Adds method to check of pointer size if bounded

[ankane]

Hi, thanks for this great library!

This PR adds a method to tell if the pointer size is bounded.

ptr = FFI::Pointer.new(0)
ptr.bounded?              # false
ptr.slice(0, 10).bounded? # true

This can be useful for libraries that accept FFI::Pointer arguments.

def load_image(ptr, ...)
  raise "Cannot load image from pointer with unbounded size" unless ptr.bounded?
  # do something with size ...
end

[eregon]

This sounds like a good idea to me.

Right now FFI::Pointer#size returns LONG_MAX for unbounded pointers.
But maybe it should return nil instead to not leak that implementation detail?
That could break some things that rely on #size always being an Integer though.

[ankane]

Thanks @eregon. I think nil or 0 would be more intuitive. Currently, code that works well with a FFI::MemoryPointer could end up reading too much data if passed an unbounded FFI::Pointer (for C functions that take a pointer and bytesize).

Edit: Somewhat related, it would also be nice to allow a size to be specified on initialization.

[larskanis]

Returning nil would be more intuitive, but I worry about compatibility. I don't like the name bounded? - maybe because I'm not a native English speaker. How about size_limit? or has_size? .

[ankane]

Hey @larskanis, thanks for the response! I thought bounded? might be good since cannot duplicate unbounded memory area is a current error message. Just pushed size_limit? and fixed the merge conflict.

[ankane]

Thanks @larskanis!