cell/docs/c-modules.md

title, description, weight, type

title	description	weight	type
Writing C Modules	Extending ƿit with native code	50	docs

ƿit makes it easy to extend functionality with C code. C files in a package are compiled into a dynamic library and can be imported like any other module.

Basic Structure

A C module exports a single function that returns a JavaScript value:

// mymodule.c
#include "cell.h"

#define CELL_USE_NAME js_mypackage_mymodule_use

static JSValue js_add(JSContext *js, JSValue self, int argc, JSValue *argv) {
  double a = js2number(js, argv[0]);
  double b = js2number(js, argv[1]);
  return number2js(js, a + b);
}

static JSValue js_multiply(JSContext *js, JSValue self, int argc, JSValue *argv) {
  double a = js2number(js, argv[0]);
  double b = js2number(js, argv[1]);
  return number2js(js, a * b);
}

static const JSCFunctionListEntry js_funcs[] = {
  MIST_FUNC_DEF(mymodule, add, 2),
  MIST_FUNC_DEF(mymodule, multiply, 2),
};

CELL_USE_FUNCS(js_funcs)

Symbol Naming

The exported function must follow this naming convention:

js_<package>_<filename>_use

Where:

• <package> is the package name with / and . replaced by _
• <filename> is the C file name without extension

Examples:

• mypackage/math.c -> js_mypackage_math_use
• gitea.pockle.world/john/lib/render.c -> js_gitea_pockle_world_john_lib_render_use
• mypackage/game.ce (AOT actor) -> js_mypackage_game_program

Actor files (.ce) use the _program suffix instead of _use.

Note: Having both a .cm and .c file with the same stem at the same scope is a build error.

Required Headers

Include cell.h for all ƿit integration:

#include "cell.h"

This provides:

• QuickJS types and functions
• Conversion helpers
• Module definition macros

Conversion Functions

JavaScript <-> C

// Numbers
double js2number(JSContext *js, JSValue v);
JSValue number2js(JSContext *js, double g);

// Booleans
int js2bool(JSContext *js, JSValue v);
JSValue bool2js(JSContext *js, int b);

// Strings (must free with JS_FreeCString)
const char *JS_ToCString(JSContext *js, JSValue v);
void JS_FreeCString(JSContext *js, const char *str);
JSValue JS_NewString(JSContext *js, const char *str);

Blobs

// Get blob data (returns pointer, sets size in bytes)
void *js_get_blob_data(JSContext *js, size_t *size, JSValue v);

// Get blob data in bits
void *js_get_blob_data_bits(JSContext *js, size_t *bits, JSValue v);

// Create new stone blob from data
JSValue js_new_blob_stoned_copy(JSContext *js, void *data, size_t bytes);

// Check if value is a blob
int js_is_blob(JSContext *js, JSValue v);

Function Definition Macros

JSC_CCALL

Define a function with automatic return value:

JSC_CCALL(mymodule_greet,
  const char *name = JS_ToCString(js, argv[0]);
  char buf[256];
  snprintf(buf, sizeof(buf), "Hello, %s!", name);
  ret = JS_NewString(js, buf);
  JS_FreeCString(js, name);
)

JSC_SCALL

Shorthand for functions taking a string first argument:

JSC_SCALL(mymodule_strlen,
  ret = number2js(js, strlen(str));
)

MIST_FUNC_DEF

Register a function in the function list:

MIST_FUNC_DEF(prefix, function_name, arg_count)

Module Export Macros

CELL_USE_FUNCS

Export an object with functions:

static const JSCFunctionListEntry js_funcs[] = {
  MIST_FUNC_DEF(mymod, func1, 1),
  MIST_FUNC_DEF(mymod, func2, 2),
};

CELL_USE_FUNCS(js_funcs)

CELL_USE_INIT

For custom initialization:

CELL_USE_INIT(
  JSValue obj = JS_NewObject(js);
  // Custom setup...
  return obj;
)

