# SWIG question: how to return an array from C to Ruby?

**URL:** <https://www.ruby-forum.com/t/swig-question-how-to-return-an-array-from-c-to-ruby/111638>\
**Category:** Ruby\
**Created:** [August 9, 2007, 12:21pm UTC](https://www.ruby-forum.com/t/swig-question-how-to-return-an-array-from-c-to-ruby/111638 "2007-08-09T12:21:36Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Joachim\_SMSSS\_nchen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/5f8ce5/32.png) [@Joachim\_SMSSS\_nchen](https://www.ruby-forum.com/u/Joachim_SMSSS_nchen)\
**Post date:** [August 9, 2007, 12:21pm UTC](https://www.ruby-forum.com/t/swig-question-how-to-return-an-array-from-c-to-ruby/111638/1 "2007-08-09T12:21:36Z")

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I need to make some C functions available to Ruby. One of them sets an  
array of doubles. I want to return this array to Ruby.

On the C side:

data\_reader.c:  
#include “hardwaredriver.h”  
…  
void read\_data( double \*data )  
{  
for( i=0; i\<8; ++i )  
data[i] = … // get values from hardware driver  
}

On the Ruby side, I would like to obtain the following

irb\> require “data\_reader”  
true  
irb\> arr = Data\_reader.read\_data  
[0.0, 0.1, …] # arr now contains eight Floats

Question 1:  
Do I understand correctly that there are basically two different ways  
to proceed ?:  
(a) wrapping the C code with SWIG,  
(b) converting the C code to an explicit Ruby extension as  
described in the Book: [http://www.rubycentral.com/pickaxe/ext\_ruby.html](http://www.rubycentral.com/pickaxe/ext_ruby.html)

IF the answer is yes, THEN  
Question 2:  
How do I return an array through C-\>SWIG-\> Ruby ?

Web search gave me some hints on how to do it in the opposite  
direction, using typemap(in); but it seems not trivial to invert  
to typemap(out).

Thanks in advance, Joachim

---

<div class="post-metadata">

**Author:** ![Joachim\_SMSSS\_nchen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/5f8ce5/32.png) [@Joachim\_SMSSS\_nchen](https://www.ruby-forum.com/u/Joachim_SMSSS_nchen)\
**Post date:** [August 9, 2007, 7:58pm UTC](https://www.ruby-forum.com/t/swig-question-how-to-return-an-array-from-c-to-ruby/111638/2 "2007-08-09T19:58:42Z")

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On Thursday 09 August 2007 03:20:04 am Joachim (München) wrote:

> ```
> for( i=0; i<8; ++i )
> 
> ```
> 
> Question 1:  
> Do I understand correctly that there are basically two different ways  
> to proceed ?:  
> (a) wrapping the C code with SWIG,  
> (b) converting the C code to an explicit Ruby extension as  
> described in the Book: [http://www.rubycentral.com/pickaxe/ext\_ruby.html](http://www.rubycentral.com/pickaxe/ext_ruby.html)
> 
> IF the answer is yes, THEN

While I don’t know much (anything) about SWIG, C code needs to be  
wrapped  
as an extension to Ruby or indirectly wrapped via Inline for it to be  
used  
by ruby code.

> Question 2:  
> How do I return an array through C-\>SWIG-\> Ruby ?
> 
> Web search gave me some hints on how to do it in the opposite  
> direction, using typemap(in); but it seems not trivial to invert  
> to typemap(out).
> 
> Thanks in advance, Joachim

I’d recommend Inline for this, as it provides nice ruby-\>C parameter  
conversions. To return ruby array data, you need a variable of type  
VALUE,  
which is a generic ruby object container. It’s also probably easiest if  
you  
know how long the array will be in advance (in fact I don’t know what to  
do  
otherwise).

ex:  
#define ARR\_LEN 31

VALUE ary;  
double data[ARR\_LEN];

… /\* do your hw-\>data[] processing here \*/

ary = rb\_ary\_new2(ARR\_LEN);  
for(i = 0; i \< ARR\_LEN; i++)  
RARRAY(ary)-\>ptr[i] = rb\_float\_new(data[i]);  
RARRAY(ary)-\>len = ARR\_LEN;  
return ary;

Something like that ought to do the trick. Happy ruby-ing!