Minimal C-Lua callback

app.lua

local foo = "INNER";
register(function() print(foo) end);

main.c

#include <lua5.3/lua.h>
#include <lua5.3/lauxlib.h>
#include <lua5.3/lualib.h>

static int reg(lua_State *L) {
    if(lua_gettop(L)==1 && lua_isfunction(L, -1)) {
        // Save callback to "callback" index
        lua_setfield(L, LUA_REGISTRYINDEX, "callback");
    }
    else {/* Handle error */ }
    return 0;
}

int main() {
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);

    lua_pushcfunction(L, reg);     // |
    lua_setglobal(L, "register");  // | Insert api function

    luaL_dofile(L, "app.lua");

    lua_getfield(L, LUA_REGISTRYINDEX, "callback"); // this would be your
    lua_pcall(L, 0, 0, 0);                          // event callback
    return 0;
}

makefile

C_FLAGS= -c -Wall -DLUA_USE_APICHECK -std=c99
L_FLAGS= -Wall
L_LIBS=  -llua5.3
CC=gcc
LD=gcc

all: main

clean:
        rm main.o

main: main.o
        $(LD) $(L_FLAGS) -o $@ $< $(L_LIBS)

main.o: main.c
        $(CC) $(C_FLAGS) -o $@ $<

install_ubuntu:
        sudo apt install liblua5.3-dev