How to import #define constants from .h file to Ada?

Question

I'm trying to move a constant from C to Ada that is located in a header file.

file.h

#define TEST 0x1234

How would I export this constant to ada? I know to export functions and import them into ada but I can't seem to figure out how to do the same thing for constants.

You can't "export" these as they are not in the compiled binary at all. — Eugene Sh., Feb 28 '18 at 17:51
Is there a way to make a constant equal to the macro and then export that to ada? How would I export and import that? — Sterisk, Feb 28 '18 at 19:03
Are you using `gcc -c -fdump-ada-spec` for the rest of your low level binding? The way I've handled this is to add a second package for the bits the automatic binding generator doesn't handle ... here I'd put `TEST : constant Integer := 16#1234#;` in that second package. — , Feb 28 '18 at 19:23
I'm currently using an .ads for c imports and a .h for c exports. — Sterisk, Feb 28 '18 at 19:25

Simon Wright · Accepted Answer · 2018-03-01T19:42:18.357

If you’re asking how to write an Ada representation of this constant by hand, then the answer is

TEST : constant := 16#1234#;

If you want to maintain the relationship between the C and the Ada automatically, you can get gcc (or, better, g++; see at end) to do the grunt work. Given sterisk.hh containing your example,

#define TEST 0x1234

compile it with

g++ -c -fdump-ada-spec sterisk.hh

which generates in sterisk_hh.ads

pragma Ada_2005;
pragma Style_Checks (Off);

with Interfaces.C; use Interfaces.C;

package sterisk_hh is

   TEST : constant := 16#1234#;  --  sterisk.hh:1

end sterisk_hh;

A third way would be to create a small C source which includes the header (here C is better; you don’t want the name to be mangled in the object file) in say sterisk.c:

#include "sterisk.hh"

const int _name_thats_unlikely_to_clash = TEST;

and compile to sterisk.o:

gcc -c sterisk.c

Then, in your Ada source:

Test : constant Integer
with
  Import,
  External_Name => "_name_thats_unlikely_to_clash";

Of course, this way means you have to decide on a specific type for the constant, and it has to match the C type.

Also, you need to include the C object file in your link:

gnatmake foo.adb -largs sterisk.o

Why is g++ better than gcc? Because g++ preserves more of the names in the C source, for example parameter names in functions.

Is there an alternative using export and import pragmas for c and ada respectively? — Sterisk, Feb 28 '18 at 21:10
See mods in answer about compiling the C source & including in the link. — Simon Wright, Mar 01 '18 at 19:41

How to import #define constants from .h file to Ada?

1 Answers1