From: Daniel Berger <djberg96@...>
Date: 2009-08-02T23:07:31+09:00
Subject: Re: ffi, struct pointer question



On Aug 2, 6:13 am, Thomas Chust <ch...@web.de> wrote:
> 2009/8/2 Daniel Berger <djber...@gmail.com>:
>
>
>
> > [...]
> >      class GroupStruct < FFI::Struct
> >         layout(
> > [...]
> >            :gr_mem,    :pointer
> >         )
> >      end
>
> >      attach_function :getgrgid, [:int], :pointer
>
> >      def self.get_group(gid)
> >         GroupStruct.new(getgrgid(gid))
> >      end
> > [...]
> > # How do I unravel the gr_mem struct member?
> > struct = Sys::Admin.get_group(84)
> > p struct[:gr_mem]
>
> Hello,
>
> the gr_mem member of the group structure is a NULL terminated array of
> zero terminated strings, so you need to read all the string pointers
> from the array until you encounter a null pointer and then read the
> string data from each string pointer.
>
> I would suggest the use of an auxiliary method, which could actually
> be added to the FFI::Pointer class for convenience:
>
>         class FFI::Pointer
>                 def read_string_array_to_null()
>                         elements = []
>
>                         psz = self.class.size
>                         loc = self
>                         until ((element = loc.read_pointer).null?)
>                                 elements << element.read_string_to_null
>                                 loc += psz
>                         end
>
>                         elements
>                 end
>         end
>
> Using this method, you can unravel the mysterious pointer member as follows:
>
>         p struct[:gr_mem].read_string_array_to_null
>
> ;-)
>
> I hope that helps,
> Thomas

Thank you for that Thomas, it worked great.

I also found nice-ffi, and I suspect it may have some convenient way
of doing what you're doing there, only I don't see it.

http://github.com/jacius/nice-ffi/tree/master

Regards,

Dan