libwebp in VB 6.0 programs.

[Александр Рощупкин]

Hello!
I want to use part of the libwebp library to view images in a program written in VB 6.0 on 32-bit systems (Windows XP x86).
I've declared declarations for two library functions:
Public Declare Function WebPGetDecoderVersion Lib "libwebp.dll" () As Long
Public Declare Function WebPGetInfo Lib "libwebp.dll" (ByRef Arr As Any, _
ByVal data_size As Long, ByRef Wwidth As Long, _
ByRef Wheight As Long) As Long
The first function works fine when called:
Label4.Caption = WebPGetDecoderVersion
The second function, when called:
Addr = WebPGetInfo(bRead(0), FSize - 1, ImgW, ImgH)
raises an error: "Bad DLL calling convention".
I tried compiling the parts I needed from the libwebp library. The decode.h file contains prototypes of the functions I need, but I couldn't find the functions themselves.
What should I do to find them?

[James Zern]

Hi,

On Wed, Oct 22, 2025 at 1:49 AM Александр Рощупкин <arosch...@gmail.com> wrote:

Hello!
I want to use part of the libwebp library to view images in a program written in VB 6.0 on 32-bit systems (Windows XP x86).
I've declared declarations for two library functions:
Public Declare Function WebPGetDecoderVersion Lib "libwebp.dll" () As Long
Public Declare Function WebPGetInfo Lib "libwebp.dll" (ByRef Arr As Any, _
ByVal data_size As Long, ByRef Wwidth As Long, _
ByRef Wheight As Long) As Long 

The first function works fine when called:
Label4.Caption = WebPGetDecoderVersion
The second function, when called:
Addr = WebPGetInfo(bRead(0), FSize - 1, ImgW, ImgH)
raises an error: "Bad DLL calling convention".

https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/error-messages/bad-dll-calling-convention has some guidance around this error. I'm not sure if the types are aligned correctly, but haven't tried this myself.

 

I tried compiling the parts I needed from the libwebp library. The decode.h file contains prototypes of the functions I need, but I couldn't find the functions themselves.
What should I do to find them?

If you're working from a git repository, you can use `git grep WebPGetInfo`. Otherwise something like `grep -R WebPGetInfo` can work. This function is defined in src/dec/webp_dec.c:

https://chromium.googlesource.com/webm/libwebp/+/refs/tags/v1.6.0/src/dec/webp_dec.c#718

 

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[Александр Рощупкин]

Hello, James!
Thanks for your reply. I don't use the repository. I just download what I need.
Apparently, the functions whose prototypes are specified in the decode.h file are generated by the preprocessor from macros.

I'm new to C++, so I don't know much yet.
I don't understand how to run the Visual Studio preprocessor to get the code for the functions I need from the decode.h file.

среда, 22 октября 2025 г. в 23:13:00 UTC+3, James Zern:

[James Zern]

Hi,

On Wed, Oct 22, 2025 at 10:14 PM Александр Рощупкин <arosch...@gmail.com> wrote:

Hello, James!
Thanks for your reply. I don't use the repository. I just download what I need.

I suggest building libwebp.dll using Makefile.vc. In a Visual Studio command prompt `nmake -f Makefile.vc CFG=release-dynamic` should work.

 

Apparently, the functions whose prototypes are specified in the decode.h file are generated by the preprocessor from macros.

No, the functions are defined in the file I mentioned: src/dec/webp_dec.c. If I'm misunderstanding, can you point to the macros you're talking about?

[Александр Рощупкин]

Hi, James!

 Macros: 

"MAKE_DECODER_VERSION(WebPDecodeRGB, RGB) MAKE_DECODER_VERSION(WebPDecodeRGBA, RGBA) MAKE_DECODER_VERSION(WebPDecodeARGB, ARGB) MAKE_DECODER_VERSION(WebPDecodeBGR, BGR) MAKE_DECODER_VERSION(WebPDecodeBGRA, BGRA)"

суббота, 25 октября 2025 г. в 01:00:46 UTC+3, James Zern:

[James Zern]

Hi,

On Wed, Oct 29, 2025 at 11:30 PM Александр Рощупкин <arosch...@gmail.com> wrote:

Hi, James!

 Macros: 

"MAKE_DECODER_VERSION(WebPDecodeRGB, RGB) MAKE_DECODER_VERSION(WebPDecodeRGBA, RGBA) MAKE_DECODER_VERSION(WebPDecodeARGB, ARGB) MAKE_DECODER_VERSION(WebPDecodeBGR, BGR) MAKE_DECODER_VERSION(WebPDecodeBGRA, BGRA)"

These aren't a part of libwebp. Are they from your implementation or somewhere else? Do you have a link to the source code?

[Александр Рощупкин]

Hi, James!

Here is the link to the source codes:

https://linuxfromscratch.org/blfs/view/8.3/general/libwebp.html

I managed to create a wrapper library for libwebp.dll and use it in a program written in VB 6.0.

четверг, 30 октября 2025 г. в 23:00:13 UTC+3, James Zern:

[James Zern]

Hi,

On Fri, Oct 31, 2025 at 1:12 AM Александр Рощупкин <arosch...@gmail.com> wrote:

Hi, James!

Here is the link to the source codes:

https://linuxfromscratch.org/blfs/view/8.3/general/libwebp.html

This looks to use a very old version of libwebp (1.0.0). If possible, it would be best to the latest release, 1.6.0, or at least 1.0.3 in that set of patch releases. They both contain import bug/security related fixes.

 

I managed to create a wrapper library for libwebp.dll and use it in a program written in VB 6.0.

That's great to hear. If you have any other details about how you did that, it may help others if you can post the steps to this thread.

[Александр Рощупкин]

Hello, James!
Here's a link to the finished result:
https://disk.yandex.ru/d/vrWheggvXeRexw
Both programs work on Windows XP, Windows 7, and Windows 10 x64.

вторник, 4 ноября 2025 г. в 06:36:49 UTC+3, James Zern: