Pure C Bridge API

v2.1.0

The C ABI in this chapter is stable across v1 and v2. C++ coroutine examples use stdexec::task; pure C callers still need no C++ dependency.

This page introduces the pure C API provided by cbridge.h and dcb_codec.h, plus cbridge_wait.hpp used together with C++ coroutines.

You can use it for three things:

• Use dcb_codec.h to encode/decode wire data, binary-compatible with the C++ side ByteWriter / ByteReader
• Use dcb_invoke_dart_fn to call registered Dart callbacks from any thread
• Use dcb_async_* with cbridge_wait.hpp to let C++ coroutines wait non-blocking for external C async operations to complete

If your code is a pure C project, can only export C symbols, or you don’t want to bring C++20 coroutines into the caller, use the C entry points in this chapter.

Other language runtimes (such as Python ctypes, Rust FFI, Go cgo) can also use this C API as the minimal entry point.

When to Use the Pure C API

Choose this entry point in the following cases:

• Your code is a pure C project, or can only export C symbols: C compilation units have no co_await, templates, or Executor, so they cannot directly use the bridge’s C++ coroutine entry points.
• You call the bridge from another language runtime: Python ctypes, Rust FFI, Go cgo, etc. can only bind to the C ABI.
• You want the caller to keep zero C++ dependencies: the C side only compiles C headers; the bridge’s internal async runtime is invisible to the caller.

C side cannot await

C code itself cannot co_await these async operations. The awaiting side of dcb_async_* is C++ coroutines (via cbridge_wait.hpp); the C side is only responsible for creating, completing, or canceling operations.

The pure C API is the lowest-common-denominator entry with zero C++ dependencies:

Any C/C++ runtime (regardless of which coroutine/event loop it uses)
    │
    │  dcb_codec.h   — encode/decode parameters/return values
    │  dcb_async_*   — async operation primitives
    │  dcb_invoke_dart_fn — invoke Dart callbacks
    ▼
bridge internals (C++ coroutine pipeline; caller need not know)
    │
    ▼
Dart Isolate executes the callback → result returned via callback

Headers

#include "dart_cpp_bridge/dcb_codec.h"  // wire payload encoding/decoding
#include "dart_cpp_bridge/cbridge.h"    // async operation primitives + invoke Dart callbacks
// Pure C99; does not introduce any C++ header files

API 1: Pure C Codec

dcb_codec.h provides a pure C99 codec that is binary compatible with the C++ side ByteWriter / ByteReader (codec.hpp).

Supported Types

C type	Wire encoding	Description
int32_t	4 bytes LE	bool also uses this (0/1)
uint32_t	4 bytes LE
int64_t	8 bytes LE
uint64_t	8 bytes LE
double	8 bytes LE (IEEE 754)
const char*	u32 len + UTF-8 bytes	Only variable-length type
Array	u32 count + N elements	Elements can be any basic type above

Array element types supported: i32, u32, i64, u64, f64, str. All elements in the same array must have the same type (caller guarantees this).

struct / pointer / nested container are not supported — serialize complex data as a string (JSON, protobuf, etc.).

Writer

Dynamically growing buffer, initially 64 bytes, doubling on demand. Internally uses malloc/realloc; release with dcb_writer_free after use.

dcb_writer w;
dcb_writer_init(&w);

dcb_write_str(&w, "hello");     // u32 strlen + UTF-8 bytes
dcb_write_i32(&w, 42);          // 4 bytes LE
dcb_write_f64(&w, 3.14);        // 8 bytes LE
dcb_write_u64(&w, 123456789ULL); // 8 bytes LE

// Array: write count first, then loop to write elements
dcb_write_arr_begin(&w, 3);
dcb_write_i32(&w, 10);
dcb_write_i32(&w, 20);
dcb_write_i32(&w, 30);

// Use w.data / w.len as the payload
dcb_invoke_dart_fn(session_id, fn_id, w.data, w.len, callback, userdata);

dcb_writer_free(&w);  // free internal buffer

Full Writer API:

void dcb_writer_init(dcb_writer* w);
void dcb_writer_free(dcb_writer* w);
void dcb_write_u32(dcb_writer* w, uint32_t v);
void dcb_write_u64(dcb_writer* w, uint64_t v);
void dcb_write_i32(dcb_writer* w, int32_t v);
void dcb_write_i64(dcb_writer* w, int64_t v);
void dcb_write_f64(dcb_writer* w, double v);
void dcb_write_len_bytes(dcb_writer* w, const void* p, uint32_t n);  // u32 len + data
void dcb_write_str(dcb_writer* w, const char* s);                    // inline: strlen + len_bytes
void dcb_write_arr_begin(dcb_writer* w, uint32_t count);             // inline: u32 count

Reader

Zero-copy reader — no malloc, no ownership of data. Out-of-bounds reads put it in an error state (subsequent reads return 0); check with dcb_reader_valid.

