lua_getinfo(L, "f", &ar);  /* retrieves function */
                 i = 1;
                 while ((name = lua_getupvalue(L, -1, i++)) != NULL) {
                   printf("upvalue %d %s\n", i-1, name);
                   lua_pop(L, 1);  /* remove upvalue value */
                 }
                 return 1;
               }
<

==============================================================================
4  THE AUXILIARY LIBRARY                                            *lua-aux*

The auxiliary library provides several convenient functions to interface C
with Lua. While the basic API provides the primitive functions for all
interactions between C and Lua, the auxiliary library provides higher-level
functions for some common tasks.

All functions from the auxiliary library are defined in header file `lauxlib.h`
and have a prefix `luaL_`.

All functions in the auxiliary library are built on top of the basic API, and
so they provide nothing that cannot be done with this API.

Several functions in the auxiliary library are used to check C function
arguments. Their names are always `luaL_check*` or `luaL_opt*`. All of these
functions raise an error if the check is not satisfied. Because the error
message is formatted for arguments (e.g., "bad argument #1"), you should not
use these functions for other stack values.

==============================================================================
4.1  Functions and Types                                   *lua-auxFunctions*

Here we list all functions and types from the auxiliary library in
alphabetical order.

luaL_addchar                                                    *luaL_addchar()*
>c
    void luaL_addchar (luaL_Buffer *B, char c);
<
        Adds the character `c` to the buffer `B` (see |luaL_Buffer|).

luaL_addlstring                                              *luaL_addlstring()*
>c
    void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);
<
        Adds the string pointed to by `s` with length `l` to the buffer `B`
        (see |luaL_Buffer|). The string may contain embedded zeros.

luaL_addsize                                                    *luaL_addsize()*
>c
    void luaL_addsize (luaL_Buffer *B, size_t n);
<
        Adds to the buffer `B` (see |luaL_Buffer|) a string of length
        `n` previously copied to the buffer area (see
        |luaL_prepbuffer()|).

luaL_addstring                                                *luaL_addstring()*
>c
    void luaL_addstring (luaL_Buffer *B, const char *s);
<
        Adds the zero-terminated string pointed to by `s` to the buffer `B`
        (see |luaL_Buffer|). The string may not contain embedded zeros.

luaL_addvalue                                                  *luaL_addvalue()*
>c
    void luaL_addvalue (luaL_Buffer *B);
<
        Adds the value at the top of the stack to the buffer `B` (see
        |luaL_Buffer|). Pops the value.

        This is the only function on string buffers that can (and must) be
        called with an extra element on the stack, which is the value to be
        added to the buffer.

luaL_argcheck                                                  *luaL_argcheck()*
>c
    void luaL_argcheck (lua_State *L,
                        int cond,
                        int narg,
                        const char *extramsg);
<
        Checks whether `cond` is true. If not, raises an error with the
        following message, where `func` is retrieved from the call stack:
>
               bad argument #<narg> to <func> (<extramsg>)
<

luaL_argerror                                                  *luaL_argerror()*
>c
    int luaL_argerror (lua_State *L, int narg, const char *extramsg);
<
        Raises an error with the following message, where `func` is retrieved
        from the call stack:
>
               bad argument #<narg> to <func> (<extramsg>)
<
        This function never returns, but it is an idiom to use it in C
        functions as `return luaL_argerror(` `args` `)`.

luaL_Buffer