XChangeProperty cannot write to clipboard - c

Question

I want to put a string or bytes to the X11 selection so that when I switch to other applications, I can directly do a ctrl+p paste.
I try to follow the documentation of the X11 clipboard mechanism. If I understand correctly, I need to use XSetSelectionOwner to obtain the XA_CLIPBOARD selection and then use XChangeProperty to put my data to the clipboard.
Here is a simple snippet, but unfortunately it does not work:
// main.c
#include <stdlib.h>
#include <stdio.h>
#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <X11/Xmu/Atoms.h>
int main() {
// try to write `hello` to the clipboard
const char *in = "hello\0";
const int n = 5;
Display* d = XOpenDisplay(0);
Window w = XCreateSimpleWindow(d, DefaultRootWindow(d), 0, 0, 1, 1, 0, 0, 0);
Atom XA_CLIPBOARD = XInternAtom(d, "CLIPBOARD", True);
XSetSelectionOwner(d, XA_CLIPBOARD, w, CurrentTime);
XEvent event;
XNextEvent(d, &event);
if (event.type != SelectionRequest) {
XCloseDisplay(d);
return 0;
}
if (event.xselectionrequest.selection != XA_CLIPBOARD) {
XCloseDisplay(d);
return 0;
}
XSelectionRequestEvent* req = &event.xselectionrequest;
XChangeProperty(d, req->requestor, req->property, XA_STRING, 8, PropModeReplace, (unsigned char *)in, n);
XEvent re = {0};
re.xselection.type = SelectionNotify;
re.xselection.display = req->display;
re.xselection.requestor = req->requestor;
re.xselection.selection = req->selection;
re.xselection.property = req->property;
re.xselection.target = req->target;
XSendEvent(d, req->requestor, 0, 0, &re); // event is sent, but data is not in my clipboard
XFlush(d);
XCloseDisplay(d);
return 0;
}
Compile: clang -o main main.c -lX11 -lXmu
What did I do wrong, and how to fix it?

Answer 1

XNextEvent retrieves any event that occured (input, pointer, configuration, property changes, ...).
So it is very unlikely that the first event, will be your desired event.
You have to iterate over the event queue to check if a SelectionRequest event occured.
Therefore you have to call XNextEvent in a loop and check everytime the event.type for the desired event.
Edit:
If you retrieve a ClientMessage event and the client data equals the Atom WM_DELETE, there was a request by the user to close the window (pressing the X). Other than that, you can quit whenever you want.
XEvent evt;
while (i_want_to_receive_and_process_events) {
XNextEvent(dpy, &evt);
switch (evt.type) {
case SelectionRequest:
if (evt.xselectionrequest.selection == XA_CLIPBOARD) {
// what you were looking for, do your thing
// i_want_to_receive_and_process_events = false (?)
}
break;
case ClientMessage:
if (evt.xclient.data.l[0] == WM_DELETE) {
i_want_to_receive_and_process_events = false;
}
break;
}
}