Ffi, struct pointer question

Ruby
1

Hi,

I’ve recently been trying to port some C code over to FFI. I’m trying
to figure out how to unravel the gr_mem group struct member. Any
suggestions?

require ‘ffi’

module Sys
class Admin
extend FFI::Library

  class GroupStruct < FFI::Struct
     layout(
        :gr_name,   :string,
        :gr_passwd, :string,
        :gr_gid,    :int,
        :gr_mem,    :pointer
     )
  end

  attach_function :getgrgid, [:int], :pointer

  def self.get_group(gid)
     GroupStruct.new(getgrgid(gid))
  end

end
end

How do I unravel the gr_mem struct member?

struct = Sys::Admin.get_group(84)
p struct[:gr_mem]

Regards,

Dan

2

2009/8/2 Daniel B. [email protected]:

 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

:wink:

I hope that helps,
Thomas

3

On Aug 2, 6:13 am, Thomas C. [email protected] wrote:

 end

    class FFI::Pointer
                    elements

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.

Regards,

Dan