Makefile for c++ program cannot be compiled properly

Question

I am quite new to c++ Makefile. Recently, I am trying to re-implement the dense trajectory algorithm introduced by this paper: https://lear.inrialpes.fr/people/wang/dense_trajectories

However, I was struggling with Makefile that they provided for quite a while. The Makefile is as follows:

# set the binaries that have to be built
TARGETS := DenseTrack Video

# set the build configuration set 
BUILD := release
#BUILD := debug

# set bin and build dirs
BUILDDIR := .build_$(BUILD)
BINDIR := $(BUILD)

# libraries 
LDLIBS = $(addprefix -l, $(LIBS) $(LIBS_$(notdir $*)))
LIBS := \
    opencv_core opencv_highgui opencv_video opencv_imgproc \
    avformat avdevice avutil avcodec swscale

# set some flags and compiler/linker specific commands
CXXFLAGS = -pipe -D __STDC_CONSTANT_MACROS -D STD=std -Wall $(CXXFLAGS_$(BUILD)) -I. -I/usr/local/include -I/home/wei/ffmpeg_build/include
CXXFLAGS_debug := -ggdb
CXXFLAGS_release := -O3 -DNDEBUG -ggdb
LDFLAGS = -L/usr/local/lib -L/home/wei/ffmpeg_build/lib -pipe -Wall $(LDFLAGS_$(BUILD))
LDFLAGS_debug := -ggdb
LDFLAGS_release := -O3 -ggdb

include make/generic.mk

I can verify that my opencv2.4.2 and ffmpeg5.4.0 are successfully installed in Ubuntu 16.04, by running an example using those two libraries.

For opencv: The lib path is: /usr/local/lib

libavcodec.a                   libopencv_gpu.so.2.4   
    libopencv_photo.so.2.4
libavdevice.a                  libopencv_gpu.so.2.4.2        libopencv_photo.so.2.4.2
libavfilter.a                  libopencv_highgui.so          libopencv_stitching.so
libavformat.a                  libopencv_highgui.so.2.4      libopencv_stitching.so.2.4
libavutil.a                    libopencv_highgui.so.2.4.2    libopencv_stitching.so.2.4.2
libopencv_calib3d.so           libopencv_imgproc.so          libopencv_ts.so
libopencv_calib3d.so.2.4       libopencv_imgproc.so.2.4      libopencv_ts.so.2.4
libopencv_calib3d.so.2.4.2     libopencv_imgproc.so.2.4.2    libopencv_ts.so.2.4.2
libopencv_contrib.so           libopencv_legacy.so           libopencv_video.so
libopencv_contrib.so.2.4       libopencv_legacy.so.2.4       libopencv_video.so.2.4
libopencv_contrib.so.2.4.2     libopencv_legacy.so.2.4.2     libopencv_video.so.2.4.2
libopencv_core.so              libopencv_ml.so               libopencv_videostab.so
libopencv_core.so.2.4          libopencv_ml.so.2.4           libopencv_videostab.so.2.4
libopencv_core.so.2.4.2        libopencv_ml.so.2.4.2         libopencv_videostab.so.2.4.2
libopencv_features2d.so        libopencv_nonfree.so          libpostproc.a
libopencv_features2d.so.2.4    libopencv_nonfree.so.2.4      libswresample.a
libopencv_features2d.so.2.4.2  libopencv_nonfree.so.2.4.2    libswscale.a
libopencv_flann.so             libopencv_objdetect.so        pkgconfig
libopencv_flann.so.2.4         libopencv_objdetect.so.2.4    python2.7
libopencv_flann.so.2.4.2       libopencv_objdetect.so.2.4.2  python3.5
libopencv_gpu.so               libopencv_photo.so

The include path is: usr/local/include:

libavcodec   libavfilter  libavutil    libswresample  opencv
libavdevice  libavformat  libpostproc  libswscale     opencv2

For ffmpeg: The lib path is: /home/user/ffmpeg_build/lib

libavcodec.a   libavfilter.a  libavutil.a   libfdk-aac.la  libswresample.a  libx265.a
libavdevice.a  libavformat.a  libfdk-aac.a  libpostproc.a  libswscale.a     pkgconfig

The include path is: /home/user/ffmpeg/include

fdk-aac     libavdevice  libavformat  libpostproc    libswscale     x265.h
libavcodec  libavfilter  libavutil    libswresample  x265_config.h

So when I make it with Makefile, I did not get error but it seems linking is working properly, nor is the compiled output.

I tried to directly compile with g++ command.

sudo g++ -o Video -pipe -D __STDC_CONSTANT_MACROS -D STD=std -Wall -I. -I/usr/local/include -O3 -DNDEBUG -ggdb -L/usr/local/lib -lopencv_core -lopencv_highgui -lopencv_video -lopencv_imgproc -lavformat -lavdevice -lavutil -lavcodec -lswscale *.h Video.cpp

However,it returns errors as:

/tmp/ccjYmwI4.o: In function `main':
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:33: undefined reference to `cvCreateFileCapture'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:48: undefined reference to `cvQueryFrame'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:57: undefined reference to `cvCopy'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:60: undefined reference to `cvShowImage'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:61: undefined reference to `cvWaitKey'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:53: undefined reference to `cvCreateImage'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:70: undefined reference to `cvDestroyWindow'
/home/wei/Documents/dt/dense_trajectory_release_v1.2/Video.cpp:41: undefined reference to `cvNamedWindow'
collect2: error: ld returned 1 exit status

I have been working on this issue for several days. Any help or suggestions would be really appreciated.

Thanks

Answer 1

The problem comes from your linker call:

The library options (-l) must come after the code to compile or to link:

Correct:

 # code in foo.cpp need function in libmylib.so
 g++ foo.cpp -lmylib

 # code in foo.o need function in libmylib.so
 g++ foo.o -lmylib

Incorrect:

 # code in foo.cpp need function in libmylib.so
 g++ -lmylib foo.cpp 

 # code in foo.o need function in libmylib.so
 g++ -lmylib foo.o