Must read in the same order as written

Reader is a sequential cursor with no random access. If the write order is str → i32 → f64, reads must also call in the order str → i32 → f64. Skipping or reading out of order causes all subsequent fields to be parsed incorrectly (offset misalignment). Types and order must match the writer side exactly.

// data / data_len come from the callback arguments of dcb_invoke_dart_fn
dcb_reader r;
dcb_reader_init(&r, data, data_len);

uint32_t slen;
const char* s = dcb_read_str(&r, &slen);  // zero-copy, points inside data
int32_t v = dcb_read_i32(&r);
double d = dcb_read_f64(&r);

// Read array
uint32_t n = dcb_read_arr_begin(&r);
for (uint32_t i = 0; i < n; i++) {
    int32_t elem = dcb_read_i32(&r);
    // ...
}

if (!dcb_reader_valid(&r)) {
    // data format mismatch, read out of bounds
}

Note

The pointer returned by dcb_read_str is not guaranteed to be NUL-terminated; its length is *out_len. If you need a C string, copy it and append '\0' yourself.

Full Reader API:

void dcb_reader_init(dcb_reader* r, const uint8_t* data, uint32_t len);
int  dcb_reader_valid(const dcb_reader* r);  // 1=ok, 0=out-of-bounds occurred
uint32_t dcb_read_u32(dcb_reader* r);
uint64_t dcb_read_u64(dcb_reader* r);
int32_t  dcb_read_i32(dcb_reader* r);
int64_t  dcb_read_i64(dcb_reader* r);
double   dcb_read_f64(dcb_reader* r);
const uint8_t* dcb_read_len_bytes(dcb_reader* r, uint32_t* out_len);  // zero-copy
const char*    dcb_read_str(dcb_reader* r, uint32_t* out_len);        // inline cast
uint32_t       dcb_read_arr_begin(dcb_reader* r);                     // inline: u32 count

Mapping to the C++ Codec

Pure C (dcb_codec.h)	C++ (codec.hpp)	Wire format
dcb_write_i32	ByteWriter::i32()	4 bytes LE
dcb_write_u32	ByteWriter::u32()	4 bytes LE
dcb_write_i64	ByteWriter::i64()	8 bytes LE
dcb_write_u64	ByteWriter::u64()	8 bytes LE
dcb_write_f64	ByteWriter::f64()	8 bytes LE
dcb_write_str	ByteWriter::str()	u32 len + bytes
dcb_write_arr_begin + loop	ByteWriter::vec() header	u32 count + N elements
dcb_read_str	ByteReader::str()	zero-copy

Encoding is identical on both sides; data written from the pure C side can be read directly by the C++ side, and vice versa.

API 2: Async Operation Primitives

These functions are used on the C side to create, complete, or cancel an async operation. The C++ coroutine side waits non-blocking via dcb::async_wait() in cbridge_wait.hpp, without occupying a bridge thread.

Function Signatures

uint64_t dcb_async_create(void);                                    // create operation
void dcb_async_complete(uint64_t op_id, const uint8_t* data, uint32_t len);  // complete successfully
void dcb_async_fail(uint64_t op_id, const char* error);             // fail
void dcb_async_cancel(uint64_t op_id);                              // cancel

One-shot operation

The op_id returned by dcb_async_create() is one-shot. After calling dcb_async_complete, dcb_async_fail, or dcb_async_cancel, that op_id becomes invalid; subsequent calls on that ID are no-ops.

Waiting on the C++ Coroutine Side

#include "dart_cpp_bridge/cbridge_wait.hpp"

// Inside a coroutine (must be bound to an Executor):
auto data = co_await dcb::async_wait(op_id);  // suspends, does not occupy a thread
// data is std::vector<uint8_t>; throws std::runtime_error on failure

Typical Scenario: C++ Coroutine Calls an Async API of an External C Library

Suppose the Dart side calls fetchUrl(url) → C++ coroutine receives the url argument → calls the async HTTP API of an external C library → the library returns the response body when done → the coroutine returns the result to Dart.

