Exporting DLL function with variable-argument lists : Windows, C / C++, DLL

May 16, 2008 11:33pm pdt

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03.19.2008 at 11:13PM PDT, ID: 23256087

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8.3

Exporting DLL function with variable-argument lists

Zones: Microsoft Visual C++.Net, C++ Programming Language

Tags: Windows, C / C++, DLL

Hello,

I'm just wondering if it is possible to have variable-argument lists on an exported DLL function i.e. a printf style function.

I would like to be able to do a logging function that may change in the future but not have to modify all of the different DLL's that use that function so exporting the one function:

int Log(char *lpszFormat, ...);

That I can then use va_start, va_end etc to get the detail out in whatever way I want.

The only question is if it is possible.

Thanks in advance.

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Question Stats

Zone: Microsoft

Question Asked By: DSXC

Solution Provided By: Gurudenis

Participating Experts: 4

Solution Grade: A

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03.20.2008 at 12:20AM PDT, ID: 21168864

Gurudenis:

Yes, it's possible AFAIK. This is supported by the __cdecl calling convention only. It means you cannot export such function via a DEF file. Therefore, your function definition may look like this, for example (__cdecl is the default here, but I explicitly specified it for clarity):

__declspec(dllexport) __cdecl MyFunc(int argc, ...)
{
// ...
}

Accepted Solution

03.20.2008 at 05:59AM PDT, ID: 21170799

davidgeldreich:

As gurudenis said, it is possible with vararg style.

But remember that your function should be able to deduce arguments number from first arguments or other way.

If you want your DLL to be "binary" compatible, you should handle the cases :

- old sofware calling a new version of the DLL
- and maybe, new software calling an older version of the DLL (i.e. with too much arguments)

Welcome in the "DLL Hell" :-)

03.20.2008 at 06:55AM PDT, ID: 21171456

Rank: Genius

itsmeandnobodyelse:

>>>> The only question is if it is possible.
variable-argument lists are a very unsafe feature which easily can crash your program if the format specifier doesn't fit to the types or count of the arguments. You should know that a many of the nowadays security leaks (buffer overflow issues) were due to sprintf and printf calls where a hijacker after manipulating the contents of the lpszFormat (or maybe it is an user input anyhow) could cause overflows and even execute malicious code.

If you would pass the arguments like commandline arguments were passed by simply using an array of zero terminated char strings (or if C++ only by passing a dynamic array of string arguments)

void function(int nargs, const char* szargs[]);

it is only a little bit less comfortable but much safer.

In C++ you could do variable formatting by passing a stream object:

stringstream oss;
format(oss << " any text " << any_variable << " any length " << any_manipulator);

You also can use your stream class for that purpose. That is all typesafe and was checked at compiletime.

03.20.2008 at 08:24AM PDT, ID: 21172462

Gurudenis:

The stream or character array solution won't work if you need the arguments to have different data types. In this case consider the following safe alternative (this time, with any calling convention you want):

__declspec(dllexport) void func(int argc, const std::vector<boost::any>& arguments)
{
// ...
}

See boost.org for more info on boost::any if you're interested.

03.20.2008 at 09:49AM PDT, ID: 21173308

Rank: Guru

ikework:

>> __declspec(dllexport) void func(int argc, const std::vector<boost::any>& arguments)

you shouldnt export STL-objects or other templates in your dll, neither using them in dll's public interface ..

here are some links about it:

http://www.codeproject.com/KB/cpp/MemberTemplateDLL.aspx
http://cygwin.com/ml/cygwin/2006-04/msg00480.html

ike

03.20.2008 at 10:38AM PDT, ID: 21173734

davidgeldreich:

This was true with VC6 but newer version of Microsoft's STL no longer have this problem. And when writing template class that you put in DLL you should follow the advice given in the cygwin link.

At this time, a solution was to use STLport to be able to use STL in DLL.

But now, it is safe to use STL in DLL on Windows.

03.20.2008 at 10:54AM PDT, ID: 21173901

Gurudenis:

ikework, I'm pretty sure it's safe in this case.

You see, the first article you linked to addresses the issue of exporting member templates, which is not directly supported by MSVC or most other compilers at all (the author suggests a workaround). The second article describes an obvious problem with static members, of which both std::vector and boost::any, I believe, have none.

The method I suggested, however, does have a limitation that you haven't pointed out. In its present form, the DLL that exports functions with classes in their signatures, especially templates, can only be used from within applications written in C++ and compiled with the same brand and (preferably) version of compiler as the DLL in question.

03.24.2008 at 03:03PM PDT, ID: 21197574

DSXC:

Well I'm surprised its able to be done.

I knew it would be an unsafe way of doing things as sprintf can easily crash your application if you pass the wrong arguments.

I am using DEF files for my DLL exports so that may count me out of using them.

Thanks for all your answers, I just wanted to know if it was possible before I started heading down that path.

03.24.2008 at 04:11PM PDT, ID: 21198352

DSXC:

After all that I can use the DEF files to export the function.

Works exactly how I wanted. Thanks for your help.

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