FFI

Use Bun’s FFI module to efficiently call native libraries from JavaScript

bun:ffi is experimental, with known bugs and limitations, and should not be relied on in production. The most stable way to interact with native code from Bun is to write a Node-API module.

Use the built-in bun:ffi module to efficiently call native libraries from JavaScript. It works with languages that support the C ABI (Zig, Rust, C/C++, C#, Nim, Kotlin, etc).

---

dlopen usage (bun:ffi)

To print the version number of sqlite3:

import { dlopen, FFIType, suffix } from "bun:ffi";

// `suffix` is either "dylib", "so", or "dll" depending on the platform
// you don't have to use "suffix", it's just there for convenience
const path = `libsqlite3.${suffix}`;

const {
  symbols: {
    sqlite3_libversion, // the function to call
  },
} = dlopen(
  path, // a library name or file path
  {
    sqlite3_libversion: {
      // no arguments, returns a string
      args: [],
      returns: FFIType.cstring,
    },
  },
);

console.log(`SQLite 3 version: ${sqlite3_libversion()}`);

---

Performance

According to our benchmark, bun:ffi is roughly 2-6x faster than Node.js FFI via Node-API.

Bun generates & just-in-time compiles C bindings that efficiently convert values between JavaScript types and native types. To compile C, Bun embeds TinyCC, a small and fast C compiler.

---

Usage

Zig

add.zig

pub export fn add(a: i32, b: i32) i32 {
  return a + b;
}

To compile:

terminal

zig build-lib add.zig -dynamic -OReleaseFast

Pass a path to the shared library and a map of symbols to import into dlopen:

import { dlopen, FFIType, suffix } from "bun:ffi";
const { i32 } = FFIType;

const path = `libadd.${suffix}`;

const lib = dlopen(path, {
  add: {
    args: [i32, i32],
    returns: i32,
  },
});

console.log(lib.symbols.add(1, 2));

Rust

// add.rs
#[no_mangle]
pub extern "C" fn add(a: i32, b: i32) -> i32 {
    a + b
}

To compile:

rustc --crate-type cdylib add.rs

C++

#include <cstdint>

extern "C" int32_t add(int32_t a, int32_t b) {
    return a + b;
}

To compile:

# Linux
clang++ -shared -fPIC add.cpp -o libadd.so

# macOS
clang++ -dynamiclib add.cpp -o libadd.dylib

---

FFI types

The following FFIType values are supported.

FFIType	C Type	Aliases
buffer	char*
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

Note: buffer arguments must be a TypedArray or DataView.

---

Strings

JavaScript strings and C-like strings are different, and that complicates using strings with native libraries.

How are JavaScript strings and C strings different?

JavaScript strings:

• UTF16 (2 bytes per letter) or potentially latin1, depending on the JavaScript engine & what characters are used
• length stored separately
• Immutable

C strings:

• UTF8 (1 byte per letter), usually
• The length is not stored. Instead, the string is null-terminated which means the length is the index of the first \0 it finds
• Mutable

To solve this, bun:ffi exports CString which extends JavaScript’s built-in String to support null-terminated strings and add a few extras:

class CString extends String {
  /**
   * Given a `ptr`, this will automatically search for the closing `\0` character and transcode from UTF-8 to UTF-16 if necessary.
   */
  constructor(ptr: number, byteOffset?: number, byteLength?: number): string;

  /**
   * The ptr to the C string
   *
   * This `CString` instance is a clone of the string, so it
   * is safe to continue using this instance after the `ptr` has been
   * freed.
   */
  ptr: number;
  byteOffset?: number;
  byteLength?: number;
}

To convert from a null-terminated string pointer to a JavaScript string:

const myString = new CString(ptr);

To convert from a pointer with a known length to a JavaScript string:

const myString = new CString(ptr, 0, byteLength);

The new CString() constructor clones the C string, so it is safe to continue using myString after ptr has been freed.

my_library_free(myString.ptr);

// this is safe because myString is a clone
console.log(myString);

When used in returns, FFIType.cstring coerces the pointer to a JavaScript string. When used in args, FFIType.cstring is identical to ptr.

