Add documentation (#2148)bun-v0.5.7

* Add documentation

* Tweaks

* Fixes

* Rearrange

* Update

1 files changed, 516 insertions, 0 deletions

diff --git a/docs/api/ffi.md b/docs/api/ffi.md
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+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).
+
+To print the version number of `sqlite3`:
+
+```ts
+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: ${sqlite3_libversion()}`);
+```
+
+## Performance
+
+According to [our benchmark](https://github.com/oven-sh/bun/tree/main/bench/ffi), `bun:ffi` is roughly 2-6x faster than Node.js FFI via `Node-API`.
+
+{% image src="/images/ffi.png" height="400" /%}
+
+Bun generates & just-in-time compiles C bindings that efficiently convert values between JavaScript types and native types. To compile C, Bun embeds [TinyCC](https://github.com/TinyCC/tinycc), a small and fast C compiler.
+
+## Usage
+
+### Zig
+
+```zig
+// add.zig
+pub export fn add(a: i32, b: i32) i32 {
+  return a + b;
+}
+```
+
+To compile:
+
+```bash
+$ zig build-lib add.zig -dynamic -OReleaseFast
+```
+
+Pass a path to the shared library and a map of symbols to import into `dlopen`:
+
+```ts
+import { dlopen, FFIType, suffix } from "bun:ffi";
+
+const path = `libadd.${suffix}`;
+
+const lib = dlopen(path, {
+  add: {
+    args: [FFIType.i32, FFIType.i32],
+    returns: FFIType.i32,
+  },
+});
+
+lib.symbols.add(1, 2);
+```
+
+### Rust
+
+```rust
+// add.rs
+#[no_mangle]
+pub extern "C" fn add(a: isize, b: isize) -> isize {
+    a + b
+}
+```
+
+To compile:
+
+```bash
+$ rustc --crate-type cdylib add.rs
+```
+
+## FFI types
+
+The following `FFIType` values are supported.
+
+| `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`         |                             |
+
+## Strings
+
+JavaScript strings and C-like strings are different, and that complicates using strings with native libraries.
+
+{% details summary="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
+
+{% /details %}
+
+To solve this, `bun:ffi` exports `CString` which extends JavaScript's built-in `String` to support null-terminated strings and add a few extras:
+
+```ts
+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:
+
+```ts
+const myString = new CString(ptr);
+```
+
+To convert from a pointer with a known length to a JavaScript string:
+
+```ts
+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.
+
+```ts
+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
+
+{% callout %}
+
+**Note** — Async functions are not yet supported.
+
+{% /callout %}
+
+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.
+
+```ts
+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`:
+
+```ts
+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.
+
+```ts
+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.
+
+{% callout %}
+
+**⚡️ Performance tip** — For a slight performance boost, directly pass `JSCallback.prototype.ptr` instead of the `JSCallback` object:
+
+```ts
+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);
+```
+
+{% /callout %}
+
+## Pointers
+
+Bun represents [pointers](<https://en.wikipedia.org/wiki/Pointer_(computer_programming)>) as a `number` in JavaScript.
+
+{% details summary="How does a 64 bit pointer fit in a JavaScript number?" %}
+64-bit processors support up to [52 bits of addressable space](https://en.wikipedia.org/wiki/64-bit_computing#Limits_of_processors). [JavaScript numbers](https://en.wikipedia.org/wiki/Double-precision_floating-point_format#IEEE_754_double-precision_binary_floating-point_format:_binary64) 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`
+{% /details %}
+
+To convert from a `TypedArray` to a pointer:
+
+```ts
+import { ptr } from "bun:ffi";
+let myTypedArray = new Uint8Array(32);
+const myPtr = ptr(myTypedArray);
+```
+
+To convert from a pointer to an `ArrayBuffer`:
+
+```ts
+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`:
+
+```ts
+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`:
+
+```ts
+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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry).
+
+#### From C, Rust, Zig, etc
+
+{% callout %}
+**Note** — Available in Bun v0.1.8 and later.
+{% /callout %}
+
+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](https://developer.apple.com/documentation/javascriptcore/jstypedarraybytesdeallocator?language=objc):
+
+```c
+typedef void (*JSTypedArrayBytesDeallocator)(void *bytes, void *deallocatorContext);
+```
+
+```ts
+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:
+
+```ts
+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](https://github.com/oven-sh/bun/blob/6a65631cbdcae75bfa1e64323a6ad613a922cd1a/src/bun.js/ffi.exports.js#L180-L182) converts the pointer to a `TypedArray`.
+
+{% details summary="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`:
+
+```ts
+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,
+);
+```
+
+{% /details %}
+
+### Reading pointers
+
+```ts
+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);
+```