make g++ -c -Wall `pkg-config --cflags --libs opencv` -std=c++11 main.cpp g++ -c -Wall `pkg-config --cflags --libs opencv` -std=c++11 inputServer.cpp g++ -c -levet `pkg-config --cflags --libs opencv` -lpthread -pthread -Wall -std=c++11 webClient.cpp g++ -c `pkg-config --cflags --libs opencv` HoughLines.cpp g++ -pthread -lopencv_imgcodecs `pkg-config --cflags --libs opencv` main.o inputServer.o HoughLines.o webClient.o -o executable HoughLines.o: In function `HoughLining(int, char**)': HoughLines.cpp:(.text+0xa0): undefined reference to `cv::imread(cv::String const&, int)' HoughLines.cpp:(.text+0x194): undefined reference to `cv::Canny(cv::_InputArray const&, cv::_OutputArray const&, double, double, int, bool)' HoughLines.cpp:(.text+0x201): undefined reference to `cv::cvtColor(cv::_InputArray const&, cv::_OutputArray const&, int, int)' HoughLines.cpp:(.text+0x2aa): undefined reference to `cv::HoughLinesP(cv::_InputArray const&, cv::_OutputArray const&, double, double, int, double, double)' HoughLines.cpp:(.text+0x413): undefined reference to `cv::line(cv::_InputOutputArray const&, cv::Point_, cv::Point_, cv::Scalar_ const&, int, int, int)' HoughLines.cpp:(.text+0x479): undefined reference to `cv::imshow(cv::String const&, cv::_InputArray const&)' HoughLines.cpp:(.text+0x4da): undefined reference to `cv::imshow(cv::String const&, cv::_InputArray const&)' HoughLines.cpp:(.text+0x502): undefined reference to `cv::waitKey(int)' HoughLines.o: In function `cv::String::String(char const*)': HoughLines.cpp:(.text._ZN2cv6StringC2EPKc[_ZN2cv6StringC5EPKc]+0x4d): undefined reference to `cv::String::allocate(unsigned long)' HoughLines.o: In function `cv::String::~String()': HoughLines.cpp:(.text._ZN2cv6StringD2Ev[_ZN2cv6StringD5Ev]+0x14): undefined reference to `cv::String::deallocate()' HoughLines.o: In function `cv::Mat::~Mat()': HoughLines.cpp:(.text._ZN2cv3MatD2Ev[_ZN2cv3MatD5Ev]+0x39): undefined reference to `cv::fastFree(void*)' HoughLines.o: In function `cv::Mat::release()': HoughLines.cpp:(.text._ZN2cv3Mat7releaseEv[_ZN2cv3Mat7releaseEv]+0x4b): undefined reference to `cv::Mat::deallocate()' collect2: error: ld returned 1 exit status makefile:4: recipe for target 'output' failed make: *** [output] Error 1