---

Function pointers

Async functions are not yet supported

To call a function pointer from JavaScript, use CFunction. This is useful if using Node-API (napi) with Bun, and you’ve already loaded some symbols.

import { CFunction } from "bun:ffi";

let myNativeLibraryGetVersion = /* somehow, you got this pointer */

const getVersion = new CFunction({
  returns: "cstring",
  args: [],
  ptr: myNativeLibraryGetVersion,
});
getVersion();

If you have multiple function pointers, you can define them all at once with linkSymbols:

import { linkSymbols } from "bun:ffi";

// getVersionPtrs defined elsewhere
const [majorPtr, minorPtr, patchPtr] = getVersionPtrs();

const lib = linkSymbols({
  // Unlike with dlopen(), the names here can be whatever you want
  getMajor: {
    returns: "cstring",
    args: [],

    // Since this doesn't use dlsym(), you have to provide a valid ptr
    // That ptr could be a number or a bigint
    // An invalid pointer will crash your program.
    ptr: majorPtr,
  },
  getMinor: {
    returns: "cstring",
    args: [],
    ptr: minorPtr,
  },
  getPatch: {
    returns: "cstring",
    args: [],
    ptr: patchPtr,
  },
});

const [major, minor, patch] = [lib.symbols.getMajor(), lib.symbols.getMinor(), lib.symbols.getPatch()];

---

Callbacks

Use JSCallback to create JavaScript callback functions that can be passed to C/FFI functions. The C/FFI function can call into the JavaScript/TypeScript code. This is useful for asynchronous code or whenever you want to call into JavaScript code from C.

import { dlopen, JSCallback, ptr, CString } from "bun:ffi";

const {
  symbols: { search },
  close,
} = dlopen("libmylib", {
  search: {
    returns: "usize",
    args: ["cstring", "callback"],
  },
});

const searchIterator = new JSCallback((ptr, length) => /hello/.test(new CString(ptr, length)), {
  returns: "bool",
  args: ["ptr", "usize"],
});

const str = Buffer.from("wwutwutwutwutwutwutwutwutwutwutut\0", "utf8");
if (search(ptr(str), searchIterator)) {
  // found a match!
}

// Sometime later:
setTimeout(() => {
  searchIterator.close();
  close();
}, 5000);

When you’re done with a JSCallback, you should call close() to free the memory.

Experimental thread-safe callbacks

JSCallback has experimental support for thread-safe callbacks. This will be needed if you pass a callback function into a different thread from its instantiation context. You can enable it with the optional threadsafe parameter.

Currently, thread-safe callbacks work best when run from another thread that is running JavaScript code, i.e. a Worker. A future version of Bun will enable them to be called from any thread (such as new threads spawned by your native library that Bun is not aware of).

const searchIterator = new JSCallback((ptr, length) => /hello/.test(new CString(ptr, length)), {
  returns: "bool",
  args: ["ptr", "usize"],
  threadsafe: true, // Optional. Defaults to `false`
});

⚡️ Performance tip — For a slight performance boost, directly pass JSCallback.prototype.ptr instead of the JSCallback object:

const onResolve = new JSCallback(arg => arg === 42, {
  returns: "bool",
  args: ["i32"],
});
const setOnResolve = new CFunction({
  returns: "bool",
  args: ["function"],
  ptr: myNativeLibrarySetOnResolve,
});

// This code runs slightly faster:
setOnResolve(onResolve.ptr);

// Compared to this:
setOnResolve(onResolve);

---

Pointers

Bun represents pointers as a number in JavaScript.

How does a 64 bit pointer fit in a JavaScript number?

64-bit processors support up to 52 bits of addressable space. JavaScript numbers support 53 bits of usable space, so that leaves us with about 11 bits of extra space.

Why not BigInt? BigInt is slower. JavaScript engines allocate a separate BigInt which means they can’t fit into a regular JavaScript value. If you pass a BigInt to a function, it will be converted to a number

Windows Note: The Windows API type HANDLE does not represent a virtual address, and using ptr for it will not work as expected. Use u64 to safely represent HANDLE values.

