freedesktop XEmbed systray client code in C, Xlib - x11

Question

I've been trying to implement a systray icon using straight C and Xlib, going along with the freedesktop specification [0]. I can't seem to get my Window to embed into my desktop manager's systray[1], while other apps seem to be able to do it. I am not sure how to go forward debugging this, but I've provided minimal sample code below.
I haven't been able to find any sample code using straight Xlib and C, and all the suggestions I've seen have been with regard to some framework like Gtk/Qt/Mono/whatever, but I want to understand what is supposed to be happening here as per the spec, and what I'm doing wrong.
#include <X11/Xutil.h>
#include <string.h>
#define MIN(A, B) ((A) < (B) ? (A) : (B))
/* --------- XEMBED and systray stuff */
#define SYSTEM_TRAY_REQUEST_DOCK 0
#define SYSTEM_TRAY_BEGIN_MESSAGE 1
#define SYSTEM_TRAY_CANCEL_MESSAGE 2
static int trapped_error_code = 0;
static int (*old_error_handler) (Display *, XErrorEvent *);
static int
error_handler(Display *display, XErrorEvent *error) {
trapped_error_code = error->error_code;
return 0;
}
void
trap_errors(void) {
trapped_error_code = 0;
old_error_handler = XSetErrorHandler(error_handler);
}
int
untrap_errors(void) {
XSetErrorHandler(old_error_handler);
return trapped_error_code;
}
void
send_systray_message(Display* dpy, Window w, long message, long data1, long data2, long data3) {
XEvent ev;
memset(&ev, 0, sizeof(ev));
ev.xclient.type = ClientMessage;
ev.xclient.window = w;
ev.xclient.message_type = XInternAtom (dpy, "_NET_SYSTEM_TRAY_OPCODE", False );
ev.xclient.format = 32;
ev.xclient.data.l[0] = CurrentTime;
ev.xclient.data.l[1] = message;
ev.xclient.data.l[2] = data1;
ev.xclient.data.l[3] = data2;
ev.xclient.data.l[4] = data3;
trap_errors();
XSendEvent(dpy, w, False, NoEventMask, &ev);
XSync(dpy, False);
if (untrap_errors()) {
/* Handle errors */
}
}
/* ------------ Regular X stuff */
int
main(int argc, char **argv) {
int width, height;
XWindowAttributes wa;
XEvent ev;
Display *dpy;
int screen;
Window root, win;
/* init */
if (!(dpy=XOpenDisplay(NULL)))
return 1;
screen = DefaultScreen(dpy);
root = RootWindow(dpy, screen);
if(!XGetWindowAttributes(dpy, root, &wa))
return 1;
width = height = MIN(wa.width, wa.height);
/* create window */
win = XCreateSimpleWindow(dpy, root, 0, 0, width, height, 0, 0, 0xFFFF9900);
send_systray_message(dpy, win, SYSTEM_TRAY_REQUEST_DOCK, win, 0, 0);
XMapWindow(dpy, win);
XSync(dpy, False);
/* run */
while(1) {
while(XPending(dpy)) {
XNextEvent(dpy, &ev); /* just waiting until we error because window closed */
}
}
}
Any help would be greatly appreciated. I think this problem is language-agnostic, and more to do with me misunderstanding the protocols, so answers in any language are acceptable, as long as they help me iron out this XEvent stuff.
[0] https://specifications.freedesktop.org/systemtray-spec/systemtray-spec-0.2.html
[1] I'm using dwm with the systray patch http://dwm.suckless.org/patches/systray

Answer 1

You are sending the message to a wrong window. The documentation isn't really helpful, it makes no sense whatsoever to send a tray embed message to your own window!. You need to send it to the tray window.
Here's a a fixed send_systray_message
void
send_systray_message(Display* dpy, long message, long data1, long data2, long data3) {
XEvent ev;
Atom selection_atom = XInternAtom (dpy,"_NET_SYSTEM_TRAY_S0",False);
Window tray = XGetSelectionOwner (dpy,selection_atom);
if ( tray != None)
XSelectInput (dpy,tray,StructureNotifyMask);
memset(&ev, 0, sizeof(ev));
ev.xclient.type = ClientMessage;
ev.xclient.window = tray;
ev.xclient.message_type = XInternAtom (dpy, "_NET_SYSTEM_TRAY_OPCODE", False );
ev.xclient.format = 32;
ev.xclient.data.l[0] = CurrentTime;
ev.xclient.data.l[1] = message;
ev.xclient.data.l[2] = data1; // <--- your window is only here
ev.xclient.data.l[3] = data2;
ev.xclient.data.l[4] = data3;
trap_errors();
XSendEvent(dpy, tray, False, NoEventMask, &ev);
XSync(dpy, False);
usleep(10000);
if (untrap_errors()) {
/* Handle errors */
}
}
and a call to it
send_systray_message(dpy, SYSTEM_TRAY_REQUEST_DOCK, win, 0, 0); // pass win only once
Credits: http://distro.ibiblio.org/vectorlinux/Uelsk8s/GAMBAS/gambas-svn/gambas2/gb.gtk/src/gtrayicon.cpp