Linking with prebuilt opencv gives errors

C++
,
1

target_link_libraries(my_app lib_opencv)

I get multiple errors like following

undefined reference to `cv::dnn::dnn4_v20200609::Net::Net()'
undefined reference to `cv::dnn::dnn4_v20200609::Net::getLayerNames[abi:cxx11]() const'
2

please edit and add some more information, like os, compiler, opencv version, and the whole cmakelists.txt you’re using to build your app, thank you !

please also compare to the cmake example

3

Thanks, I have added full details but the forum did not allow me to post the full message.

OS: Linux Ubuntu 18.04

I am building opencv 5.4.1

wget latest opencv
wget latest opencv contrib

so in a folder we must have opencv and opencv_contrib folders

cmake -DCMAKE_BUILD_TYPE=RELEASE -DCMAKE_INSTALL_PREFIX=$cwd/install -DINSTALL_C_EXAMPLES=ON -DINSTALL_PYTHON_EXAMPLES=OFF -DWITH_TBB=OFF -DWITH_V4L=OFF -DWITH_QT=OFF -DWITH_OPENGL=ON -DOPENCV_EXTRA_MODULES_PATH=../../opencv_contrib/modules -DBUILD_EXAMPLES=OFF -DBUILD_opencv_world=ON -DOPENCV_DNN_OPENCL=ON ..

Once I have libopencv_world.so, I am linking in my cmakelists.txt file as

set(OPENCV_PATH ${LIB_DIR}/opencv)
add_library(lib_opencv SHARED IMPORTED)

# sets the location of the prebuilt opencv .so
set_target_properties( lib_opencv
        PROPERTIES IMPORTED_LOCATION
        ${LIB_DIR}/opencv/lib/libopencv_world.so )

include_directories(${LIB_DIR}/opencv/include)

target_link_libraries(smaple lib_opencv)

I get multiple errors like following

undefined reference to `cv::dnn::dnn4_v20200609::Net::Net()'
undefined reference to `cv::dnn::dnn4_v20200609::Net::getLayerNames[abi:cxx11]() const' ```
4

disable opencv_world

5

I did try without opencv_world but still face the same issue.
Isn’t the purpose of opencv_world is to combine all the modules?

6

use this sample
and if it’s you please close this issue

7

yes, sure. however, there are some known issues, when using the contrib modules, too, so, if you need the contrib stuff, rather try building single-module libs, not world

also, can it be, you still have a previous opencv version around ?

4.5.1 actually has this as version tag (in the namespace):

#define OPENCV_DNN_API_VERSION 20201117

while 20200609 (from error msg) would be 4.4.0

so, again, – can it be, you’re using (or cmake is finding) 4.4.0 includes, but 4.5.1 libs ?

(which would explain the mismatch)