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标题: Using Visual C++ DLLs in a C++Builder Project [打印本页]

作者: 韩冰 时间: 2005-1-26 12:51
标题: Using Visual C++ DLLs in a C++Builder Project
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It is likely that one day your boss will ask you if you can create a GUI with C++Builder that interfaces to an existing 32 bit DLL compiled with Microsoft Visual C++. Often times, the original DLL source code won't be available to you, either because the DLL comes from a third party vendor, or because the 22 year old intern just deleted the \DLL\SRC directory from the network. Given a DLL and a header file, this article shows you how to call the DLL from your C++Builder project. 
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>Calling DLL functions from a C++Builder project
The problem with Visual C++ DLLs
Step 1: Identify calling conventions used by the Visual C++ DLL
Step 2: Examine the linker names in the DLL
Step 3: Generate an import library for the Visual C++ DLL
Step 4: Add the import library to your project
Conclusion 

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>Calling DLL functions from a C++Builder project
Calling a DLL that was created with Visual C++ presents C++Builder programmers with some unique challenges. Before we attempt to tackle DLLs generated by Visual C++, it may be beneficial to review how you call a DLL that was created with C++Builder. A DLL that was created with C++Builder presents fewer roadblocks than one that was made by Visual C++. 
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>You need to gather three ingredients in order to call a DLL function from your C++Builder program: the DLL itself, a header file with function prototypes, and an import library (you could load the library at runtime instead of using an import library, but we will stick to the import library method for simplicity). To call a DLL function, add the import library to your C++Builder project by selecting the Project | Add To Project menu option in C++Builder. Next, insert a #include statement for the DLL header file in the C++ source file that needs to call one of the DLL functions. The last step is to add the code that calls the DLL function. 
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>Listings A and B contain source code for a DLL that can serve as a test DLL. Notice that the test code implements two different calling conventions (__stdcall and __cdecl). This is for a very good reason. When you try to call a DLL that was compiled with Visual C++, most of your headaches will result from disagreements due to calling conventions. Also notice that one function does not explicitly list the calling convention that it uses. This unknown function will act as a measuring stick for DLL functions that don't list their calling convention. 
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>//------------------------------------------
// Listing A: DLL.H
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>#ifdef __cplusplus
extern "C" {
#endif
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>#ifdef _BUILD_DLL_
#define FUNCTION __declspec(dllexport)
#else
#define FUNCTION __declspec(dllimport)
#endif
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>FUNCTION int __stdcall StdCallFunction(int Value);
FUNCTION int __cdecl CdeclFunction (int Value);
FUNCTION int UnknownFunction(int Value);
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>#ifdef __cplusplus
}
#endif
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>
//------------------------------------------
//Listing B: DLL.C
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>#define _BUILD_DLL_
#include "dll.h"
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>FUNCTION int __stdcall StdCallFunction(int Value)
{
return Value + 1;
}
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>FUNCTION int __cdecl CdeclFunction(int Value)
{
return Value + 2;
}
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>FUNCTION int UnknownFunction(int Value)
{
return Value;
}
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>To create a test DLL from Listing A and Listing B, open up C++Builder and bring up the Object Repository by selecting the File | New menu option. Select the DLL icon and click the OK button. C++Builder responds by creating a new project with a single source file. That file will contain a DLL entry point function and some include statements. Now select File | New Unit. Save the new unit as DLL.CPP. Cut and paste the text from Listing A and insert it into the header file DLL.H. Then copy the code from Listing B and insert it into DLL.CPP. Make sure that the #define for _BUILD_DLL_ is placed above the include statement for DLL.H. 
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>Save the project as BCBDLL.BPR. Next, compile the project and take a look at the files produced. C++Builder generates both a DLL and an import library with a .LIB extension. 
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>At this point, you have the three ingredients needed to call a DLL from a C++Builder project: the DLL itself, a header file for function prototypes, and an import library to link with. Now we need a C++Builder project that will try to call the DLL functions. Create a new project in C++Builder and save it to your hard drive. Copy the DLL, the import library, and the DLL.H header file from DLL project to this new project. Select the Project | Add To Project menu option and add the LIB file to the project. Next, add a #include statement in the main unit that includes DLL.H. Finally, add code that calls the DLL functions. Listing C shows code that calls each the DLL functions from Listing A and B. 
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>//------------------------------------------
// Listing C: MAINFORM.CPP - DLLTest program
#include <vcl\vcl.h>
#pragma hdrstop
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>#include "MAINFORM.h"
#include "dll.h"
//---------------------------------------------------------
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
: TForm(Owner)
{
}
//---------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
int Value = StrToInt(Edit1->Text);
int Result= StdCallFunction(Value);
ResultLabel->Caption = IntToStr(Result);
}
//---------------------------------------------------------
void __fastcall TForm1::Button2Click(TObject *Sender)
{
int Value = StrToInt(Edit1->Text);
int Result= CdeclFunction(Value);
ResultLabel->Caption = IntToStr(Result);
}
//---------------------------------------------------------
void __fastcall TForm1::Button3Click(TObject *Sender)
{
int Value = StrToInt(Edit1->Text);
int Result= UnknownFunction(Value);
ResultLabel->Caption = IntToStr(Result);
}

