[ruby-core:23414] Re: ruby-ffi problem

If you want to change the string value of a field in a struct through  
FFI, I suggest using the ":pointer" rather than ":string".

=cut

require 'rubygems'
require 'ffi'

module LibTest
   class Info < FFI::Struct
     layout :name, :pointer,
            :val, :double
   end

   extend FFI::Library
   ffi_lib './libtest.dylib'
   attach_function :create, [:string, :double], :pointer
   attach_function :show, [:pointer], :int
end

include FFI

ptr = LibTest.create( "test string", 11.27)
obj = LibTest::Info.new(ptr)
LibTest.show(ptr)
obj[:val] = 27.11
obj[:name].put_bytes(0,'new string')
LibTest.show(ptr)

=end


In the long run, as Charlie suggested, your question will be better  
answered by ruby-ffi project.

-- 
Hirotsugu Asari



On May 9, 2009, at 4:48 AM, Aston wrote:

> Hello,
>
> I am trying to learn ruby-ffi now (after sadly leaving dl library),  
> I have some trouble here too
>
> I will present a simple example where I want to pass a struct  
> between ruby and C
> I am facing problems while setting fields of struct if that field is  
> of type string(char*, not char[])
> I understand we have to allocate the char* field of struct first  
> than assign some literal srting
>
> consider the C file
>
> ********************** C FILE**********************
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
>
> typedef struct
> {
>     char *name;
>     double val;
> } INFO, *INFO_PTR;
>
> INFO_PTR create( char* name, double val)
> {
>     INFO_PTR p = (INFO_PTR)malloc( sizeof(INFO));
>     p->val = val;
>     p->name = (char*)malloc( strlen(name) + 1);
>     strcpy( p->name, name);
>     return p;
> }
>
> int show( INFO_PTR pInfo)
> {
>     return printf( "%s - %f\n", pInfo->name, pInfo->val);
> }
>
> ********************** END C FILE**********************
>
> ********************** RUBY FILE**********************
> require 'ffi'
>
> module LibTest
>   class Info < FFI::Struct
>     layout :name, :string,
>            :val, :double
>   end
>
>   extend FFI::Library
>   ffi_lib "./libtest.so"
>   attach_function :create, [:string, :double], :pointer
>   attach_function :show, [:pointer], :int
> end
>
> include FFI
>
> ptr = LibTest.create( "test string", 11.27)
> obj = LibTest::Info.new(ptr)
> LibTest.show(ptr)
> obj[:val] = 27.11 # this works
> # obj[:name] = "new string" # this gives error, `[]=': Cannot  
> set :string fields (ArgumentError)
> LibTest.show(ptr)
> ********************** END RUBY FILE**********************
>
> how can I set string values from ruby ?
> If I refuse to declare name field as type char[] from char* then I  
> have to allocate before I can assign
> then my approach below core dumps :(
>
> 1 str = "test string"
> 2 p = MemoryPointer.new( str.size)
> 3 p.write_string(str)
> 4 puts p.read_string # "test string"
> 5 obj[:name].write_pointer(p) # core dumps here!
>
> line # 5 core dumps everytime one tries to write anything there,  
> since pointer is invalid I guess
> how do I allocate memory there ? once allocated can i treat that  
> allocated memory as ruby string ?
>
> Goal I want to achieve is I shall allocate in ruby, assign in ruby  
> and pass in to C code only for modification or read only purpose
> how do I go from here ? can you comment on this ?
>
> Aston
>
>
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> Edition * Click here!