使用opencv时找不到lopencv_core

Question

I have opencv3 installed through home-brew and pkg-config can find the linkers too by

pkg-config --cflags --libs opencv

the outputs contains -lopencv_core, but when I add that in Makefile like this

CC=clang++
CFLAGS= -Wall -g -std=c++0x
LFLAGS= -I/usr/local/Cellar/opencv3/3.2.0/include -I/usr/local/Cellar/opencv3/3.2.0/bin -I/usr/local/Cellar/opencv3/3.2.0/lib -lopencv_core

CFILES=blah.cpp
HFILES=blah.hpp
OFILES=blah.o

all:    main

%.o:    %.cpp $(HFILES)                                                                                   
        $(CC) -c $(CFLAGS) $< -o $@ $(LFLAGS)

main:   $(OFILES) $(HFILES)
    $(CC) $(CFLAGS) $(OFILES) -o main $(LFLAGS)

it says

ld: library not found for -lopencv_core
clang: error: linker command failed with exit code 1 (use -v to see invocation)

How should I link that to gcc?

Answer 1

The -l option looks in the usual places for the mentioned library. Where and what it looks varies by OS, which you didn't mention. Possibly it can be fixed by adding -L /usr/local/lib or some other path to where you know the libraries are.

Answer 2

Assuming you want CFLAGS to be the options for compilation and LFLAGS to be the options for linkage, the settings:

CFLAGS= -Wall -g -std=c++0x
LFLAGS= -I/usr/local/Cellar/opencv3/3.2.0/include -I/usr/local/Cellar/opencv3/3.2.0/bin -I/usr/local/Cellar/opencv3/3.2.0/lib -lopencv_core

are confused.

The -I path option is meaningful for compilation and meaningless for linkage. It tells the preprocessor to search in path for header files that you #include in the source code. There will be header files in:

/usr/local/Cellar/opencv3/3.2.0/include

There will be no header files in:

/usr/local/Cellar/opencv3/3.2.0/bin

just executables, which are irrelevant to building your program. And there will be no header files in:

/usr/local/Cellar/opencv3/3.2.0/lib

just static and or dynamic libraries.

The library libopencv_core that linker can't find is presumably in /usr/local/Cellar/opencv3/3.2.0/lib . The way to tell the linker to search there for the library is:

-L/usr/local/Cellar/opencv3/3.2.0/lib -lopencv_core

So these settings make sense:

CFLAGS := -Wall -g -std=c++0x -I/usr/local/Cellar/opencv3/3.2.0/include
LFLAGS := -L/usr/local/Cellar/opencv3/3.2.0/lib -lopencv_core

However, to compile and link an opencv program you would probably be better leaving it to pkg-config to get the options right:

CFLAGS := -Wall -g -std=c++0x $(shell pkg-config --cflags opencv)
LFLAGS := $(shell pkg-config --libs opencv)