Undefined reference to XOpenDisplay in a Qt project

Question

Now I am feeling quite stupid. I am trying to do some stuff with xlib in Qt Creator.

My code:

#include <QtCore/QCoreApplication>
#include <X11/Xlib.h>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Display *display = XOpenDisplay(NULL);

    return 0;
}

Just one line of code and gives me:

/main.cpp:8: undefined reference to `XOpenDisplay'

It is defined in Xlib.h as

extern Display *XOpenDisplay(
    _Xconst char* /* display_name */
);

I feel I am missing something very basic.

Answer 1

Found it... compiler problem

added -lX11 to the make file

Answer 2

@КодСерфинг145 I added LIBS += -lX11 to the make file (the .pro file) Adding Additional arguments to Build steps inside Projects did not work for me either and neither did QMAKE_CXXFLAGS += -lX11 like many suggests.