#include "dart_cpp_bridge/cbridge.h"
#include "dart_cpp_bridge/cbridge_wait.hpp"
#include "dart_cpp_bridge/dcb_codec.h"   // pure C codec (for callbacks)
#include "dart_cpp_bridge/codec.hpp"     // C++ ByteReader (for coroutines)
#include <string>
#include <thread>
#include <chrono>
#include <cstring>

// ─── Simulate an external C library (represents any third-party async library, e.g. libcurl, libuv, etc.) ───
// Callback type: status=0 means success, body/body_len is the response data
typedef void (*http_callback)(void* ctx, int status,
                              const uint8_t* body, uint32_t body_len);

// Async interface of the external library: it calls the callback after doing work on its own thread
static void http_client_get_async(const char* url, http_callback cb, void* ctx) {
    // Simulate: library internally spawns a thread to process the request
    // (real library may use a thread pool / event loop)
    std::thread([url = std::string(url), cb, ctx] {
        // Simulate network latency...
        std::this_thread::sleep_for(std::chrono::milliseconds(50));

        // Simulate response body
        std::string response = "response from " + url;
        cb(ctx, 0, (const uint8_t*)response.data(), (uint32_t)response.size());
    }).detach();
}

// ─── Our callback (triggered on the library's thread) ───
// ctx is the op_id we passed in
static void http_done(void* ctx, int status, const uint8_t* body, uint32_t body_len) {
    uint64_t op_id = (uint64_t)(uintptr_t)ctx;
    if (status == 0) {
        // Use dcb_codec to encode into wire format (string: u32 len + bytes)
        dcb_writer w;
        dcb_writer_init(&w);
        dcb_write_len_bytes(&w, body, body_len);  // wrap raw bytes as a string
        dcb_async_complete(op_id, w.data, w.len);  // pass to coroutine
        dcb_writer_free(&w);
    } else {
        dcb_async_fail(op_id, "http request failed");
    }
}

// ─── bridge coroutine (called by wire dispatch; arguments already decoded from Dart) ───
// url is the std::string decoded from the Dart request by wire dispatch
stdexec::task<std::string> fetch_url(std::string url) {
    // 1. create async operation
    uint64_t op_id = dcb_async_create();

    // 2. start the external C library's async operation (non-blocking)
    //    pass in url and callback; the library will call http_done when done
    http_client_get_async(url.c_str(), http_done,
                          (void*)(uintptr_t)op_id);

    // 3. coroutine suspends, does not occupy the io thread. Automatically resumes when the library completes.
    auto payload = co_await dcb::async_wait(op_id);
    // payload are the bytes passed to dcb_async_complete in http_done

    // 4. decode result (use ByteReader directly inside the C++ coroutine)
    dcb::ByteReader r(payload.data(), payload.size());
    co_return r.str();  // corresponds to dcb_write_len_bytes in http_done
}

Data flow:

Dart                     C++ coroutine               External C library
──────────────────────────────────────────────────────────────
fetchUrl("https://...")
    │  wire payload
    ▼
wire dispatch decodes → url = "https://..."
                         │
                         dcb_async_create() → op_id
                         http_client_get_async(url, cb, op_id)
                         │                          │
                         co_await async_wait(op_id)  │  (library works on its own thread)
                         │  coroutine suspends ─────────────  │
                         │                          http_done(op_id, body)
                         │                            dcb_write_len_bytes(body)
                         │                            dcb_async_complete(op_id, ...)
                         │  ◀───────────────────  coroutine resumes
                         ByteReader(payload).str() → body
                         co_return body
    ◀─── wire response
Dart receives response body

Comparison with spawn_blocking

	dcb_async_*	spawn_blocking
Occupies thread?	No (coroutine suspends)	Yes (occupies a pool thread)
Concurrency limit	None (only consumes one op_id)	thread_pool size (default 4)
Use case	External C library has async API	External C library only has sync API
C++ dependency	Coroutine side needs cbridge_wait.hpp	Needs runtime.hpp

