how to import opencv lib into other lib using g++ - c++

Question

How can I set properly the makefile to import some OpenCV lib into the libmat.o?
this is the make file:
# Define a variable for classpath
CLASS_PATH = ../bin
# Debug: -g3=compile with extra debugg infos. -ggdbg3=include things like macro defenitions. -O0=turn off optimizations.
DEBUGFLAGS = -g3 -ggdb3 -O0
CFLAGS = $(DEBUGFLAGS)
# Define a virtual path for .class in the bin directory
vpath %.class $(CLASS_PATH)
all : libMat.so
# $# matches the target, $< matches the first dependancy
libMat.so : libMat.o
g++ $(CFLAGS) -W -shared -o $# $<
# $# matches the target, $< matches the first dependancy
libMat.o : Mat2Image.cpp Mat2Image.h
g++ $(CFLAGS) -fPIC -I/usr/lib/jvm/jdk1.8.0_111/include -I/usr/lib/jvm/jdk1.8.0_111/include/linux -c $< -o $# -L/usr/local/lib
# $* matches the target filename without the extension
# manually this would be: javah -classpath ../bin HelloJNI
HelloJNI.h : Mat2Image.class
javah -classpath $(CLASS_PATH) $*
clean :
rm -f Mat2Image.h libMat.o libMat.so
i need some OpenCV lib located in /usr/local/lib
for example libopencv_imgproc.so

Answer 1

I think what you want to do is get (e.g.) libopencv_imgproc.so,
located in /usr/local/lib, to be linked with your shared library libMat.so.
The way you are trying to do this in your makefile is by "linking"
libopencv_imgproc.so with the object file libMat.o by the recipe:
libMat.o : Mat2Image.cpp Mat2Image.h
g++ $(CFLAGS) -fPIC -I/usr/lib/jvm/jdk1.8.0_111/include \
-I/usr/lib/jvm/jdk1.8.0_111/include/linux -c $< -o $# \
-L/usr/local/lib
You think that -L/usr/local/lib at the end will in some way let the linker
know you want to link libopencv_imgproc.so and do so.
That won't work for more than one reason:
-L/usr/local/lib will just tell the linker that /usr/local/lib is
a directory where you want it search for libraries that you ask it to
link. It doesn't ask it to link any libraries from there, and if you
don't tell it to, it won't. To tell the linker
to search for libraries in /usr/local/lib and ask it to link
libopencv_imgproc.so you would use:
`-L/usr/local/lib -lopencv_imgproc
But you don't need to tell it to search for libraries in /usr/local/lib,
because that is one of the directories the linker searches for libraries
by default. So -lopencv_imgproc is all you would need.
But more important than that,
g++ $(CFLAGS) -fPIC -I/usr/lib/jvm/jdk1.8.0_111/include \
-I/usr/lib/jvm/jdk1.8.0_111/include/linux -c $< -o $# \
-L/usr/local/lib
is the command that simply compiles libMat.o from Mat2Image.cpp. It
doesn't do any linkage. It is not possible and does not make sense to "link"
anything with an object file. So adding any linkage options to your
compile command is pointless. The compiler will ignore them. This command is
effectively no different from:
g++ $(CFLAGS) -fPIC -I/usr/lib/jvm/jdk1.8.0_111/include \
-I/usr/lib/jvm/jdk1.8.0_111/include/linux -c $< -o $#
and that's what it should be.
The recipe that does your linkage is:
libMat.so : libMat.o
g++ $(CFLAGS) -W -shared -o $# $<
So this is where you have to tell the linker that you want libopencv_imgproc.so
to be linked:
libMat.so : libMat.o
g++ $(CFLAGS) -W -shared -o $# $< -lopencv_imgproc
With this, libMat.so will be linked containing libMat.o and a runtime
dependency on the shared library libopencv_imgproc.so - that is, an
instruction to the runtime loader that it must load libopencv_imgproc.so
into the same process when it loads libMat.so.
Your makefile has various other faults but what I've said is enough to
explain and fix the particular problem I think you're asking about. You
can learn how to write a better makefile, if you want, by searching for tutorials
and ariticles about GNU Make and studying the documentation
I'll just point out one other fault that is serious. Your recipe:
clean:
rm -f Mat2Image.h libMat.o libMat.so
is going to delete the header file Mat2Image.h when you run
make clean. That header file is one of your original source files. It
isn't generated by this makefile - like libMat.o and libMat.so - unless
you've ommitted some bits of the real makefile. So
if you delete it you'll have to recover it from you source-control system; and
if its not under a source-control system, you'll have to rewrite it. So
I think you just want:
clean:
rm -f libMat.o libMat.so