DllMain/DllMainCRTStartup does not execute in DLL

Question

The initialization code in a DLL build with gnat is not running automatically when imported. I did a MCVE that consists on:

division.ads

with System;
with Interfaces.C;

package Division is
   --Neither of these work
   procedure DllMainCRTStartup ;
   pragma Export (StdCall, DllMainCRTStartup , "DllMainCRTStartup"); --Edited as noticed by Brian
   -- procedure DllMain
   -- pragma Export (StdCall, DllMain , "DllMain ");

   function Div (A : in INTEGER; B : in INTEGER) return INTEGER;
   pragma Export (C, Div, "MyDivision");

   -- --If I put this, it does not compile... maybe a wrong linkage option set?
   -- procedure AdaInit; 
   -- pragma Import (C, AdaInit, "adainit");
end Division;

division.adb

with text_io;

package body Division is
   procedure DllMainCRTStartup  is begin --DllMain or DllMainCRTStartup
      text_io.put("INIT CODE YEAH!!!*************!"); --This does not execute :(
      --AdaInit;
   end DllMainCRTStartup ;

   function Div(A : in INTEGER; B : in INTEGER) return INTEGER is
      X : INTEGER := A/B;
   begin
      return X;
   end Div;
end Division;

and the gpr:

library project Proj_Name is
  for Library_Name use "math";
  for Object_Dir use "obj";
  for Source_Dirs use ("src");
  for Library_Dir use "lib";
  for Library_Interface use ("Division");
  for Library_Kind use "dynamic";
  for Library_Options use ("-LC:\GNAT\2015\lib\gcc\i686-pc-mingw32\4.9.3\adalib",
                           "-LC:\GNAT\2015\lib\gcc\i686-pc-mingw32\4.9.3\adalib\libgnat");
end Proj_Name;

I am testing the dll from python, with ctypes. I import it with ctypes.CDLL and I'm able to use MyDivision. However, the init code does not run when importing the dll, as the text_io is not executed.

On the other hand, if I add the AdaInit procedure to the code I get something like this when compiling:

undefined reference to `adainit'

Thank you very much!

Answer 1

I’m not sure how you know that the initialization code isn’t being run?

I’m running on macOS, but the Ada aspects should be similar. I wrote this package spec/body as a simpler version of yours:

package Division is

   function Div (A : in INTEGER; B : in INTEGER) return INTEGER;
   pragma Export (C, Div, "MyDivision");

end Division;

with Ada.Text_IO;
package body Division is

   function Div(A : in INTEGER; B : in INTEGER) return INTEGER is
      X : INTEGER := A/B;
   begin
      return X;
   end Div;

   procedure Test_For_Elaboration is
   begin
      Ada.Text_IO.Put_Line ("hello world!");
   end Test_For_Elaboration;

begin
   Test_For_Elaboration;
end Division;

with this simpler GPR

library project Proj_Name is
  for Library_Name use "math";
  for Object_Dir use "obj";
  for Source_Dirs use ("src");
  for Library_Dir use "lib";
  for Library_Interface use ("Division");
  for Library_Kind use "dynamic";
end Proj_Name;

and tested with this C code:

#include <stdio.h>

extern int MyDivision(int, int);

int main()
{
  printf("42 / 2 => %d\n", MyDivision(42, 2));
  return 0;
}

and the result was

$ ./caller 
hello world!
42 / 2 => 21

so clearly, for me, library elaboration was called without my having to do anything.

The reason is that you specified Library_Interface in your project file, which means you’re building a stand-alone library, which

is a library that contains the necessary code to elaborate the Ada units that are included in the library. A stand-alone library is a convenient way to add an Ada subsystem to a more global system whose main is not in Ada since it makes the elaboration of the Ada part mostly transparent.

You can specify a stand-alone dynamic library which is not automatically initialized, using

for Library_Auto_Init use "false";

in which case you need to call the library’s initialization procedure yourself; it's called {library-name}init (in your case, mathinit). But then you need to call it from your main program; it’d need to be declared, in C

extern void mathinit();