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>The problem with Visual C++ DLLs
In an ideal world, calling a DLL created with Visual C++ would be no more difficult than calling a DLL built with C++Builder. Unfortunately, Borland and Microsoft disagree on several points. For starters, Borland and Microsoft disagree on file formats for OBJs and import library files (Visual C++ uses the COFF library format while Borland uses OMF). This means that you can't add a Microsoft generated import library to a C++Builder project. Thanks to the Borland IMPLIB utility, the file format differences are surmountable. 
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>The two products also disagree on linker naming conventions. This turns out to be the primary hurdle when trying to call a Visual C++ DLL from C++Builder. Every function in a DLL or OBJ has a linker name. The linker uses the linker name to resolve functions that were protoyped during compile time. The linker will generate an unresolved external error if it can't find a function with a linker name that it thinks is needed by the program. 
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>With regard to linker function names, Borland and Microsoft disagree on these points: 
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>1- Visual C++ sometimes decorates exported __stdcall functions.
2- Borland C++Builder expects imported __cdecl functions to be decorated. 
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>So why is this such a big deal? Take disagreement #1 regarding the __stdcall calling convention. If you create a DLL with Visual C++ that contains a __stdcall function called MyFunction(), Visual C++ will give the function a linker name that looks like <a href="mailt_MyFunction@4" target="_blank" >_MyFunction@4</A>. When the Borland linker tries to resolve calls made to this function, it expects to find a function with the name MyFunction. Since the import library for the Visual C++ DLL doesn't contain a function called MyFunction, the Borland linker reports an unresolved external, which means it couldn't find the function. 
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>Your attack strategy for overcoming these three problems will depend on how the Visual C++ DLL was compiled. I have broken the process into four steps. 

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>Step 1: Identify calling conventions used by the Visual C++ DLL
In order to combat the naming convention entanglements, you must first determine what calling conventions are used by functions in the DLL. You can determine this by investigating the header file for the DLL. The function prototypes in the DLL header should look something like this: 
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> __declspec(dllimport) void CALLING_CONVENTION MyFunction(int nArg);
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>CALLING_CONVENTION should be __stdcall or __cdecl (see Listing A for concrete examples). In many cases, the calling convention won't be specified, in which case it defaults to __cdecl. 

