Passing a table from Lua into C

Archive - Originally posted on "The Horse's Mouth" - 2008-10-18 14:49:17 - Graham Ellis

In a previous article, I showed you a simple example of how you can call a function that's written in C from Lua, and a second example that extended that - passing simple parameters in to the C function, and returning results.

But it's often more complex that that - for example, you may with to pass a table from your Lua script into a C function, so that the C function can make use of a value from the table code like this:

stuff = {hotel = 48, hq = 404, town = 1}

summat = extratasks.dothis(stuff)

This isn't as easy as just passing a value, since the size of a table can vary, and Lua's dynamic memory allocation doesn't sit well, in a simple interface, with C's static model. The "trick" is to tackle it within the C code by using function calls to get (and if necessary) reset values from the table - with the data required by those function calls being processed via the Lua stack.

Here's an example of what I mean in the C code:

/* Looking up based on the key */

/* Add key we're interested in to the stack*/

lua_pushstring(L,"hq");

/* Have Lua functions look up the keyed value */

lua_gettable(L, -2 );

/* Extract the result which is on the stack */

double workon = lua_tonumber(L,-1);

/* Tidy up the stack - get rid of the extra we added*/

lua_pop(L,1);

That code retrieves the "hq" value from the table that's been passed in (called stuff in my Lua example above) and stores it into a C variable called workon ... objective achieved!

The full source code of the Lua is available here and the C source code is here.

Here's sample output from my testing of the code:

[trainee@easterton three]$ lua wrapper.lua

Lua code running ...

Violets are Blue

Table passed to humbug

Roses are Red

... back into Lua code

Values in Lua now       527     -511

Table contains ...

hq      404

town    1

hotel   48

[trainee@easterton three]$

Looking around, I have found a number of very much more complex examples of how to embed C in Lua, but nothing as easy as the 1 - 2 - 3 steps I have shown you in this article and the previous one. If you're looking to pass strings (strings in Lua are not null terminated, so differ), to iterate through a Lua table within C, or to modify a table, you'll find that covered in Lua's reference documentation. If you would like to learn more about Lua - from the basics through to the topics covered in this article - and you feel I could help you, please have a look at our public Lua Courses or ask about a private course if you're looking at more specialised needs, or if you have a group of delegates.

Illustration - Delegates on a private Lua course. Although we'll run private courses for larger groups, we limit public courses to just eight delegates so that we can provide good coverage and support for the wide variety of applications to which Lua is put

Main Content

Passing a table from Lua into C