Ruby on Rails

I have trouble even getting adding value n returned when I tried to use
Fiddle::Function.new,  I returned to my old way of using Fiddle:

class SamplesizeController < ApplicationController
require 'fiddle'
require 'fiddle/import'

  module Libm
    extend Fiddle::Importer
    dlload '/var/www/myapp/smart/lib/lib_add.so'
    extern 'long add(long, double, double, char *)'
  end

  def compute
        session[:n] = Libm.add( session[:nmax], session[:delta],
                        session[:conf], session[:errmsg] )

 end

...
end

This way, my session[:n] is returned correctly, even I couldn't
get session[:errmsg].

Before, I was able to pass session[:errmsg] and get out
errormsg (don't need to deal with Fiddle::Pointer), my
c code, I didn't use malloc for errmsg, but copy another
char * to errmsg using a for loop...

Liz



On Monday, November 23, 2015 at 1:02:36 PM UTC-5, Liz Huang wrote:

If I don't allocate a new buffer, then I don't need pointer to a pointer? For example, I
just copy an existing char * iarr to errMsg?


long add(long maxn, double delta, double conf, char *errMsg)
{
        long answer;
        int i;
        char *iarr;

        //errMsg = (char *) malloc(6*sizeof(char));
        answer = (long)(maxn + delta + conf);
        iarr = "Hello!";
        for(i=0;i<6;i++) errMsg[i] = (char) iarr[i];

        return answer;
}

The document for Fiddle is so limited, wonder where I could find an example doing 
what you said? Now I can just guess and try, guess I use

buf = Fiddle::Pointer.malloc(8)

then pass buf to C function,

then try to convert string from buf returned?



Liz


On Monday, November 23, 2015 at 12:21:27 PM UTC-5, Frederick Cheung wrote:

On Monday, November 23, 2015 at 3:25:07 PM UTC, Liz Huang wrote:

Thanks!

Strange that I am able to get error message before just using char *, thought
it meant passing string by pointer already.

it does, but if you want to allocate a new buffer and have the caller access that then you need pointer to a pointer

 

I change the argument to char ** errMsg, but in Ruby, when I tried to
convert the pointer to string, I got error message:

[snip]

 

if options.is_a?(::JSON::State) # Called from JSON.{generate,dump}, forward it to JSON gem's to_json self.to_json_without_active_support_encoder(options) else # to_json is being invoked directly, use ActiveSupport's encoder ActiveSupport::JSON.encode(self, options)

Do I need to  change anything in ruby file when use a pointer?

Yes. You need to allocate a pointer size bit of memory for the C function (via Fiddle::Pointer). The C function fills that in, you then use the ptr method on Fiddle::Pointer to get the memory allocated by the C function, and create your string from there. (don't forget to free the memory too)

Fred