Complete Example

// vector.c - Simple 2D vector operations
#include "cell.h"
#include <math.h>

#define CELL_USE_NAME js_mypackage_vector_use

JSC_CCALL(vector_length,
  double x = js2number(js, argv[0]);
  double y = js2number(js, argv[1]);
  ret = number2js(js, sqrt(x*x + y*y));
)

JSC_CCALL(vector_normalize,
  double x = js2number(js, argv[0]);
  double y = js2number(js, argv[1]);
  double len = sqrt(x*x + y*y);
  if (len > 0) {
    JS_FRAME(js);
    JS_ROOT(result, JS_NewObject(js));
    JS_SetPropertyStr(js, result.val, "x", number2js(js, x/len));
    JS_SetPropertyStr(js, result.val, "y", number2js(js, y/len));
    JS_RestoreFrame(_js_ctx, _js_gc_frame, _js_local_frame);
    ret = result.val;
  }
)

JSC_CCALL(vector_dot,
  double x1 = js2number(js, argv[0]);
  double y1 = js2number(js, argv[1]);
  double x2 = js2number(js, argv[2]);
  double y2 = js2number(js, argv[3]);
  ret = number2js(js, x1*x2 + y1*y2);
)

static const JSCFunctionListEntry js_funcs[] = {
  MIST_FUNC_DEF(vector, length, 2),
  MIST_FUNC_DEF(vector, normalize, 2),
  MIST_FUNC_DEF(vector, dot, 4),
};

CELL_USE_FUNCS(js_funcs)

Usage in ƿit:

var vector = use('vector')

var len = vector.length(3, 4)  // 5
var n = vector.normalize(3, 4) // {x: 0.6, y: 0.8}
var d = vector.dot(1, 0, 0, 1) // 0

Build Process

C files are automatically compiled when you run:

cell --dev build

Each C file is compiled into a per-file dynamic library at a content-addressed path in ~/.cell/build/<hash>. A manifest is written for each package so the runtime can find dylibs without rerunning the build pipeline — see Dylib Manifests.

Compilation Flags (cell.toml)

Use the [compilation] section in cell.toml to pass compiler and linker flags:

[compilation]
CFLAGS = "-Isrc -Ivendor/include"
LDFLAGS = "-lz -lm"

Include paths

Relative -I paths are resolved from the package root:

CFLAGS = "-Isdk/public"

If your package is at /path/to/mypkg, this becomes -I/path/to/mypkg/sdk/public.

Absolute paths are passed through unchanged.

The build system also auto-discovers include/ directories — if your package has an include/ directory, it is automatically added to the include path. No need to add -I$PACKAGE/include in cell.toml.

Library paths

Relative -L paths work the same way:

LDFLAGS = "-Lsdk/lib -lmylib"

Target-specific flags

Add sections named [compilation.<target>] for platform-specific flags:

[compilation]
CFLAGS = "-Isdk/public"

[compilation.macos_arm64]
LDFLAGS = "-Lsdk/lib/osx -lmylib"

[compilation.linux]
LDFLAGS = "-Lsdk/lib/linux64 -lmylib"

[compilation.windows]
LDFLAGS = "-Lsdk/lib/win64 -lmylib64"

Available targets: macos_arm64, macos_x86_64, linux, linux_arm64, windows.

Sigils

Use sigils in flags to refer to standard directories:

• $LOCAL — absolute path to .cell/local (for prebuilt libraries)
• $PACKAGE — absolute path to the package root

CFLAGS = "-I$PACKAGE/vendor/include"
LDFLAGS = "-L$LOCAL -lmyprebuilt"

Example: vendored SDK

A package wrapping an external SDK with platform-specific shared libraries:

mypkg/
├── cell.toml
├── wrapper.cpp
└── sdk/
    ├── public/
    │   └── mylib/
    │       └── api.h
    └── lib/
        ├── osx/
        │   └── libmylib.dylib
        └── linux64/
            └── libmylib.so

[compilation]
CFLAGS = "-Isdk/public"

[compilation.macos_arm64]
LDFLAGS = "-Lsdk/lib/osx -lmylib"

[compilation.linux]
LDFLAGS = "-Lsdk/lib/linux64 -lmylib"

// wrapper.cpp
#include "cell.h"
#include <mylib/api.h>
// ...

