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DLL Conventions: Issues and Solutions, Part I


Have you ever experienced compatibility issues between Dynamic Link Libraries developed using different tools? It is not irrational to use a DLL developed with one tool in a different tool; sometimes there are very good reasons to do so. This first article in a three-part series explaining how to resolve the issues presents the problem plainly.

Author Info:
By: Digvijay Chauhan
Rating: 5 stars5 stars5 stars5 stars5 stars / 16
April 04, 2005
TABLE OF CONTENTS:
  1. · DLL Conventions: Issues and Solutions, Part I
  2. · A simple problem
  3. · Where is the actual problem?
  4. · In the Next Article

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DLL Conventions: Issues and Solutions, Part I - Where is the actual problem?
(Page 3 of 4 )

In a world where several vendors compete to make fabulous developer tools, it is certainly impossible to have interoperability, especially when there are no standards in place. In short, calling a DLL produced by Visual C++ would be not much different than calling a DLL built with Borland C++ Builder. But to my surprise, Borland and Microsoft disagree on several issues regarding the techniques they follow and embed in the developer tools they produce.

For instance, Borland and Microsoft disagree on file formats for intermediate object files (.OBJ files) and import library files (Visual C++ uses the COFF library format while Borland uses OMF). COFF stands for Common Object File Format and OMF stands for Object Metafile Format. This means that you can't add a Microsoft generated import library to a Borland C++ Builder project. But we developers owe a big thanks to Borland for providing the IMPLIB (Import Library) utility, which means the file format differences are no longer an issue of great concern.

Further the two C++ compilers produced by these two companies also disagree on naming conventions of the exported symbols produced as a result of linking the final DLL or Libraries. This is the root cause of most problems which we developers face while we try to bridge the gap between the two.

Each exported function/symbol in a DLL or OBJect file bears a linker name. The linker uses this linker name to resolve function references that remained unresolved during compile time in order to make the final output executable or DLL. The linker generates an unresolved external error if it is unable to find a function with a linker name in the program's code itself or in the list of import libraries provided to it.

As far as the linker names are concerned, Borland and Microsoft compilers disagree on the scheme that is used to generate the linker name. For example, the Microsoft Visual C++ compiler sometimes decorates exported __stdcall functions, while the Borland C++ Builder expects that only imported __cdecl functions be decorated. If you don't understand what __stdcall and __cdecl mean, don't worry; we'll be discussing them shortly in detail.

So "How does that creates problems for us developers?" might be the question coming to your mind. Okay, let's talk about that in plain words. Say you've created a DLL in VC++ that exports a function Foo() which uses the __stdcall convention. The linker name generated for this function looks like _Foo@4. When you try using this DLL in the Borland environment, the Borland linker first expects the .lib file (import library) to be in OMF format, and then looks for a name Foo only because it is not expecting to see a decorated name for a function using the __stdcall calling convention. Okay I told you that __cdecl and __stdcall are calling conventions, but let's leave them here before we discuss them in detail. Next the Borland compiler reports an unresolved external for the name Foo.


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