API 3: Calling Dart Callbacks

Function Signatures

typedef void (*dcb_dart_fn_callback)(
    void* userdata,
    int ok,              // 1=success, 0=failure
    const uint8_t* data, // encoded return value on success (wire payload)
    uint32_t data_len,
    const char* error    // error message on failure (NUL-terminated)
);

int dcb_invoke_dart_fn(
    uint64_t session_id,   // target session
    uint64_t fn_id,        // Dart closure ID
    const uint8_t* args,   // encoded arguments (wire payload format)
    uint32_t args_len,
    dcb_dart_fn_callback callback,
    void* userdata);

Behavior

• Non-blocking: returns 0 immediately (successfully initiated) or -1 (invalid session)
• Callback is guaranteed to be called exactly once (success or failure)
• Callback is triggered on the bridge’s io thread — don’t block in the callback
• Can be called from any thread (including external runtime loop threads)

Example: Full Flow of C Calling a Dart Function

Full chain: Dart calls C++ → C++ passes IDs to the C layer → C layer calls Dart callback → Dart returns data → C layer processes and wakes C++ → C++ optionally returns a result.

① Dart side — user writes a normal async function + calls the bridge API:

// User-written callback: receives a name and returns a greeting
Future<String> greet(String name) async {
  if (name.isEmpty) throw Exception('name cannot be empty');
  return 'Hello, $name!';
}

// Call the bridge function (a codegen-generated top-level function)
// Pass the callback + arguments together to C++
final result = await startGreetTask(callback: greet, name: 'World');
print(result); // "Dart said: Hello, World!"

② C++ side — receives DartFn + arguments, creates an async operation, passes everything to the C layer:

#include "dart_cpp_bridge/dart_fn.hpp"
#include "dart_cpp_bridge/session.hpp"
#include "dart_cpp_bridge/cbridge_wait.hpp"

// Forward declaration for the pure C layer
void c_start_greet(uint64_t session_id, uint64_t fn_id,
                   uint64_t op_id, const char* name);

// wire dispatch calls this coroutine after decoding
stdexec::task<std::string> start_greet_task(
    dcb::DartFn<std::string(std::string)> callback,
    std::string name) {
  // 1. extract the IDs needed by the pure C API
  uint64_t session_id = dcb::SessionRegistry::instance().find_id(callback.session());
  uint64_t fn_id = callback.fn_id();

  // 2. create async operation (used to wake this coroutine when the C layer completes)
  uint64_t op_id = dcb_async_create();

  // 3. pass IDs + arguments to the C layer; the C layer will call back Dart at some point
  c_start_greet(session_id, fn_id, op_id, name.c_str());

  // 4. suspend and wait for the C layer to complete (does not occupy the io thread)
  auto payload = co_await dcb::async_wait(op_id);

  // 5. decode the final result returned by the C layer (optional)
  dcb::ByteReader r(payload.data(), payload.size());
  co_return r.str();
}

③ Pure C side — at some point call the Dart callback, then wake C++ after getting the result:

#include "dart_cpp_bridge/cbridge.h"
#include "dart_cpp_bridge/dcb_codec.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// Context: the C layer needs to remember op_id (used to wake C++)
typedef struct {
    uint64_t op_id;
} greet_ctx;

// After Dart finishes executing the callback, the bridge triggers this function on the io thread
// Parameter description:
//   userdata  — the user pointer passed when calling dcb_invoke_dart_fn (passed back unchanged)
//   ok        — 1=Dart returned normally, 0=Dart threw an exception
//   data/len  — on success: encoded bytes of Dart's return value; on failure: NULL/0
//   error     — on failure: the message of the exception thrown by the Dart closure (NUL-terminated); on success: NULL
//               For example, on the Dart side throw Exception('name cannot be empty')
//               → error = "Exception: name cannot be empty"
static void on_dart_reply(void* userdata, int ok, const uint8_t* data,
                          uint32_t len, const char* error) {
    greet_ctx* ctx = (greet_ctx*)userdata;

    if (ok) {
        // decode the string returned by Dart
        dcb_reader r;
        dcb_reader_init(&r, data, len);
        uint32_t slen;
        const char* greeting = dcb_read_str(&r, &slen);

        // The C layer does its own processing after getting the data (concatenate final result)
        char result[256];
        snprintf(result, sizeof(result), "Dart said: %.*s", (int)slen, greeting);

