nim-ffi

Expose a Nim library to C, C++ and Rust by annotating ordinary Nim procs.

You write async Nim; nim-ffi provides the whole FFI runtime — a dedicated worker thread, a request channel, CBOR (de)serialization, an event queue and a context/handle registry — and generates the foreign-language bindings for you. No hand-written .h files, no manual request enums, no shared-memory plumbing.

Mental model

• You declare a library once with declareLibrary(name, LibType).
• You annotate procs and types with pragmas ({.ffi.}, {.ffiCtor.}, {.ffiDtor.}, {.ffiEvent.}).
• You call genBindings() last, which emits the foreign bindings.

By default, every request/response crosses the boundary as a single CBOR blob (the wire format is configurable per library or per annotation — see ABI format); the ctx handle returned by the constructor is the only pointer that crosses. Each {.ffi.} proc runs on the library's own chronos event loop, so bodies can await freely.

Minimal example

import ffi, chronos

# 1. The library's main state. The FFI context owns one instance.
type Counter = object
  value: int

declareLibrary("counter", Counter)

# 2. Request/response shapes. Any {.ffi.} object type becomes a first-class
#    struct/class in the generated bindings and rides the wire in the library's
#    ABI format (CBOR by default).
type BumpRequest {.ffi.} = object
  by: int

type BumpResponse {.ffi.} = object
  newValue: int

# 3. Constructor: returns Future[Result[LibType, string]].
proc counterNew*(): Future[Result[Counter, string]] {.ffiCtor.} =
  return ok(Counter(value: 0))

# 4. A method: first param is the library value, then any typed params.
#    Return type is always Future[Result[T, string]].
proc counterBump*(
    c: Counter, req: BumpRequest
): Future[Result[BumpResponse, string]] {.ffi.} =
  await sleepAsync(1.milliseconds)
  return ok(BumpResponse(newValue: c.value + req.by))

# 5. Destructor: exactly one param (the library value).
proc counterDestroy*(c: Counter) {.ffiDtor.} =
  discard

# 6. genBindings() must be the LAST FFI call in the file (see below).
genBindings()

The generated C export names are the snake_case form of the proc names, e.g. counterBump → counter_bump.

Pragma reference

Pragma	Applies to	Purpose
declareLibrary(name, LibType[, defaultABIFormat])	call	Registers the library, its state type, and the default wire format. Must run before any annotation.
{.ffi.} on a type	object	Registers the type for binding generation; it serializes via the library's ABI format (CBOR by default).
{.ffi.} on a proc	proc	Exposes a method. First param is the library value, then typed params; returns Future[Result[T, string]].
{.ffiCtor.}	proc	The constructor. Returns Future[Result[LibType, string]]; creates the FFI context.
{.ffiDtor.}	proc	The destructor. Exactly one param (x: LibType); tears the context down.
{.ffiEvent[: "wire_name"].}	proc (empty body)	A library-initiated callback. Call the proc from any {.ffi.} handler to fire it. The wire name is optional — see below.
{.ffiHandle.}	ref object	Marks a type as an opaque handle: it stays server-side and crosses the wire as a uint64 id.
genBindings()	call	Emits the bindings. Must be the last FFI call in the compilation root.

The return-type contract

Every {.ffi.} / {.ffiCtor.} proc must have an explicit Future[Result[T, string]] return type — even for synchronous logic (just return ok(...) without awaiting). The Result's error string is delivered to the foreign caller as the failure message.

Events

An event is a proc with an empty body annotated {.ffiEvent.}. You fire it by calling it with a typed payload from inside any {.ffi.} handler; the foreign side receives it through a registered callback.

type PeerConnected {.ffi.} = object
  id: string

proc onPeerConnected*(peer: PeerConnected) {.ffiEvent.}  # wire name: "on_peer_connected"

proc counterBump*(c: Counter, req: BumpRequest): Future[Result[BumpResponse, string]] {.ffi.} =
  onPeerConnected(PeerConnected(id: "p-1"))
  return ok(BumpResponse(newValue: c.value + req.by))

The wire name is optional: when omitted it is derived from the proc name (onPeerConnected → on_peer_connected), matching how {.ffi.} derives its C export symbol. Pass a string literal ({.ffiEvent: "custom_name".}) only when you need a name that differs from the proc.

ABI format

The default wire format is cbor. Override the library default with declareLibrary("lib", Lib, defaultABIFormat = "c"), or per annotation with an "abi = ..." spec, e.g. {.ffi: "abi = c".}.

An abi = c proc whose whole signature is scalar — fixed-width integer, float, or bool params (a string return is fine, a string param is not) and no structs, handles, or pointers — dispatches through a CBOR-free scalar fast path. Foreign-binding codegen for that shape isn't implemented yet, so under -d:ffiGenBindings such a proc would be omitted from the generated bindings — and genBindings() fails with an error naming the affected procs. Resolve it by switching the proc to abi = cbor, adding a non-scalar param so it takes the CBOR wire shape, or passing -d:ffiAllowScalarSkip to accept the omission (the proc still works over the scalar fast path; it's just absent from the generated foreign bindings).

Placement of genBindings()

genBindings() reads the compile-time registries that the pragmas populate as the compiler expands them, so it must come after every annotation. Since Nim resolves imports before running the importing module's body, a multi-file library keeps its annotations in imported sub-modules and calls genBindings() once at the bottom of the top-level root file.

An annotation that expands after genBindings() is now a compile error (previously it was silently dropped from the bindings).

Generating bindings

Binding emission is gated behind -d:ffiGenBindings; without it, genBindings() is a no-op and the library just builds normally.

nim c -d:ffiGenBindings -d:targetLang=c \
      -d:ffiOutputDir=path/to/output -d:ffiSrcPath=../mylib.nim mylib.nim

• -d:targetLang — rust (default), cpp, c, or cddl.
• -d:ffiOutputDir — where the generated files land.
• -d:ffiSrcPath — the Nim source path embedded in the generated build files.

Examples

• examples/timer — a self-contained Nimble project covering the ctor, sync and async methods, multi-param methods, events, and C / C++ / Rust / CDDL bindings with runnable clients. Start here:

  cd examples/timer && nimble install -y ../.. && nimble build
  

• examples/echo — a second minimal library, loaded alongside timer in the C++ end-to-end test to prove two libraries coexist in one process.