From: Thomas Chust <chust@...>
Date: 2009-08-02T21:13:35+09:00
Subject: Re: ffi, struct pointer question

2009/8/2 Daniel Berger <djberg96@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


-- 
When C++ is your hammer, every problem looks like your thumb.