Platform-Specific Code

Use filename suffixes for platform variants:

audio.c           # default
audio_playdate.c  # Playdate
audio_emscripten.c # Web/Emscripten

ƿit selects the appropriate file based on the target platform.

Multi-File C Modules

If your module wraps a C library, place the library's source files in a src/ directory. Files in src/ are compiled as support objects and linked into your module's dylib — they are not treated as standalone modules.

mypackage/
  rtree.c       # module (exports js_mypackage_rtree_use)
  src/
    rtree.c      # support file (linked into rtree.dylib)
    rtree.h      # header

The module file (rtree.c) includes the library header and uses cell.h as usual. The support files are plain C — they don't need any cell macros.

GC Safety

ƿit uses a Cheney copying garbage collector. Any JS allocation — JS_NewObject, JS_NewString, JS_NewInt32, JS_SetPropertyStr, js_new_blob_stoned_copy, etc. — can trigger GC, which moves heap objects to new addresses. Bare C locals holding JSValue become dangling pointers after any allocating call. This is not a theoretical concern — it causes real crashes that are difficult to reproduce because they depend on heap pressure.

Checklist (apply to EVERY C function you write or modify)

1. Count the JS_New*, JS_SetProperty*, and js_new_blob* calls in the function
2. If there are 2 or more, the function MUST use JS_FRAME / JS_ROOT / JS_RETURN
3. Every JSValue held in a C local across an allocating call must be rooted

When you need rooting

If a function creates one heap object and returns it immediately, no rooting is needed:

JSC_CCALL(mymod_name,
  ret = JS_NewString(js, "hello");
)

If a function creates an object and then sets properties on it, you must root it — each JS_SetPropertyStr call is an allocating call that can trigger GC:

// UNSAFE — will crash under GC pressure:
JSValue obj = JS_NewObject(js);
JS_SetPropertyStr(js, obj, "x", JS_NewInt32(js, 1));  // can GC → obj is stale
JS_SetPropertyStr(js, obj, "y", JS_NewInt32(js, 2));  // obj may be garbage
return obj;

// SAFE:
JS_FRAME(js);
JS_ROOT(obj, JS_NewObject(js));
JS_SetPropertyStr(js, obj.val, "x", JS_NewInt32(js, 1));
JS_SetPropertyStr(js, obj.val, "y", JS_NewInt32(js, 2));
JS_RETURN(obj.val);

Patterns

Object with properties — the most common pattern in this codebase:

JS_FRAME(js);
JS_ROOT(result, JS_NewObject(js));
JS_SetPropertyStr(js, result.val, "width", JS_NewInt32(js, w));
JS_SetPropertyStr(js, result.val, "height", JS_NewInt32(js, h));
JS_SetPropertyStr(js, result.val, "pixels", js_new_blob_stoned_copy(js, data, len));
JS_RETURN(result.val);

Array with loop — root the element variable before the loop, then reassign .val each iteration:

JS_FRAME(js);
JS_ROOT(arr, JS_NewArray(js));
JS_ROOT(item, JS_NULL);
for (int i = 0; i < count; i++) {
  item.val = JS_NewObject(js);
  JS_SetPropertyStr(js, item.val, "index", JS_NewInt32(js, i));
  JS_SetPropertyStr(js, item.val, "data", js_new_blob_stoned_copy(js, ptr, sz));
  JS_SetPropertyNumber(js, arr.val, i, item.val);
}
JS_RETURN(arr.val);

WARNING — NEVER put JS_ROOT inside a loop. JS_ROOT declares a JSGCRef local and calls JS_PushGCRef(&name), which pushes its address onto a linked list. Inside a loop the compiler reuses the same stack address, so on iteration 2+ the list becomes self-referential (ref->prev == ref). When GC triggers it walks the chain and hangs forever. This bug is intermittent — it only manifests when GC happens to run during the loop — making it very hard to reproduce.