        // encode the result and wake the C++ coroutine
        dcb_writer w;
        dcb_writer_init(&w);
        dcb_write_str(&w, result);
        dcb_async_complete(ctx->op_id, w.data, w.len);
        dcb_writer_free(&w);
    } else {
        // The Dart closure threw an exception (e.g. throw Exception('...'))
        // The bridge catches it and passes exception.toString() as error here
        // → forward to the C++ coroutine; the co_await site will throw std::runtime_error(error)
        dcb_async_fail(ctx->op_id, error);
    }

    free(ctx);
}

// C++ calls this function to start the whole flow
void c_start_greet(uint64_t session_id, uint64_t fn_id,
                   uint64_t op_id, const char* name) {
    // encode the arguments to be passed to the Dart callback
    dcb_writer w;
    dcb_writer_init(&w);
    dcb_write_str(&w, name);  // Dart side receives the name argument

    // save context (op_id needed in the callback)
    greet_ctx* ctx = (greet_ctx*)malloc(sizeof(greet_ctx));
    ctx->op_id = op_id;

    // initiate the call (can be done on any thread, at any time)
    int rc = dcb_invoke_dart_fn(session_id, fn_id,
                                w.data, w.len,
                                on_dart_reply, ctx);
    dcb_writer_free(&w);

    if (rc != 0) {
        dcb_async_fail(op_id, "invoke failed: invalid session");
        free(ctx);
    }
}

Data flow:

Dart                    C++ coroutine               Pure C layer
──────────────────────────────────────────────────────────────
startGreetTask(
  callback: greet,
  name: "World")
    │ wire transfer
    ▼
                    start_greet_task(callback, "World")
                      find_id() → session_id
                      fn_id()
                      dcb_async_create() → op_id
                      │
                      c_start_greet(sid, fn_id, op_id, "World")
                                              │
                                              dcb_write_str("World")
                                              dcb_invoke_dart_fn(...)
    ◀────────────────────────────────────  invoke greet("World")
    greet executes → "Hello, World!"
    ────────────────────────────────────▶  on_dart_reply receives result
                                              │
                                              concatenate "Dart said: Hello, World!"
                                              dcb_async_complete(op_id, ...)
                                              │
                    co_await resumes ◀───────────┘
                    ByteReader.str()
                    co_return "Dart said: Hello, World!"
    ◀─── final result returned to Dart
print(result)

Internal Implementation

dcb_async_create()
  → create oneshot channel, store Sender + Receiver in the global registry
  → return op_id

C++ coroutine: co_await dcb::async_wait(op_id)
  → take Receiver from registry
  → co_await rx.recv() suspends coroutine

dcb_async_complete(op_id, data, len)    [any thread]
  → take Sender from registry
  → tx.send(OpResult{ok=true, data})
  → wake_waiter → coroutine resumes on its Executor

dcb_async_fail / dcb_async_cancel similar.

Inside dcb_invoke_dart_fn:

dcb_invoke_dart_fn(session_id, fn_id, args, len, callback, userdata)
  → look up Session
  → spawn_on_asio: start coroutine on the io thread
    → co_await session->invoke_dart_fn_async(gen, fn_id, args)
      → send DartFnCall frame to Dart
      → suspend and wait for Dart reply
    → Dart replies → coroutine resumes
    → callback(userdata, ok, data, len, error)

Thread Safety

Function	Thread-safe	Note
dcb_invoke_dart_fn	Yes	Can be called from any thread
dcb_async_create	Yes	Protected by global mutex
dcb_async_complete/fail/cancel	Yes	Can be called from any thread
dcb::async_wait	—	Only use inside a coroutine

Design Constraints

Caution

• dcb_invoke_dart_fn’s callback is triggered on the bridge io thread, don’t block in it
• If you need to marshal results to another thread, post to your event loop in the callback
• Only the first call to dcb_async_complete/fail/cancel on a given op_id is effective; subsequent calls are no-op
• After dcb_async_cancel, the coroutine receives an “operation cancelled” error

Full Examples

• Test cases: examples/foreign_runtime_demo — test_cbridge_async / test_cbridge_invoke
• Codec: dart/native/include/dart_cpp_bridge/dcb_codec.h
• Header: dart/native/include/dart_cpp_bridge/cbridge.h
• C++ helper: dart/native/include/dart_cpp_bridge/cbridge_wait.hpp