bun:ffi has experimental support for compiling and running C from JavaScript with low overhead.

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Usage (cc in bun:ffi)

See the introduction blog post for more information.

JavaScript:

import { cc } from "bun:ffi";
import source from "./hello.c" with { type: "file" };

const {
  symbols: { hello },
} = cc({
  source,
  symbols: {
    hello: {
      args: [],
      returns: "int",
    },
  },
});

console.log("What is the answer to the universe?", hello());

C source:

int hello() {
  return 42;
}

When you run hello.js, it will print:

bun hello.js
What is the answer to the universe? 42

Under the hood, cc uses TinyCC to compile the C code and then link it with the JavaScript runtime, efficiently converting types in-place.

Primitive types

The same FFIType values in dlopen are supported in cc.

FFIType	C Type	Aliases
cstring	char*
function	(void*)(*)()	fn, callback
ptr	void*	pointer, void*, char*
i8	int8_t	int8_t
i16	int16_t	int16_t
i32	int32_t	int32_t, int
i64	int64_t	int64_t
i64_fast	int64_t
u8	uint8_t	uint8_t
u16	uint16_t	uint16_t
u32	uint32_t	uint32_t
u64	uint64_t	uint64_t
u64_fast	uint64_t
f32	float	float
f64	double	double
bool	bool
char	char
napi_env	napi_env
napi_value	napi_value

Strings, objects, and non-primitive types

To make it easier to work with strings, objects, and other non-primitive types that don't map 1:1 to C types, cc supports N-API.

To pass or receive a JavaScript values without any type conversions from a C function, you can use napi_value.

You can also pass a napi_env to receive the N-API environment used to call the JavaScript function.

Returning a C string to JavaScript

For example, if you have a string in C, you can return it to JavaScript like this:

import { cc } from "bun:ffi";
import source from "./hello.c" with { type: "file" };

const {
  symbols: { hello },
} = cc({
  source,
  symbols: {
    hello: {
      args: ["napi_env"],
      returns: "napi_value",
    },
  },
});

const result = hello();

And in C:

#include <node/node_api.h>

napi_value hello(napi_env env) {
  napi_value result;
  napi_create_string_utf8(env, "Hello, Napi!", NAPI_AUTO_LENGTH, &result);
  return result;
}

You can also use this to return other types like objects and arrays:

#include <node/node_api.h>

napi_value hello(napi_env env) {
  napi_value result;
  napi_create_object(env, &result);
  return result;
}

cc Reference

library: string[]

The library array is used to specify the libraries that should be linked with the C code.

type Library = string[];

cc({
  source: "hello.c",
  library: ["sqlite3"],
});

symbols

The symbols object is used to specify the functions and variables that should be exposed to JavaScript.

type Symbols = {
  [key: string]: {
    args: FFIType[];
    returns: FFIType;
  };
};

source

The source is a file path to the C code that should be compiled and linked with the JavaScript runtime.

type Source = string | URL | BunFile;

cc({
  source: "hello.c",
  symbols: {
    hello: {
      args: [],
      returns: "int",
    },
  },
});

flags: string | string[]

The flags is an optional array of strings that should be passed to the TinyCC compiler.

type Flags = string | string[];

These are flags like -I for include directories and -D for preprocessor definitions.

define: Record<string, string>

The define is an optional object that should be passed to the TinyCC compiler.

type Defines = Record<string, string>;

cc({
  source: "hello.c",
  define: {
    NDEBUG: "1",
  },
});

These are preprocessor definitions passed to the TinyCC compiler.