On Friday, November 20, 2015 at 5:34:33 PM UTC, Liz Huang wrote:
> Hi,
>
> I am trying to use Fiddle to call C function in dynamic library, I used to be
> able to
> pass a return long variable and an error message, but now only return long
> variable
> is returned, can't get the error message, I create a very simple example to
> test,
> this is my add.c
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
> long add(long maxn, double delta, double conf, char *errMsg)
> {
> long answer;
>
> errMsg = (char *) malloc(6*sizeof(char));
> answer = (long)(maxn + delta + conf);
> errMsg = "Hello!";
Arguments are passed by value in C, so if you assign to a variable in a
function it does not change its value in the calling function. You need to
change your C function so that either:
- the argument is a pointer to some memory allocated by the caller and you copy
into it by strncpy or similar
- the argument is a pointer to a pointer size block of memory. Your function
would then allocate a buffer, write the error message to that buffer and then
write the value of the pointer to the argument, ie the last argument to the
function is now char **message and your code does *message = malloc(...)
Fred
>
> return answer;
> }
>
> and add.h
>
> #ifndef add_h
> #define add_h
>
> long add(long maxn, double delta, double conf, char *errMsg);
>
> #endif
>
> I created lib_add.so and part of my .rb file calling fiddle:
>
> class SamplesizeController < ApplicationController
> require 'fiddle'
>
> .....
> def compute
>
> str = "there!"
> libm = Fiddle.dlopen('/var/www/myapp/smart/lib/lib_add.so')
> add = Fiddle::Function.new(libm['add'],[Fiddle::TYPE_LONG,
> Fiddle::TYPE_DOUBLE,
> Fiddle::TYPE_DOUBLE,
> Fiddle::TYPE_VOIDP],
> Fiddle::TYPE_LONG)
>
> add.call(session[:nmax], session[:delta], session[:conf], str)
> session[:errmsg] = str.to_s
> redirect_to :action => "results"
> end
>
> ....
>
> end
>
> session[:errmsg] can be modified? only return variable can be changed or
> something I did
> was wrong? Any advice will be greatly appreciated.
>
> Thanks!
> Liz
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