Quick and dirty example of how to wrap a callback registration

CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(luac C)
set(CMAKE_C_STANDARD 11)
find_package(PkgConfig REQUIRED)
pkg_search_module(LUA REQUIRED IMPORTED_TARGET luajit luajit2)
add_executable(retval retval.c)
target_link_libraries(retval PUBLIC PkgConfig::LUA)

retval.c

#include <stdio.h>
#include <string.h>
#include <lua.h>
#include <lauxlib.h>
#include <lualib.h>

// This is the code registered by the user as a string
char usercode[] = "return function() return 42 end";

char header[] = "callback = (function()\n";
char footer[] = "\nend)()";

char buf[1024] = "";

// Call wrapper we use to avoid checking the retval with LuaWrappers.

int main (void) {
  strcat(buf, header);
  strcat(buf, usercode);
  strcat(buf, footer);

  int error;
  lua_State *L = lua_open();   /* opens Lua */
  luaL_openlibs(L);

  error = luaL_loadbuffer(L, buf, strlen(buf), "line") || lua_pcall(L, 0, 0, 0);
  if (error) {
    fprintf(stderr, "%s", lua_tostring(L, -1));
    lua_pop(L, 1);  /* pop error message from the stack */
    return 1;
  }

  lua_getglobal(L, "callback");
  if (lua_pcall(L, 0, 1, 0) != 0) {
    fprintf(stderr, "%s", lua_tostring(L, -1));
    return 1;
  }
  if (!lua_isnumber(L, -1)) {
    fprintf(stderr, "not a number\n");
    return 1;
  }
  int n = lua_tonumber(L, -1);
  lua_pop(L, 1);
  fprintf(stderr, "res: %i\n", n); // prints 42

  lua_close(L);
  return 0;
}