File descriptor interfacing

File descriptor interfacing

[Richard Beare]

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

Re: File descriptor interfacing

[Geert Bosch]

Richard Beare <Richard.Beare@cmis.csiro.au> wrote:
   The file control block is a private time, so I guess I should
   be using unchecked conversion. Is this the only way?

If you think the file control block is just a private type to
make your life harder, you can use unchecked conversion. 
But then, you could as well change the relevant run-time source to
make the type non-private.

In the case that you think the type was made private on purpose
to prevent you from depending on GNAT internals that can change
without notice then just use the provided interface in the package
spec. BTW, note that you never should use packages in the system
hierarchy that are not described in the Ada RM or the GNAT RM!

Regards,
   Geert

Re: File descriptor interfacing

[Robert Dewar]

Richard Beare asks

<<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.
>>

Functions to extract file descriptors from Ada files are standardly 
available in GNAT, as are functions to open files givenm C file descriptors.
How do you find out about all this? You consult the ultra-secret source
of information, namely the GNAT documentation that comes with GNAT :-)
Have a look at the sceion in the manual hidden under the obscure 
title "Interfacing to C Streams"