Making and Using a DLL with Assembly program

[Robert Collins]

This question is mainly for Vortex.

The other day you gave me an example of making a DLL and using it in an Assembly language program. After looking over it for awhile I finally got the hang of it up to a point. The part of assembling the program, using the 'Build.bat' file was really straight forward and works without any problems as long as I put the include statement in the source code and have the 'mydll2.dll' file and the 'mydll2.lib' file available. However, I am still a little vague about the making of the DLL and the .Lib file. If I run the 'BuildDLL.bat' file I get the correct DLL and .LIB file as output and that is what I use when I assemble the program. I see that in order to run the 'BuildDLL.bat' I have to have the 'MyDLL2.Obj' and the 'MyDLL2.def' file as input. My question is how did you get the 'MyDLL2.OBJ file. I know that it must be the output of the Assembler so I tried using the 'MyDll2.c' program and the 'MyDLL2.def' file and recompile it using MS VC++. I got a link error but was able to correct it so I did get a .OBJ file from the compilation of 'MyDLL2.c'. I used this .OBJ as the input to 'BuildDLL.bat but it didnt work. So, I figured it then must be the output of assembling the 'MyDLL2.ASM' file using MASM. When I tried to do that I got an error from MASM about the 'include listing.inc' statement because that file is missing from the sample you gave me. The other question I have is how did you make the 'MyDLL2.ASM' file? If you made it from scratch then I am completely lost since I would have no idea how to make one myself if I needed to make another DLL. It looks though as if it was made by parsing the original file I made and converting it to an assembly source code. If this is the case then what did you use to get the .ASM file results?

Hope you can help me with this but thanks anyway for what you already have done.

[six_L]

win32asm.cjb.net
Read the Turor by Iczelion

• Tutorial 17: Dynamic Link Libraries
• Tutorial 26: Splash Screen

Then you do something what you want to do at making and using a DLL with Assembly language program

regards

[hutch--]

Robert,

Two things to watch out for between C and MASM, make sure the calling conventions are the same in the way you prototype them and check the compiler syntax for EXTERN C so you don't get any name mangling. A DLL created in MASM should be fine with Microsoft C.

[Vortex]

Hi Robert,

Yes, you are right about mydll2.asm. If you examine build.bat, you will see that the first line of batch file launches Cl.exe , the Visual C++ compiler. One of the switches of cl.exe is aimed to output (/FAs) an assembly listing of the original C source file. This listing helps me to understand how the compiler deals with the source file at low level. When I will be back home, I will provide you more information about building the DLL.

[Vortex]

Hi Robert,

To make it clearer, I removed the options generating the assembly listing file mydll2.asm, you can check the attachment.

[attachment deleted by admin]

[Robert Collins]

win32asm.cjb.net
Read the Turor by Iczelion

• Tutorial 17: Dynamic Link Libraries
• Tutorial 26: Splash Screen

Then you do something what you want to do at making and using a DLL with Assembly language program

regards

Where is Tutorial 26? I downloaded Steve's 'masm32' but it only goes up to Tutorial 24 and also Tutorial 01 is missing.

Also, the example in Tutorial 17 is fine if one is creating a DLL in Assembly language but I have to create the DLL using MS VC++ (boss's orders) and that is where I am have a problem getting the DLL to work in an Assembly program.

[John]

You can get any of the missing tutorials from here:
http://win32asm.cjb.net/

Just click the link for tutorials.

[John]

[...]
Also, the example in Tutorial 17 is fine if one is creating a DLL in Assembly language but I have to create the DLL using MS VC++ (boss's orders) and that is where I am have a problem getting the DLL to work in an Assembly program.

What part are you having trouble with?

The BuildDLL.bat file will build the C dll from the Mydll2.c source code. The Msgbox.asm file then will use that dll.

[John]

In an effort to simplify things for you I have removed anything that is not strictly necessary to build this and uploaded it here as a zip file.

Make sure the c compiler (cl.exe) and linker (link.exe) are on your path and then simply execute builddll.bat to build the dll. Then execute build.bat to build the asm app that uses the dll you just created. When you execute the resulting executable (Msgbox.exe) you will see that it executes a procedure in the C dll.

I hope this helps clear it up for you.

[attachment deleted by admin]

[Robert Collins]

You can get any of the missing tutorials from here:
http://win32asm.cjb.net/

Just click the link for tutorials.

Thanks for the link, John. That's a pretty cool Web Site.

[Robert Collins]

[...]
Also, the example in Tutorial 17 is fine if one is creating a DLL in Assembly language but I have to create the DLL using MS VC++ (boss's orders) and that is where I am have a problem getting the DLL to work in an Assembly program.

What part are you having trouble with?

The BuildDLL.bat file will build the C dll from the Mydll2.c source code. The Msgbox.asm file then will use that dll.