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>Step 2: Examine the linker names in the DLL
If step 1 reveals that the DLL utilizes the __stdcall calling convention, you will need to examine the DLL to determine the naming convention that Visual C++ followed when it created the DLL. Visual C++ decorates __stdcall functions by default, but the DLL programmer can prohibit name decorations if they add a DEF file to their project. Your work will be slightly more tedious if the DLL supplier did not use a DEF file. 
The command line TDUMP utility allows you to examine the linker names of functions exported by the DLL. The following command invokes TDUMP on a DLL. 
 TDUMP -ee MYDLL.DLL > MYDLL.LST
TDUMP can report a ton of information about the DLL. We're only interested in functions exported by the DLL. The -ee command option instructs TDUMP to list only export information. If the DLL is large, you may want to redirect the output of TDUMP to a file (via the > MYDLL.LST appendage). 
The TDUMP output for the test DLL in Listing A and B looks like this: 
 Turbo Dump Version 5.0.16.4 Copyright (c) 1988, 1998 Borland International
 Display of File DLL.DLL
 EXPORT ord:0000='CdeclFunction'
EXPORT ord:0002='UnknownFunction'
EXPORT ord:0001='_StdCallFunction@4'
Notice the leading underscore and the trailing @4 on the __stdcall function. The __cdecl and the unknown function don't contain any decorations. If the Visual C++ DLL had been compiled with a DEF file, the decorations on the __stdcall function would not be present. 

Step 3: Generate an import library for the Visual C++ DLL
Here comes the hard part. Due to the library file format differences between C++Builder and Visual C++, you cannot add an import library created with Visual C++ to your C++Builder project. You must create an OMF import library using the command line tools that come with C++Builder. Depending out what you found in the first two steps, this step will either go smoothly, or it could take some time. 
As stated earlier, C++Builder and Visual C++ don't agree on how functions should be named in a DLL. Due to naming convention differences, you will need to create an aliased import library if the DLL implements calling conventions where C++Builder and Visual C++ disagree. Table A lists the areas of disagreement. 
Table A: Visual C++ and C++Builder naming conventions
Calling convention VC++ name VC++ (DEF used) C++Builder Name
-----------------------------------------------------------------------
__stdcall <a href="mailt_MyFunction@4" target="_blank" >_MyFunction@4</A> MyFunction MyFunction
__cdecl MyFunction MyFunction _MyFunction

The C++Builder column lists function names that the Borland linker expects to see. The first Visual C++ column lists the linker names that Visual C++ generates when the Visual C++ project does not utilize a DEF file. The second Visual C++ column contains linker names that Visual C++ creates when a DEF file is used. For things to go smoothly, the C++Builder name should agree with the Visual C++ name. Notice that the two products agree in only one place: __stdcall functions where the Visual C++ project contained a DEF file. For the remaining scenarios, you will need to create an import library that aliases the Visual C++ name to a C++Builder compatible name. 
Table A shows that there are several combinations that you may need to deal with when creating the import library. I have separated the combinations into two cases. 
Case 1: The DLL contains only __stdcall functions and the DLL vendor utilized a DEF file.
Table A reveals that VC++ and C++Builder agree only when the DLL uses __stdcall functions. Furthermore, the DLL must be compiled with a DEF file to prevent VC++ from decorating the linker names. The header file will tell you if the __stdcall calling convention was used (Step 1), and TDUMP will reveal whether or not the functions are decorated (Step 2). If the DLL contains __stdcall functions that are not decorated, then Visual C++ and C++Builder agree on how the functions should be named. You can create an import library by running IMPLIB on the DLL. No aliases are needed. 
IMPLIB works like this: 
 IMPLIB (destination lib name) (source dll)
For example,
 IMPLIB mydll.lib mydll.dll
Create the import library and move on to step 4.
Case 2: The DLL contains __cdecl functions or decorated __stdcall functions.
If your DLL vendor is adamant about creating DLLs that are compiler independent, then you have a good chance of falling into the Case 1 category. Unfortunately, odds are you won't fall into the Case 1 group for several reasons. For one, the calling convention defaults to __cdecl if the DLL vendor omits a calling convention when prototyping the functions, and __cdecl forces you into Case 2. Secondly, even if your vendor has utilized the __stdcall calling convention, they probably neglected to utilize a DEF file to strip the Visual C++ decorartions. 
However you got here, Good Day, and welcome to Case 2. You're stuck with a DLL whose function names don't agree with C++Builder. Your only way out of this mess is to create an import library that aliases the Visual C++ function names into a format compatible with C++Builder. Fortunately, the C++Builder command line tools allow you to create an aliased import library. 
The first step is to create a DEF file from the Visual C++ DLL by using the IMPDEF program that comes with C++Builder. IMPDEF creates a DEF file that lists all of the functions exported by the DLL. You invoke IMPDEF like this: 
 IMPDEF (Destination DEF file) (source DLL file).
For example
 IMPDEF mydll.def mydll.dll
After running IMPDEF, open the resulting DEF file using the editor of your choice. When the DLL source in Listing A and B is compiled with Visual C++, the DEF file created by IMPDEF looks like this: 

