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c_abi.md

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Layout requirements on the platform's C ABI

Disclaimer: This chapter summarizes the consensus about Rust's layout requirements on the platforms' C ABI.

These requirement assume that the platform has a C implementation that can be interfaced with. This C implementation is required to satisfy the following requirements.

Integer and floating point requirements

  • CHAR_BITS == 8
  • sizeof(_Bool) == 1
  • true = 1 and false = 0
  • two's complement integers

Layout of structs

See the C-compatible layout ("repr C") section.

Unresolved questions

The unresolved questions that don't contain a link to an issue have no issue open yet. This document shall be updated as issues are resolved and new issues are opened:

  • minimum platform pointer size: should Rust require that the C's platform pointer size must be at lest, e.g., 16-bits wide?

  • float types (32 and 64-bit wide only for now): see [#9]

    • Are these optional?

    • Should we require anything about them beyond what the C standard requires?

    • These probably need to be IEEE754:2018 compliant and we should specify their binary representation.

    • Do we need to specify anything else on top of IEEE754:2018, e.g., with respect to implementation-defined behavior?

  • atomics:

    • Are these optional?

    • Should we require anything about them beyond what the C standard requires?

  • NULL pointer bitpattern:

    • Does the C standard allow platforms without a run-time NULL representation?

    • Some platforms do not have any pointer value that compares equal to NULL. Should we support these platforms? For example, an OS kernel like Linux that wants to use the 0x0 address. Rust is used for these platforms already.

    • Some platforms have pointer values that compare equal to NULL but whose bit representation is not 0x0. Should we support these platforms?

      • That is, should we only support C platforms where the only pointer representation that compares equal to NULL is (void*)intptr_t(0) ?

      • This interacts which the bit-patterns:

        • for which &T and &mut T have niches, and therefore Option<&T> is None
        • <*[mut,const] T>::is_null(self) returns true - note that NonNull does not convey any information about the exact representation of its niche, that is, it is different from NonZero.

  • targets "without C":

    • What should we do about targets whose C implementation is not standard conforming? Do we require full standard conformance? Or only conformance of some specific aspects that are required for FFI ?

    • What should we do about targets that do not have a C implementation at all?