Re: nabbasi@pacbell.net

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

how to read C extern variables from Ada? <how@newsguy.com> writes:

> Hello,
> 
> It is easy to interface to C from Ada. But one problem that I could not
> see mentioned in annex B, is how to get access to 'extern' variables.
> 
> For example, I can call getopt() from Ada. But the problem is that getopt()
> sets values into an extern variables that are defined is C header files, 
> and so can be read from C client programs after the call to getopt() 
> returns.
> 
> --------------------
> SYNOPSIS
>     #include <unistd.h>
> 
>     int getopt(int argc, char * const argv[], const char *optstring);
>     extern char *optarg;
>     extern int optind, opterr, optopt;
> -----------------------
> 
> So, in the above, after I call getopt(), I need a way to 'read' optarg. There
> is no C API to read optarg. So, How to do this in Ada??

Ada code to import 'optarg':

optarg : Interfaces.C.strings.Chars_Ptr;
pragma Import (C, optarg, "optarg");

I haven't compiled this, but it should work with GNAT.

> ps. offcourse I could write a small C stub that the Ada program calls, and
> then it calls getopt(), and this C stub could read the extern
> variables. and I define an API to this C stub function to return these
> variables via function call. But I wanted to see if I can do the whole thing
> in 100% pure Ada :), it also makes the Makefile simpler.  This is GNAT 3.11p.

I have tried to import an object exported by a DLL, using ObjectAda on
Windows NT, and that doesn't work. So I had to add an access function
to the DLL. But in the above, getopt() is probably not in a DLL, but
just a normal library, so you should be ok.

-- Stephe