"Undefined reference" error when passing variable by reference - c++

Question

I'm fairly new to C++ but this thing has me baffled by any logic. My code is as follows:
#include "stdlib.h"
#include "syslog.h"
#include "unistd.h"
#include "sys/stat.h"
#include "X11/Xlib.h"
#include "cstdio"
void process();
void startTracker();
Display *display;
Window rootWindow;
XEvent xevent;
I have the Xlib header included and if I click on member functions in Eclipse it navigates to the definitions.
int main(int argc, char *argv[])
{
// set logging up
openlog("unison", LOG_CONS|LOG_PID|LOG_NDELAY, LOG_LOCAL1);
syslog(LOG_NOTICE, "Starting Unison Handler");
pid_t pid, sid;
pid = fork();
// fork failed
if (pid < 0) {
exit(EXIT_FAILURE);
}
if (pid > 0) {
exit(EXIT_SUCCESS);
}
umask(0);
sid = setsid();
if (sid < 0) {
exit(EXIT_FAILURE);
}
if (chdir("/") < 0) {
exit(EXIT_FAILURE);
}
close(STDIN_FILENO);
close(STDOUT_FILENO);
close(STDERR_FILENO);
startTracker();
while (true) {
process();
}
closelog();
return(EXIT_SUCCESS);
}
Then I assign the variables for input selection
void startTracker() {
display = XOpenDisplay(0);
rootWindow = XRootWindow(display, 0);
XSelectInput(display, rootWindow, PointerMotionMask);
}
void process()
{
...but when i add the &event here...
XNextEvent(display, &xevent);
switch (xevent.type) {
case MotionNotify:
syslog(
LOG_NOTICE,
"Mouse position is %dx%d",
xevent.xmotion.x_root, xevent.xmotion.y_root
);
}
}
...the whole thing falls apart.
For some reason passing the xevent as reference throws off the entire Xlib header and gives me this:
00:16:15 **** Incremental Build of configuration Debug for project unisond ****
make all
Building file: ../unisond.cpp
Invoking: GCC C++ Compiler
g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"unisond.d" -MT"unisond.d" -o "unisond.o" "../unisond.cpp"
Finished building: ../unisond.cpp
Building target: unisond
Invoking: GCC C++ Linker
g++ -o "unisond" ./unisond.o
./unisond.o: In function `startTracker()':
/home/ancarius/workspace/unisond/Debug/../unisond.cpp:97: undefined reference to `XOpenDisplay'
/home/ancarius/workspace/unisond/Debug/../unisond.cpp:98: undefined reference to `XRootWindow'
/home/ancarius/workspace/unisond/Debug/../unisond.cpp:99: undefined reference to `XSelectInput'
./unisond.o: In function `process()':
/home/ancarius/workspace/unisond/Debug/../unisond.cpp:105: undefined reference to `XNextEvent'
collect2: error: ld returned 1 exit status
make: *** [unisond] Error 1
00:16:15 Build Finished (took 159ms)
At the risk of getting downvoted could someone please explain what I've done wrong? I've tried everything I could think of but no luck.

Answer 1

It looks like you are missing the X11 library for linking.
add -lX11 to the g++ invocation.
This provides the steps required.
Right click on Project Folder> Properties> C/C++ Build > Settings > GCC C++ Linker > Libraries > add "X11"