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Date: Tue, 10 Apr 2012 23:02:15 +0300
From: Tero Koskinen <tero.koskinen@iki.fi>
Newsgroups: comp.lang.ada
Subject: Re: Importing C function with variable argument list
Message-Id: <20120410230215.502473d3a9f0287850900775@iki.fi>
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Date: 2012-04-10T23:02:15+03:00
List-Id: <comp.lang.ada>

Hi,

On Tue, 10 Apr 2012 02:21:59 -0700 (PDT)
Maxim Reznik <reznikmm@gmail.com> wrote:

> Hi, All
> 
> It seems it's impossible to call C function variable argument lists
> from Ada.
> Even when one knows in advace all arguments at compile time.
> 
> Functions with Variable Argument Lists have their own ABI. For
> instance, in x86_64:
> 
> > When a function taking variable-arguments is called, %rax must be set to the
> > total number of floating point parameters passed to the function in vector registers.
...
> But it's impossible to specify this fact in pragma Import (C, ...
...
> What do you think?

To work-around this, I use a small wrapper for x86_64 (Linux)
environments in my curl bindings:
https://bitbucket.org/tkoskine/curl-ada/src/ebd5156f7334/Makefile#cl-6
https://bitbucket.org/tkoskine/curl-ada/src/ebd5156f7334/utils/easywrap.pl
https://bitbucket.org/tkoskine/curl-ada/src/ebd5156f7334/src/template/curl-easy.adb.tmpl#cl-144

Ada code is following:
         procedure curl_easy_setopt
           (C_Handle : CURL_Private_Access;
            C_Option : CURL.Options.CURL_Option_Type;
            C_Action : C_Action_Callback);
         pragma Import (C, curl_easy_setopt, "wrap_setopt");

         procedure curl_easy_setopt_data
           (C_Handle : CURL_Private_Access;
            C_Option : CURL.Options.CURL_Option_Type;
            Data     : Callback_Interface_Class_Access);
         pragma Import (C, curl_easy_setopt_data, "wrap_setopt");

And C wrapper:
CURLcode wrap_setopt(CURL* handle, CURLoption option, void* param)
{
        return curl_easy_setopt(handle, option, param);
}

For i386, I do s/wrap_setopt/curl_easy_setopt/ replacement before
compiling:

if ($Config{archname} =~ m/x86_86/) { $use_wrap = 1; }
while (<>) {
        if ($use_wrap == 0) {
                $_ =~ s/wrap_setopt/curl_easy_setopt/g;
        }
        print $_;
}

-- 
Tero Koskinen - http://iki.fi/tero.koskinen/