A MemoryPointer is an allocated Pointer.

i.e. it handles allocation and deallocation of the memory.
FFI::MemoryPointer.new(x) tries to allocate x bytes.

FFI::Pointer.new on the other hand, just wraps whatever you give it in
a Pointer instance

btw, you should be checking for Integer in your code, not Fixnum.  A
Fixnum in MRI cannot hold all the bits in a pointer.

e.g. on 32bit:
a = 0xfee1dead # this is a valid 32bit address
puts "a.is_a?(Fixnum)=#{a.is_a?(Fixnum)}"
# => false
puts "a.is_a?(Integer)=#{a.is_a?(Integer)}"
# => true

Or the better way is to do:

pointer = address.is_a?(FFI::Pointer) ? address : FFI::Pointer.new(address)

Or you could always just wrap it using FFI::Pointer.new(address),
since FFI::Pointer.new can take pointer arguments as well as Integer
arguments

2009/9/16 Brett Blackham <brett.blackham@gmail.com>:

>
> Ah ha! Thanks.
>
> pointer = FFI::Pointer.new(address) # Works
> pointer = FFI::MemoryPointer.new(address) # Fails.
>
> Thanks again!
>
> Now I am curious what is the difference between a Pointer and
> MemoryPointer. It was my understanding that a pointer is a pointer.
>