Nested objects — root every object that persists across an allocating call:

JS_FRAME(js);
JS_ROOT(outer, JS_NewObject(js));
JS_ROOT(inner, JS_NewArray(js));
// ... populate inner ...
JS_SetPropertyStr(js, outer.val, "items", inner.val);
JS_RETURN(outer.val);

Inside JSC_CCALL — use JS_RestoreFrame and assign to ret:

JSC_CCALL(mymod_make,
  JS_FRAME(js);
  JS_ROOT(obj, JS_NewObject(js));
  JS_SetPropertyStr(js, obj.val, "x", number2js(js, 42));
  JS_RestoreFrame(_js_ctx, _js_gc_frame, _js_local_frame);
  ret = obj.val;
)

Macros

Macro	Purpose
JS_FRAME(js)	Save the GC frame. Required before any JS_ROOT.
JS_ROOT(name, init)	Declare a JSGCRef and root its value. Access via name.val.
JS_LOCAL(name, init)	Declare a rooted JSValue (GC updates it through its address).
JS_RETURN(val)	Restore the frame and return a value.
JS_RETURN_NULL()	Restore the frame and return JS_NULL.
JS_RETURN_EX()	Restore the frame and return JS_EXCEPTION.
JS_RestoreFrame(...)	Manual frame restore (for JSC_CCALL bodies that use ret =).

Error return rules

• Error returns before JS_FRAME can use plain return JS_ThrowTypeError(...) etc.
• Error returns after JS_FRAME must use JS_RETURN_EX() or JS_RETURN_NULL() — never plain return, which would leak the GC frame.

Migrating from gc_mark

The old mark-and-sweep GC had a gc_mark callback in JSClassDef for C structs that held JSValue fields. This no longer exists. The copying GC needs to know the address of every pointer to update it when objects move.

If your C struct holds a JSValue that must survive across GC points, root it for the duration it's alive:

typedef struct {
  JSValue callback;
  JSLocalRef callback_lr;
} MyWidget;

// When storing:
widget->callback = value;
widget->callback_lr.ptr = &widget->callback;
JS_PushLocalRef(js, &widget->callback_lr);

// When done (before freeing the struct):
// The local ref is cleaned up when the frame is restored,
// or manage it manually.

In practice, most C wrappers hold only opaque C pointers (like SDL_Window*) and never store JSValues in the struct — these need no migration.

Static Declarations

Keep internal functions and variables static:

static int helper_function(int x) {
  return x * 2;
}

static int module_state = 0;

This prevents symbol conflicts between packages.

Troubleshooting

Missing header / SDK not installed

If a package wraps a third-party SDK that isn't installed on your system, the build will show:

module.c: fatal error: 'sdk/header.h' file not found (SDK not installed?)

Install the required SDK or skip that package. These warnings are harmless — other packages continue building normally.

CFLAGS not applied

If your cell.toml has a [compilation] section but flags aren't being picked up, check:

1. The TOML syntax is valid (strings must be quoted)
2. The section header is exactly [compilation] (not [compile] etc.)
3. Target-specific sections use valid target names: macos_arm64, macos_x86_64, linux, linux_arm64, windows

API changes from older versions

If C modules fail with errors about function signatures:

• JS_IsArray takes one argument (the value), not two — remove the context argument
• Use JS_GetPropertyNumber / JS_SetPropertyNumber instead of JS_GetPropertyUint32 / JS_SetPropertyUint32
• Use JS_NewString instead of JS_NewAtomString
• There is no undefined — use JS_IsNull and JS_NULL only