To convert from a TypedArray to a pointer:

import { ptr } from "bun:ffi";
let myTypedArray = new Uint8Array(32);
const myPtr = ptr(myTypedArray);

To convert from a pointer to an ArrayBuffer:

import { ptr, toArrayBuffer } from "bun:ffi";
let myTypedArray = new Uint8Array(32);
const myPtr = ptr(myTypedArray);

// toArrayBuffer accepts a `byteOffset` and `byteLength`
// if `byteLength` is not provided, it is assumed to be a null-terminated pointer
myTypedArray = new Uint8Array(toArrayBuffer(myPtr, 0, 32), 0, 32);

To read data from a pointer, you have two options. For long-lived pointers, use a DataView:

import { toArrayBuffer } from "bun:ffi";
let myDataView = new DataView(toArrayBuffer(myPtr, 0, 32));

console.log(
  myDataView.getUint8(0, true),
  myDataView.getUint8(1, true),
  myDataView.getUint8(2, true),
  myDataView.getUint8(3, true),
);

For short-lived pointers, use read:

import { read } from "bun:ffi";

console.log(
  // ptr, byteOffset
  read.u8(myPtr, 0),
  read.u8(myPtr, 1),
  read.u8(myPtr, 2),
  read.u8(myPtr, 3),
);

The read function behaves similarly to DataView, but it’s usually faster because it doesn’t need to create a DataView or ArrayBuffer.

FFIType	read function
ptr	read.ptr
i8	read.i8
i16	read.i16
i32	read.i32
i64	read.i64
u8	read.u8
u16	read.u16
u32	read.u32
u64	read.u64
f32	read.f32
f64	read.f64

Memory management

bun:ffi does not manage memory for you. You must free the memory when you’re done with it.

From JavaScript

If you want to track when a TypedArray is no longer in use from JavaScript, you can use a FinalizationRegistry.

From C, Rust, Zig, etc

If you want to track when a TypedArray is no longer in use from C or FFI, you can pass a callback and an optional context pointer to toArrayBuffer or toBuffer. This function is called at some point later, once the garbage collector frees the underlying ArrayBuffer JavaScript object.

The expected signature is the same as in JavaScriptCore’s C API:

typedef void (*JSTypedArrayBytesDeallocator)(void *bytes, void *deallocatorContext);

import { toArrayBuffer } from "bun:ffi";

// with a deallocatorContext:
toArrayBuffer(
  bytes,
  byteOffset,

  byteLength,

  // this is an optional pointer to a callback
  deallocatorContext,

  // this is a pointer to a function
  jsTypedArrayBytesDeallocator,
);

// without a deallocatorContext:
toArrayBuffer(
  bytes,
  byteOffset,

  byteLength,

  // this is a pointer to a function
  jsTypedArrayBytesDeallocator,
);

Memory safety

Using raw pointers outside of FFI is extremely not recommended. A future version of Bun may add a CLI flag to disable bun:ffi.

Pointer alignment

If an API expects a pointer sized to something other than char or u8, make sure the TypedArray is also that size. A u64* is not exactly the same as [8]u8* due to alignment.

Passing a pointer

Where FFI functions expect a pointer, pass a TypedArray of equivalent size:

import { dlopen, FFIType } from "bun:ffi";

const {
  symbols: { encode_png },
} = dlopen(myLibraryPath, {
  encode_png: {
    // FFIType's can be specified as strings too
    args: ["ptr", "u32", "u32"],
    returns: FFIType.ptr,
  },
});

const pixels = new Uint8ClampedArray(128 * 128 * 4);
pixels.fill(254);
pixels.subarray(0, 32 * 32 * 2).fill(0);

const out = encode_png(
  // pixels will be passed as a pointer
  pixels,

  128,
  128,
);

The auto-generated wrapper converts the pointer to a TypedArray.

Hardmode

If you don’t want the automatic conversion or you want a pointer to a specific byte offset within the TypedArray, you can also directly get the pointer to the TypedArray:

import { dlopen, FFIType, ptr } from "bun:ffi";

const {
  symbols: { encode_png },
} = dlopen(myLibraryPath, {
  encode_png: {
    // FFIType's can be specified as strings too
    args: ["ptr", "u32", "u32"],
    returns: FFIType.ptr,
  },
});

const pixels = new Uint8ClampedArray(128 * 128 * 4);
pixels.fill(254);

// this returns a number! not a BigInt!
const myPtr = ptr(pixels);

const out = encode_png(
  myPtr,

  // dimensions:
  128,
  128,
);

Reading pointers

const out = encode_png(
  // pixels will be passed as a pointer
  pixels,

  // dimensions:
  128,
  128,
);

// assuming it is 0-terminated, it can be read like this:
let png = new Uint8Array(toArrayBuffer(out));

// save it to disk:
await Bun.write("out.png", png);