FFMPEG and Qt mix

[limentas]

Hi. In the project I'm working on we use Qt we install via official installer and some other libraries we install via vcpkg+cmake.
At this moment I'm working on updating Qt version from 6.8.0 to 6.10.1 and I faced a problem when my project doesn't build for iOS. The issue is that Qt provides own FindFFmpeg.cmake and it cannot properly find FFmpeg libraries. It fails in particular because it cannot find libavutil.xcframework:

2025-12-04T08:29:09.3610470Z Call Stack (most recent call first):
2025-12-04T08:29:09.3610640Z   /Users/x/Qt/6.10.1/ios/lib/cmake/Qt6/FindFFMPEG.cmake:230 (find_component)
2025-12-04T08:29:09.3610950Z   /Users/x/work/x/build-x/vcpkg_installed/arm64-ios/share/ffmpeg/vcpkg-cmake-wrapper.cmake:27 (_find_package)
2025-12-04T08:29:09.3611190Z   /Users/x/work/x/build-x/_deps/vcpkg-src/scripts/buildsystems/vcpkg.cmake:850 (include)
2025-12-04T08:29:09.3611260Z   CMakeLists.txt:663 (find_package)

The issue is that Qt carries ffmpeg itself and they install xcframework files. Here is the change: qt/qtmultimedia@7c44857
Not sure if this is correct but it seems to me it is not (considering that this is module cmake package and cmake doesn't provide a fallback solution for them).

Now what options I have to resolve the issue:

1. modify CMAKE_MODULE_PATH before looking for ffmpeg and reverting it back after.
2. modify Qt FindFFmpeg.cmake
3. modify vcpkg-cmake-wrapper.cmake

My question to you. Why vcpkg looks for its own ffmpeg installation as a last resort? In particular why here:

vcpkg/ports/ffmpeg/vcpkg-cmake-wrapper.cmake

Line 2 in 74e6536

list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR})

vcokg_installed dir is appended and not prepended?
To me as a vcpkg user it would be more expected if vcpkg would look for its own installed packages at first. And then fallback to other options.

[dg0yt]

If you use non-vcpkg binaries for Qt, shouldn't you also use non-vcpkg binaries for Qt's dependencies? If you resolve the current build-time issue, you still have to ensure that the binaries interact well at runtime.

Why vcpkg looks for its own ffmpeg installation as a last resort?

• There is no official Kitware CMake Find module for ffmpeg. Consequently, it is something which downstream projects (here: external Qt) generally have to bring along themselves. The projects are the authorities which defined the interface of module (imported targets, names of variables etc.).
• It is true that the ffmpeg port brings a convenient Find module, and it is made available via the wrapper. AFAICT this is a legacy pattern in vcpkg which would not be accepted today without prefix the name with unofficial-. The prefix would signal users that they are locking them into vcpkg.
• By appending to the module path, the wrapper ensures that the particual module in vcpkg doesn't take precedence over the project-supplied module in the consumer's code base.
• Consumers in the vcpkg registry (e.g. vcpkg ports for Qt) are covered by adaptations and vcpkg CI.

[limentas]

If you use non-vcpkg binaries for Qt, shouldn't you also use non-vcpkg binaries for Qt's dependencies?

FFmpeg is our project direct dependency. And Qt allows to use own deployments of FFmpeg: https://doc.qt.io/qt-6/qtmultimedia-index.html#the-ffmpeg-backend.

By appending to the module path, the wrapper ensures that the particual module in vcpkg doesn't take precedence over the project-supplied module in the consumer's code base.

I see the point of not taking precedence over project supplied modules. But at the same time I see that I don't have an option to enforce vcpkg find own installed modules. And it seems to me that if I would like to use vcpkg as a last chance dependency resolver I would not use its cmake integration.

I'm sorry just to be clear I don't want to dispute design decisions or something. I want to understand the philosophy behind these decisions.

[dg0yt]

I hope I could provide enough useful context. Again, even forcing modules from vcpkg often wouldn't be enough, due to mismatch in "cmake interfaces".

[dg0yt]

I would suggest that you seek a solution in your project. I don't know how Qt interacts with CMAKE_MODULE_PATH. If Qt config prepends the location of Qt's FindFFmpeg.cmake, then this is the module you will have to work with. It might be done like that:

• Call find_package(FFMPEG) first. This will use the vcpkg setup and create targets and variables accordingly.
• Set input (cache) variables of Qt's FindFFMPEG.cmake to values provided from vcpkg's module.
• Call find_package(Qt) as needed.

I didn't study the details for ffmpeg vs. Qt. But some vcpkg ports use a similar pattern via CMAKE_PROJECT_INCLUDE, avoiding patching for injecting vcpkg-provided information from installed CMake config etc.