Dynamically loading MSVC DLL from MinGW code

[Paul Steckler]

Hi,

  I want to dynamically load a DLL created by MSVC from a program compiled
with MinGW.

I see the FAQ page:

   http://www.mingw.org/wiki/MSVC_and_MinGW_DLLs

which says how to to create a Windows .LIB file from DLL sources that links against
a MinGW-compiled C file.  This approach loads the DLL code automatically when
the main program runs.

  But I want to load the DLL dynamically, depending on the execution of the program.
Originally, I had compiled the DLL using MinGW, and loaded the DLL using
dlopen(). Once loaded, the code used dlsym() to obtain function entry points.
The MinGW code was linked against libdl.dll.a.

  Will that approach work with MSVC-compiled DLLs? Is the format of an MSVC
DLL the same a MinGW library created with the -shared flag?

-- Paul
--
Paul Steckler
National ICT Australia
paul DOT steckler AT nicta.com.au

The information in this e-mail may be confidential and subject to legal professional privilege and/or copyright. National ICT Australia Limited accepts no liability for any damage caused by this email or its attachments.

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[JonY-3]

On 7/15/2011 13:41, Paul Steckler wrote:

> Hi,
>
>   I want to dynamically load a DLL created by MSVC from a program compiled
> with MinGW.
>
> I see the FAQ page:
>
>    http://www.mingw.org/wiki/MSVC_and_MinGW_DLLs
>
> which says how to to create a Windows .LIB file from DLL sources that links against
> a MinGW-compiled C file.  This approach loads the DLL code automatically when
> the main program runs.
>
>   But I want to load the DLL dynamically, depending on the execution of the program.
> Originally, I had compiled the DLL using MinGW, and loaded the DLL using
> dlopen(). Once loaded, the code used dlsym() to obtain function entry points.
> The MinGW code was linked against libdl.dll.a.
>
>   Will that approach work with MSVC-compiled DLLs? Is the format of an MSVC
> DLL the same a MinGW library created with the -shared flag?
>

Yes, be sure to take care of passing objects between between two
different C libraries.


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[George Brink]

On 7/15/2011 1:41 AM, Paul Steckler wrote:
>    Will that approach work with MSVC-compiled DLLs? Is the format of an MSVC
> DLL the same a MinGW library created with the -shared flag?
DLL format is defined by OS and does not depend on compiler or language.
As long as you have the correct function signature - you would be fine.



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[NightStrike]

On Fri, Jul 15, 2011 at 10:31 AM, George Brink <[hidden email]> wrote:
> On 7/15/2011 1:41 AM, Paul Steckler wrote:
>>    Will that approach work with MSVC-compiled DLLs? Is the format of an MSVC
>> DLL the same a MinGW library created with the -shared flag?
> DLL format is defined by OS and does not depend on compiler or language.
> As long as you have the correct function signature - you would be fine.

Really?  What about c++ name mangling, and exception handling?

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[George Brink]

On 7/17/2011 2:57 PM, NightStrike wrote:
> On Fri, Jul 15, 2011 at 10:31 AM, George Brink<[hidden email]>  wrote:
>> On 7/15/2011 1:41 AM, Paul Steckler wrote:
>>>     Will that approach work with MSVC-compiled DLLs? Is the format of an MSVC
>>> DLL the same a MinGW library created with the -shared flag?
>> DLL format is defined by OS and does not depend on compiler or language.
>> As long as you have the correct function signature - you would be fine.
>
> Really?  What about c++ name mangling, and exception handling?
>

Yes, really.
Name mangling comes from function/variable types and I said "Correct
function signature", didn't I?

Exception handling is a completely different matter. It depends on which
CRLs were used to compile DLL and EXE. And in general exceptions should
be restricted to inside DLL-only and EXE-only. Same problem as with
memory management, by the way. But those problems do not depend on how
you link EXE with DLL, you would have them in both cases.


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[d3x0r]

There's really no magic to LoadLibrary and GetProcAddress.  I'll
assume you don't want to do silly things like pass a FILE* from a
fopen in mingw to this other library, cause the other will have no
clue about it.

posix I guess is dlopen and dlsym either one of which can be #defined
to look like the other.



On Thu, Jul 14, 2011 at 10:41 PM, Paul Steckler
<[hidden email]> wrote:

> Hi,
>
>  I want to dynamically load a DLL created by MSVC from a program compiled
> with MinGW.
>
> I see the FAQ page:
>
>   http://www.mingw.org/wiki/MSVC_and_MinGW_DLLs
>
> which says how to to create a Windows .LIB file from DLL sources that links against
> a MinGW-compiled C file.  This approach loads the DLL code automatically when
> the main program runs.
>
>  But I want to load the DLL dynamically, depending on the execution of the program.
> Originally, I had compiled the DLL using MinGW, and loaded the DLL using
> dlopen(). Once loaded, the code used dlsym() to obtain function entry points.
> The MinGW code was linked against libdl.dll.a.
>
>  Will that approach work with MSVC-compiled DLLs? Is the format of an MSVC
> DLL the same a MinGW library created with the -shared flag?
>
> -- Paul
> --
> Paul Steckler
> National ICT Australia
> paul DOT steckler AT nicta.com.au
>
> The information in this e-mail may be confidential and subject to legal professional privilege and/or copyright. National ICT Australia Limited accepts no liability for any damage caused by this email or its attachments.
>
> ------------------------------------------------------------------------------
> AppSumo Presents a FREE Video for the SourceForge Community by Eric
> Ries, the creator of the Lean Startup Methodology on "Lean Startup
> Secrets Revealed." This video shows you how to validate your ideas,
> optimize your ideas and identify your business strategy.
> http://p.sf.net/sfu/appsumosfdev2dev
> _______________________________________________
> MinGW-users mailing list
> [hidden email]
>
> This list observes the Etiquette found at
> http://www.mingw.org/Mailing_Lists.
> We ask that you be polite and do the same.  Disregard for the list etiquette may cause your account to be moderated.
>
> _______________________________________________
> You may change your MinGW Account Options or unsubscribe at:
> https://lists.sourceforge.net/lists/listinfo/mingw-users
> Also: mailto:[hidden email]?subject=unsubscribe
>

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[Paul Steckler]

I wrote:

> But I want to load the DLL dynamically, depending on the execution of the program.
> Originally, I had compiled the DLL using MinGW, and loaded the DLL using
> dlopen(). Once loaded, the code used dlsym() to obtain function entry points.
> The MinGW code was linked against libdl.dll.a.

I went ahead and did the experiment -- and everything works!

Our DLLs are wrappers around a commercial library. The code that loads the
DLLs uses some header files provided for using that library.

It would've been nice to compile the whole thing in MinGW. Unfortunately,
the MinGW header files don't provide the datatype _locale_t, referenced in the
commercial library's header files, and defined in the MSVC headers, and
the MinGW libraries don't provide some locale-related functions offered in MSVC
libraries.

The solution was to copy some of the MSVC datatypes into the MinGW build, which
allowed the client code to compile. That client code doesn't actually use any locale-related
functions. The DLL code does use locale-related functions, so it needs to be built with
MSVC and link with its libraries.

-- Paul
--
Paul Steckler
National ICT Australia
paul DOT steckler AT nicta.com.au

The information in this e-mail may be confidential and subject to legal professional privilege and/or copyright. National ICT Australia Limited accepts no liability for any damage caused by this email or its attachments.

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