File descriptor interfacing

[Richard Beare <Richard.Beare@cmis.csiro.au>]

I am producing some bindings to image libraries, like libtiff. Some of
the functions in these libraries take file descriptors as arguments. I
definitely want to retain functions operating at this level (file_type
rather than file names), but I am not sure about the most appropriate
way to interface.

The gnat os_lib package is one option, but that means the library user
would have to start using file descriptors as well. I think it would be
better if the user could pass a "file_type" from which the c file
descriptor could be extracted.

This information is in the system.file_control_block package, but I am
unsure about the best way of extracting the "stream" information. The
file control block is a private time, so I guess I should be using
unchecked conversion. Is this the only way?

-- 
Richard Beare
Richard.Beare@cmis.csiro.au