Makefile error causes undefined reference to symbol 'GOMP_parallel@@GOMP_4.0'

Question

Warning: Newbie Ubuntu/g++/makefile user

I'm trying to build a C++ app (C++11, OpenMP, OpenCV3, experimental filesystem) that works if I build directly, but fails if I use a makefile with a linker error.

This works:

g++ \
-std=c++11 -std=c++1y -fopenmp -DNDEBUG \
-O3 -fprofile-generate -march=native -fomit-frame-pointer \
docproc.cpp \
-flto -L/usr/local/lib -lstdc++fs \
-lpthread -lboost_thread -lboost_system \
-lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_calib3d -lopencv_ccalib -lopencv_core -lopencv_datasets -lopencv_dnn -lopencv_dpm -lopencv_face -lopencv_features2d -lopencv_flann -lopencv_fuzzy -lopencv_highgui -lopencv_imgcodecs -lopencv_imgproc -lopencv_line_descriptor -lopencv_ml -lopencv_objdetect -lopencv_optflow -lopencv_photo -lopencv_plot -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_shape -lopencv_stereo -lopencv_stitching -lopencv_structured_light -lopencv_superres -lopencv_surface_matching -lopencv_text -lopencv_tracking -lopencv_xfeatures2d -lopencv_ximgproc -lopencv_xobjdetect -lopencv_xphoto

But my makefile produces this, and fails:

g++ -g -L/usr/local/lib -o docproc docproc.o taina.o logging.o \
-lpthread -lboost_thread -lboost_system \
-lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_calib3d -lopencv_ccalib -lopencv_core -lopencv_datasets -lopencv_dnn -lopencv_dpm -lopencv_face -lopencv_features2d -lopencv_flann -lopencv_fuzzy -lopencv_highgui -lopencv_imgcodecs -lopencv_imgproc -lopencv_line_descriptor -lopencv_ml -lopencv_objdetect -lopencv_optflow -lopencv_photo -lopencv_plot -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_shape -lopencv_stereo -lopencv_stitching -lopencv_structured_light -lopencv_superres -lopencv_surface_matching -lopencv_text -lopencv_tracking -lopencv_xfeatures2d -lopencv_ximgproc -lopencv_xobjdetect -lopencv_xphoto

/usr/bin/ld: /tmp/ccNDMpfK.ltrans2.ltrans.o: undefined reference to symbol 'GOMP_parallel@@GOMP_4.0'
//usr/lib/x86_64-linux-gnu/libgomp.so.1: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Here's my makefile

CC=gcc
CXX=g++
RM=rm -f

CPPCONFIG=-std=c++11 -std=c++1y -fopenmp -flto -lstdc++fs -DNDEBUG
CPP_OPT=-O3 -fprofile-generate -march=native -fomit-frame-pointer
CPPFLAGS=-g $(CPPCONFIG) $(CPP_OPT)

THREAD_LIBS=-lpthread -lboost_thread -lboost_system
OPENCV_LIBS=-lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_calib3d -lopencv_ccalib -lopencv_core -lopencv_datasets -lopencv_dnn -lopencv_dpm -lopencv_face -lopencv_features2d -lopencv_flann -lopencv_fuzzy -lopencv_highgui -lopencv_imgcodecs -lopencv_imgproc -lopencv_line_descriptor -lopencv_ml -lopencv_objdetect -lopencv_optflow -lopencv_photo -lopencv_plot -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_shape -lopencv_stereo -lopencv_stitching -lopencv_structured_light -lopencv_superres -lopencv_surface_matching -lopencv_text -lopencv_tracking -lopencv_xfeatures2d -lopencv_ximgproc -lopencv_xobjdetect -lopencv_xphoto
LDFLAGS=-g -L/usr/local/lib
LDLIBS=$(THREAD_LIBS) $(OPENCV_LIBS)

SRCS=docproc.cpp taina.cpp logging.cpp
OBJS=$(subst .cpp,.o,$(SRCS))

all: docproc

docproc: $(OBJS)
    $(CXX) $(LDFLAGS) -o docproc $(OBJS) $(LDLIBS)

depend: .depend

.depend: $(SRCS)
    rm -f ./.depend
    $(CXX) $(CPPFLAGS) -MM $^>>./.depend;

clean:
    $(RM) $(OBJS)

dist-clean: clean
    $(RM) *~ .depend

include .depend

Answer 1

You need to link with -fopenmp. Your makefile doesn't provide that flag on the linker step. Just add -fopenmp to your LDFLAGS.

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All your rules that aren't building files should be indicated as .PHONY.