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Lua Academy · Lesson

Registering C Functions in Lua

Expose C functions to Lua scripts using luaL_register and lua_pushcfunction.

Lesson 3 of 413 steps

Registering C Functions in Lua is a free Lua Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, and your progress syncs across the web and the CoddyKit app. The Lua Academy course includes 4 lessons in total.

Overview

C functions can be exposed to Lua scripts. They must have the signature int myFunc(lua_State *L). They read arguments from the stack and push return values.

C Function Signature

Every function callable from Lua is a lua_CFunction: it takes a lua_State*, reads arguments, pushes results, and returns the count of results.

// C code
static int l_add(lua_State *L) {
  double a = luaL_checknumber(L, 1);
  double b = luaL_checknumber(L, 2);
  lua_pushnumber(L, a + b);
  return 1;  // one result
}

Registering as a Global

lua_pushcfunction(L, l_add) pushes the C function, then lua_setglobal(L, "add") registers it as a global in Lua.

// C code
lua_pushcfunction(L, l_add);
lua_setglobal(L, "add");
// Now Lua can call: add(3, 4)

luaL_newlib / luaL_register

Register a batch of C functions as a Lua module using a luaL_Reg array and luaL_newlib.

// C code
static const luaL_Reg mylib[] = {
  {"add", l_add},
  {"mul", l_mul},
  {NULL, NULL}
};
luaL_newlib(L, mylib);
lua_setglobal(L, "mylib");
// Lua: mylib.add(1, 2)

Reading Arguments

luaL_checknumber(L, n), luaL_checkstring(L, n), etc. read and validate arguments, raising a Lua error if the type is wrong.

Returning Multiple Values

Push multiple values and return their count. Lua will assign them as multiple return values.

// C code
static int l_divmod(lua_State *L) {
  int a = luaL_checkinteger(L, 1);
  int b = luaL_checkinteger(L, 2);
  lua_pushinteger(L, a / b);
  lua_pushinteger(L, a % b);
  return 2;  // two results
}

Upvalues for C Closures

C closures capture upvalues at registration time using lua_pushcclosure(L, fn, n). Access upvalues with lua_upvalueindex(n).

Error Reporting from C

Call luaL_error(L, format, ...) to raise a Lua error from a C function. This never returns — it longjmps to the protected call handler.

// C code
if (divisor == 0)
  luaL_error(L, "division by zero at argument %d", 2);

C Module as a .so/.dll

A C module exposes a luaopen_modname function. Lua's require loads the shared library and calls it to initialize the module.

// C code (shared library entry point)
int luaopen_mymod(lua_State *L) {
  luaL_newlib(L, mymod_funcs);
  return 1;
}

Argument Count Checking

Use lua_gettop(L) to check the number of arguments passed. luaL_checkany(L, n) ensures at least n arguments were provided.

LuaJIT FFI Alternative

LuaJIT's FFI lets you call C functions directly from Lua without writing C wrapper code. It is often simpler for pure function calls. The C API is needed for deeper integration.

C Function Question

What must a Lua-callable C function return?

Answer it in the interactive lesson

Recap: Registering C Functions

C functions callable from Lua have signature int f(lua_State*). Read args with luaL_check*, push results, return count. Register individually with lua_pushcfunction or in bulk with luaL_newlib.

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Frequently Asked Questions

Is the “Registering C Functions in Lua” lesson free?

Yes — any 3 lessons of the Lua Academy learning path, including “Registering C Functions in Lua”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Lua Academy course includes 4 lessons in total.

What will I learn in “Registering C Functions in Lua”?

Expose C functions to Lua scripts using luaL_register and lua_pushcfunction. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Registering C Functions in Lua” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Lua Academy lesson?

Yes. Every Lua Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. The Lua C API Stack Model
  2. Calling Lua Functions from C
  3. Registering C Functions in Lua
  4. Managing State and Error Handling
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