Lua C API Examples

lua_examples.c

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

// Helper function to print the stack contents
// Values other than numbers and strings are printed as "(null)"
void print_stack(lua_State *L, char* title){
  int nargs = lua_gettop(L);
  printf("   --- STACK at %s ---\n", title);
  for(int i=1; i<=nargs; i++) {
    printf("   - S[%d] = %s\n", i, lua_tostring(L, i));
  }
  printf("   ---\n");
}

int l_cls(lua_State *L) {
  print_stack(L, "entry cls");
  lua_settop(L,0); // clear stack
  lua_pushstring(L, "Not returned");
  lua_pushvalue(L, lua_upvalueindex(1));      // ret1
  lua_pushvalue(L, lua_upvalueindex(2));      // ret2
  lua_pushvalue(L, lua_upvalueindex(3));      // ret3
  lua_pushstring(L, "Hello world from CLS!"); // ret4
  print_stack(L, "exit cls");
  return 4;
}

int l_closure(lua_State *L) {
  print_stack(L, "entry l_closure");
  lua_pushstring(L, "a");
  lua_pushstring(L, "b");
  lua_pushstring(L, "c");
  lua_pushcclosure(L, l_cls, 3);
  print_stack(L, "exit l_closure");
  return 1;
}

int l_simple(lua_State *L) {
  print_stack(L, "entry l_simple");
  // push some more garbage onto the stack
  lua_pushstring(L, "some ");
  lua_pushstring(L, "more");
  lua_pushstring(L, "garbage");
  // push some return values
  lua_pushstring(L, "ret1");
  lua_pushstring(L, "ret2");
  lua_pushstring(L, "ret3");
  // return the top 3 values on the stack
  // in order S[-3], S[-2], S[-1]
  print_stack(L, "exit l_simple");
  return 3;
}

int main(int argc, char **argv){
  static const struct luaL_Reg l_lib[] = {
    {"simple", l_simple},
    {"closure", l_closure},
    {NULL, NULL}
  };
  lua_State *L = lua_open();
  luaL_openlibs(L);
  luaL_openlib(L, "lib", l_lib, 0);
  luaL_dostring(L, "print(\"Simple function call:\")");
  luaL_dostring(L, "print(lib.simple(1,2,3))");
  luaL_dostring(L, "print(\"Call a closure\")");
  luaL_dostring(L, "print(lib.closure()(1,2,3))");
  return 0;
}

Example run:

$ gcc -O0 -g -std=c99 \
   -I/<path to luajit>/include/luajit-2.1 -L/<path to luajit>/lib/ -lluajit-5.1 \
   lua_example.c -o lua_example && \
   ./lua_example

Simple function call:
   --- STACK at entry l_simple ---
   - S[1] = 1
   - S[2] = 2
   - S[3] = 3
   ---
   --- STACK at exit l_simple ---
   - S[1] = 1
   - S[2] = 2
   - S[3] = 3
   - S[4] = some
   - S[5] = more
   - S[6] = garbage
   - S[7] = ret1
   - S[8] = ret2
   - S[9] = ret3
   ---
ret1    ret2    ret3
Call a closure
   --- STACK at entry l_closure ---
   ---
   --- STACK at exit l_closure ---
   - S[1] = (null)
   ---
   --- STACK at entry cls ---
   - S[1] = 1
   - S[2] = 2
   - S[3] = 3
   ---
   --- STACK at exit cls ---
   - S[1] = Not returned
   - S[2] = a
   - S[3] = b
   - S[4] = c
   - S[5] = Hello world from CLS!
   ---
a       b       c       Hello world from CLS!