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From: Maxim Reznik <reznikmm@gmail.com>
Newsgroups: comp.lang.ada
Subject: Importing C function with variable argument list
Date: Tue, 10 Apr 2012 02:21:59 -0700 (PDT)
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Date: 2012-04-10T02:21:59-07:00
List-Id: <comp.lang.ada>

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, ...

Small example:

C declaration:
void syslog (int __pri, __const char *__fmt, ...);

Supposed Import declaration:

procedure C_Syslog (Priority : int; Format : chars_ptr; Msg :
chars_ptr);
pragma Import (C, C_Syslog, "syslog");

Call to C_Syslog fails, because %rax has not zero value.

What do you think?