"No rule to make target '../libapriltag.a' " when compiling demo codes

[CarmackMo]

Hi Everybody:
I am using the Ubuntu18.04 virtual machine (virtual box) as my environment. I have successfully installed OpenCV 3.4.8 and Apriltag library. But when I try to cmake the demo by following the official instructions "make opencv_demo" and "make apriltag_demo", the following errors show up:

$ cd example/
$ make opencv_demo 
make: *** No rule to make target '../libapriltag.a', needed by 'opencv_demo'.  Stop.
$ make apriltag_demo 
make: *** No rule to make target '../libapriltag.a', needed by 'apriltag_demo'.  Stop.

I try to ignore the error and directly run the demos, the following errors show up:

$ ./apriltag_demo 
apriltag_demo: error while loading shared libraries: libapriltag.so.3: cannot open shared object file: No such file or directory
$ ./opencv_demo
opencv_demo: error while loading shared libraries: libapriltag.so.3: cannot open shared object file: No such file or directory

But in fact, I have libapriltag.so.3 installed in /usr/local/lib as Apriltag library is successfully installed. Is anyone has any idea with these issues? Thanks for any advice

[jeremi]

@WaterFriend I had the same issue, running sudo ldconfig resolved it.

[Reihold-Chen]

I meet the same mistake, did you have solved it ?

[Zhang-Qinghui]

I meet the same mistake, did you have solved it ?

[christian-rauch]

@Reihold-Chen @Zhang-Qinghui Did running sudo ldconfig work for you?

Are you using cmake or make? The documentation has instructions for both. You can also try to run CMake with Ninja instead:

cmake . -GNinja
ninja

[Hola39e]

I met the same mistake and got a solution.
the ../libapriltag.a file doesnt exsit in the right path when using cmake to build apriltag library, the lib was directly added to /usr/local/lib.
so change the Makefile file in /example, delete ../libapriltag.a text and add pkg-config --libs apriltag instead, then is could be compiled successfully.

apriltag_demo: apriltag_demo.o  
      @echo "   [$@]"
      @$(CC) -o $@ $^ $(LDFLAGS) `pkg-config --libs apriltag`
 
opencv_demo: opencv_demo.o
     @echo "   [$@]"
     @$(CXX) -o $@ $^ $(LDFLAGS) `pkg-config --libs opencv` `pkg-config --libs apriltag`

[christian-rauch]

I met the same mistake and got a solution.

What did you do?

The cmake instructions on the main page work for me, I can run ./opencv_demo afterwards. The make file exists in parallel to the cmake system. But don't use them in parallel since running cmake in the source folder will override the Makefile.

[Hola39e]

I get what's your meaning. Thanks. What I did was:

$cmake .
$sudo make install
$cd ./example
$make all
make: *** No rule to make target '../libapriltag.a', needed by 'apriltag_demo'.  Stop.

After I used cmake method to install, I didn't find static lib libapriltag.a in the source code dir, that's why I cannot run make all instructions in /example.
I just tried to use another way make + sudo make install to install in another virtual environment, then I found make all in /example works.
Thanks for helping, I have solved my problem.

[christian-rauch]

Yes, don't mix the build systems.

The Makefile already builds the example, it prints: make[1]: Entering directory '/tmp/apriltag/example'.

If you need the static library, you can either use the Makefile as per the README instructions, or you use cmake with the BUILD_SHARED_LIBS set to OFF (introduced in #172):

cmake . -DBUILD_SHARED_LIBS=OFF
make -j

[christian-rauch]

The documentation has been updated.