Undefined reference

I am trying to connect to Jack (Kubuntu 18.04, so it is Jack2) from a Qt app:

Code:

---

void JackPortInfo::signal_handler ( int sig )
{
    jack_client_close ( client );
    qDebug() << "JackPortInfo: signal received, exiting ...";
    exit ( 0 );
}

---

But I ran into a problem:
CMakeFiles/TestApp.dir/src/jackportinfo.cpp.o: In function `JackPortInfo::signal_handler(int)':
/home/me/projects/TestApp/src/jackportinfo.cpp:32: undefined reference to `jack_client_close'
collect2: error: ld returned 1 exit status

I thought
#include <jack/jack.h>
should resolve this, but it doesn't.

There is a bunch of jack related .h files in
/usr/src/linux-headers-4.15.0-74-generic/include/config/snd/
so it seems that what should be there is present.
Maybe I ought to post this question in a different forum?

An "undefined reference" means that you have not linked to whatever library it is that contains the function. Including the header file only fixes compile-time errors, not link errors.

I have no idea what name your missing library has. Look in your Jack2 documentation or distribution. You will need to modify your CMakeLists.txt file to add the library reference to the target_link_libraries() directive..