Linker problem: undefined reference to `cv::imread(std::string const&, int)'

[TheTimKiely]

Thanks for the great sample.
I can't seem to this project to link if I include both OpenCV and Torch.
When both libraries are included in the cmake file, I get the error:
PyTorch-CPP/prediction.cpp:22: undefined reference to `cv::imread(std::string const&, int)'
collect2: error: ld returned 1 exit status

If I remove all references to Torch, the project builds, links, and runs.

Any suggestions would be appreciated.
I asked this question on StackOverflow, but I haven't gotten any responses.

Here the CMakeError.txt file:
erforming C SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_3c4f7/fast && /usr/bin/make -f CMakeFiles/cmTC_3c4f7.dir/build.make CMakeFiles/cmTC_3c4f7.dir/build
make[1]: Entering directory '/home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_3c4f7.dir/src.c.o
/usr/bin/cc -DCMAKE_HAVE_LIBC_PTHREAD -o CMakeFiles/cmTC_3c4f7.dir/src.c.o -c /home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp/src.c
Linking C executable cmTC_3c4f7
/home/tim/.local/share/JetBrains/Toolbox/apps/CLion/ch-0/201.7223.86/bin/cmake/linux/bin/cmake -E cmake_link_script CMakeFiles/cmTC_3c4f7.dir/link.txt --verbose=1
/usr/bin/cc -DCMAKE_HAVE_LIBC_PTHREAD -rdynamic CMakeFiles/cmTC_3c4f7.dir/src.c.o -o cmTC_3c4f7
/usr/bin/ld: CMakeFiles/cmTC_3c4f7.dir/src.c.o: in function main': src.c:(.text+0x46): undefined reference to pthread_create'
/usr/bin/ld: src.c:(.text+0x52): undefined reference to pthread_detach' /usr/bin/ld: src.c:(.text+0x63): undefined reference to pthread_join'
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_3c4f7.dir/build.make:87: cmTC_3c4f7] Error 1
make[1]: Leaving directory '/home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_3c4f7/fast] Error 2

Source file was:
#include <pthread.h>

void* test_func(void* data)
{
return data;
}

int main(void)
{
pthread_t thread;
pthread_create(&thread, NULL, test_func, NULL);
pthread_detach(thread);
pthread_join(thread, NULL);
pthread_atfork(NULL, NULL, NULL);
pthread_exit(NULL);

return 0;
}

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_6b9bf/fast && /usr/bin/make -f CMakeFiles/cmTC_6b9bf.dir/build.make CMakeFiles/cmTC_6b9bf.dir/build
make[1]: Entering directory '/home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_6b9bf.dir/CheckFunctionExists.c.o
/usr/bin/cc -DCHECK_FUNCTION_EXISTS=pthread_create -o CMakeFiles/cmTC_6b9bf.dir/CheckFunctionExists.c.o -c /home/tim/.local/share/JetBrains/Toolbox/apps/CLion/ch-0/201.7223.86/bin/cmake/linux/share/cmake-3.16/Modules/CheckFunctionExists.c
Linking C executable cmTC_6b9bf
/home/tim/.local/share/JetBrains/Toolbox/apps/CLion/ch-0/201.7223.86/bin/cmake/linux/bin/cmake -E cmake_link_script CMakeFiles/cmTC_6b9bf.dir/link.txt --verbose=1
/usr/bin/cc -DCHECK_FUNCTION_EXISTS=pthread_create -rdynamic CMakeFiles/cmTC_6b9bf.dir/CheckFunctionExists.c.o -o cmTC_6b9bf -lpthreads
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_6b9bf.dir/build.make:87: cmTC_6b9bf] Error 1
make[1]: Leaving directory '/home/tim/samples/PyTorch-CPP/cmake-build-debug/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_6b9bf/fast] Error 2

I looked for the cause of the error "/usr/bin/ld: cannot find -lpthreads".
I don't think that is relevant. The linker command is looking for 'pthread', not 'pthreads:
$ cat link.txt
/usr/bin/c++ -g -rdynamic CMakeFiles/classifier.dir/prediction.cpp.o -o classifier -Wl,-rpath,/usr/local/lib:/home/tim/tools/libtorch/lib /usr/local/lib/libopencv_dnn.so.4.3.0 /usr/local/lib/libopencv_gapi.so.4.3.0 /usr/local/lib/libopencv_highgui.so.4.3.0 /usr/local/lib/libopencv_ml.so.4.3.0 /usr/local/lib/libopencv_objdetect.so.4.3.0 /usr/local/lib/libopencv_photo.so.4.3.0 /usr/local/lib/libopencv_stitching.so.4.3.0 /usr/local/lib/libopencv_video.so.4.3.0 /usr/local/lib/libopencv_videoio.so.4.3.0 /home/tim/tools/libtorch/lib/libtorch.so /home/tim/tools/libtorch/lib/libc10.so /usr/local/lib/libopencv_imgcodecs.so.4.3.0 /usr/local/lib/libopencv_calib3d.so.4.3.0 /usr/local/lib/libopencv_features2d.so.4.3.0 /usr/local/lib/libopencv_flann.so.4.3.0 /usr/local/lib/libopencv_imgproc.so.4.3.0 /usr/local/lib/libopencv_core.so.4.3.0 -Wl,--no-as-needed,/home/tim/tools/libtorch/lib/libtorch_cpu.so -Wl,--as-needed /home/tim/tools/libtorch/lib/libc10.so -lpthread -Wl,--no-as-needed,/home/tim/tools/libtorch/lib/libtorch.so -Wl,--as-needed

libpthread is in "/usr/lib/x86_64-linux-gnu":
$ ls /usr/lib/x86_64-linux-gnu/libpth*.*
/usr/lib/x86_64-linux-gnu/libpthread-2.31.so /usr/lib/x86_64-linux-gnu/libpthread.so
/usr/lib/x86_64-linux-gnu/libpthread.a /usr/lib/x86_64-linux-gnu/libpthread.so.0

And that directory is found by "ld":
ld --verbose | grep SEARCH_DIR
SEARCH_DIR("=/usr/local/lib/x86_64-linux-gnu"); SEARCH_DIR("=/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu64"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib"); SEARCH_DIR("=/usr/x86_64-linux-gnu/lib64"); SEARCH_DIR("=/usr/x86_64-linux-gnu/lib");

[TheTimKiely]

I found that the object file created with a reference to Torch looks different than the object file without a reference to Torch.
My C++ experience is not very deep, so I don't know what to do with that.
The good object file has a reference to imread like this:
_ZN2cv6imreadERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEi

The bad object file:
_ZN2cv6imreadERKSsi

Is there any way to troubleshoot the compiler to figure out what's happenting?