The improper_ctypes lints, and which mistakes they do and do not catch

The trap it exists for

Treating the extern block as checked. Rust takes the declaration on trust; the header it was copied from can change, and nothing between the two will notice.

See also

• Calling C — the round trip, verified, including edition 2024's unsafe extern
• The linker and Targets and triples — who resolves the symbol, and which ABI a target means
• Building and linking — the next page
• What the symbol table lists ↗ — symbols from C's side, in the C learning library
• External blocks ↗ and ABI ↗ in the Reference

Po polsku

C i Rust spotykają się na poziomie binarnym, nie źródłowym: łączy je nazwa symbolu w pliku obiektowym i konwencja wywołań (calling convention), której przestrzegają oba kompilatory. Blok extern "C" to obietnica złożona kompilatorowi w sprawie funkcji, której on nigdy nie widzi — dlatego błędna sygnatura nie daje błędu kompilacji, tylko zachowanie niezdefiniowane.

Szukaj po polsku: interfejs binarny C · konwencja wywołań · rust extern c no_mangle · rust repr c struct ffi