Ask, how to implement the function like ipairs with C api?

[sagasw <sagasw@...>]

Hi guys, 
I have a question when I read the manual.
lua_gettable could get table, and lua_getfield could get specific key.

If I only know the table in top of stack like：

local tbl_sample = {a = "aaa", b = "bbb", c= "ccc", 15}

How could I get the information of table key-value pairs in c? And the case is I don't know the key.

Just like the function ipairs(), it doesn't need to know the key before using the table.

I read the source code of ipairs(), it says to use upvalue to aux the function.

Who could give me c code sample for it?

Thanks,