LIBRARY DLL.DLL
 EXPORTS
 CdeclFunction @1
 UnknownFunction @3
 <a href="mailt_StdCallFunction@4" target="_blank" >_StdCallFunction@4</A> =_StdCallFunction @2
The next step is to alter the DEF file so it aliases the DLL functions into names that C++Builder will like. You can alias a function by listing a C++Builder compatible name followed by the original Visual C++ linker name. For the test DLL in Listing A and B, the aliased DEF looks like this: 
 EXPORTS
 ; use this type of aliasing
 ; (Borland name) = (Name exported by Visual C++)
 _CdeclFunction = CdeclFunction
 _UnknownFunction = UnknownFunction
 StdCallFunction = <a href="mailt_StdCallFunction@4" target="_blank" >_StdCallFunction@4</A>
Notice that the function names on the left match the Borland compatible names from Table A. The function names on the right are the actual linker names of the functions in the Visual C++ DLL. 
The final step is to create an aliased import library from the aliased DEF file. Once again, you rely on the IMPLIB utility, except that this time, you pass IMPLIB the aliased DEF file as its source file instead of the original DLL. The format is 
 IMPLIB (dest lib file) (source def file)
For example
 IMPLIB mydll.lib mydll.def
Create the import library and move on to step 4. You may want to examine the import library first to ensure that each DLL function appears in a naming format that C++Builder agrees with. You can use the TLIB utility to inspect the import library. 
 TLIB mydll.lib, mydll.lst
The list file for the test DLL looks like this: 
 Publics by module
 StdCallFunction size = 0
 StdCallFunction
 _CdeclFunction size = 0
 _CdeclFunction
 _UnknownFunction size = 0
 _UnknownFunction

Step 4: Add the import library to your project
Once you create an import library for the Visual C++ DLL, you can add the import library to your C++Builder project using the Project | Add to Project menu option. You use the import library without regard to whether the import library contains aliases or not. After adding the import library to your project, build your project and see if you can successfully link. 

Conclusion:
This article demonstrated how you can call functions in a Visual C++ DLL from a C++Builder project. The techniques work with C++Builder 1 and C++Builder 3, and DLLs built with Visual C++ 4.X or Visual C++ 5 (I haven't tested Visual C++ 6 yet). 
You may have noticed that this article only discusses how to call C style functions in a DLL. No attempt is made to call methods of an object where the code for the class resides in a Visual C++ DLL. C++ DLLs present an even greater array of problems because linker names for member functions are mangled. The compiler employs a name mangling scheme in order to support function overloading. Unfortunately, the C++ standard does not specify how a compiler should mangle class methods. Without a strict standard in place, Borland and Microsoft have each developed their own techniques for name mangling, and the two conventions are not compatible. In theory, you could use the same aliasing technique to call member functions of a class that resides in a DLL. However, you may want to consider creating a COM object instead. COM introduces many of its own problems, but it does enforce a standard way of calling methods of an object. A COM object created by Visual C++ can be called from any development environment, including both Delphi and C++Builder. 
C++Builder 3.0 introduced a new command line utility called COFFtoOMF.EXE. This utility can convert a Visual C++ import library to a C++Builder import library. Furthermore, the program will automatically alias __cdecl functions from the Visual C++ format to the C++Builder format. The automatic aliasing can simplify Step 3 if the DLL exclusively uses the __cdecl calling convention.

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