The original problem (now because of you and Vortex, I'm not sure it's a problem anymore) was that I had to take a DLL that was written by the Systems Programming Dept where I work and use it in an Assembly application that has been assigned to me. All DLLs are written in MS VC++ (no exceptions). These DLLs are used by the company in all applications written by the Applications Programming Dept (which I am part of) and they all work when used in other programs written in VB, C/C++, VJ++ 6.0, Delphi, etc. except I have been unsuccessful in getting them to work in an Assembly program. The DLLs are valid but I guess I am doing something wrong in trying to get them to work because during the assembling of the program I always get an unresolved external reference during the linking. Either the problem lies in the .LIB file or it's me.  I'm begining to think that there is no way to use the DLLs as they are given to me in an Assembly program. However, I can take the DLL (that is, its C++ source -->MyDLL.cpp, for example) and use it as input to the BuildDLL.BAT file and this creates a DLL and a LIB file that does work. Now, I have to convince the Systems Programming Dept that they are going to have to run their DLL source programs through the BuildDLL.bat but because of strict company policies I'm not sure they will agree. If there is a way to use the DLLs as written then I have to find the answer to how to use them in Assembly but if it can't be done then the only choise left is what you gave me in the VCDLL.ZIP file.       

[John]

I thought it was said earlier but I'm not sure. I believe the problem you are having is caused by the fact that you don't have the .LIB file for the DLL. The lib file will be generated by the C compiler (cl.exe) when it is run, as demonstrated here. You will need this when you build your ASM app.

Can't you just ask them to either recompile the dll and give you the lib file or ask them for the lib file provided they are saved? Far as I know, admittedly not much, you can't do anything without it.

[Robert Collins]

I thought it was said earlier but I'm not sure. I believe the problem you are having is caused by the fact that you don't have the .LIB file for the DLL. The lib file will be generated by the C compiler (cl.exe) when it is run, as demonstrated here. You will need this when you build your ASM app.

Can't you just ask them to either recompile the dll and give you the lib file or ask them for the lib file provided they are saved? Far as I know, admittedly not much, you can't do anything without it.

No, that is and was not the problem. I always had both the .DLL file and the .LIB file. The problem is that when I assemble and link the 'MessageBox.asm' I get a link error saying 'unresolved external ShowMessageBox@0'. Either I am not doing the source code incorrectly or the linker cannot read a .LIB file from VC++. I think it's the .LIB file becaise VC++ outputs both the DLL and the LIB files but they are probably not in the correct format that the linker expects or something.

As far as dealing with the Systems Programming Dept is that they are bound by company policies and have to create all DLLs using MS VC++. They probably cant or wont give me the source code so I can use it as input to the BuildDLL.bat file. (P.S. I have never worked for a company that makes it easy for programmers to get their stuff done. Always some piss-ass policy that says you can't do this and you can't do that). Anyway, the problem still exists untill I find a way to use their DLLs or I convince the manager of programming that in order to use their DLLS I have to or they have to run them through the BuildDLL.bat file and create another DLL and LIB that works with assembly. Now, you see, the company would have to maintain two versions of the same DLL, one for all the other applications and one for the assembly programs. I don't think they want to do that.

[John]

Out of curiosity, what calling convention is being used in the dl and the ASM app?

[edit]/me takes note that hutch-- mentioned this in the beginning[/edit]

[Robert Collins]

Out of curiosity, what calling convention is being used in the dl and the ASM app?

[edit]/me takes note that hutch-- mentioned this in the beginning[/edit]

Here's the files I am using.......Note: The below C DLL is C++, not C. I think the #include "stdafx.h" statement is what makes it a C++ instead of a regular C program but that is what the System Programming Dept uses so I have to use C++ too.

Source for MyDLL2.cpp and MyDll2.def files

Code Select Expand


#include <windows.h>
#include "stdafx.h"

BOOL APIENTRY DllMain(HANDLE hModule, DWORD  ul_reason_for_call, LPVOID lpReserved)
{
  return TRUE;
}

void _stdcall ShowMessageBox()
{
  MessageBox (0,"HELLO FROM DLL","MYDLL.DLL",MB_OK);
  return;
}


Code Select Expand


;mydll2.def
LIBRARY mydll2

EXPORTS
ShowMessageBox



Source for the MessageBox.asm file

Code Select Expand


.386
.model flat, stdcall
option casemap:none

ShowMessageBox   PROTO
ExitProcess      PROTO :DWORD                           

includelib \masm32\include\user32.lib
includelib \masm32\include\kernel32.lib
includelib mydll2.lib

.data

adr_MsgBox      DD      ShowMessageBox
adr_ExitProcess DD      ExitProcess             

.code
start:
                call    Do_MessageBox
                push    0                   
                call    Do_Exit             
;---------------------------------------------------------------------------
;
;----------------------------------------------------------------------------               
Do_MessageBox:                     
                jmp     dword ptr ds:[adr_MsgBox]
Do_Exit: 
                jmp     dword ptr ds:[adr_ExitProcess]  ;Invoke the ExitProcess function

                end     start


Someone told me, not sure who it was, probably Hutch, that C++ uses a name mangleing technique and this is what I think is making the .LIB file
unreadable by the MASM Linker.

Now, when I assemble and link the above .asm I always get that 'unresolved external 'ShowMessageBox@0' message.

If the DLL was created using MS VC++ as a '.c' program(instead of a .cpp) and the #include stdafx.h statement removed then the output MyDLL2.dll and MyDLL2.lib files will work as is in the above Assembly program but not if the DLL is compiled as a C++ program. But the problem remains that the programmers in the System Programming are always going to make DLLs using .cpp files and not .c files. So, tha't my situation.