Handle missing XDG_RUNTIME_DIR explicitly

Also update the documentation to indicate that this is a Wayland thing
and not dwl-specific.

README.md

@@ -74,16 +74,20 @@ this process a SIGTERM and wait for it to terminate (if it hasn't already).
 This makes it ideal not only for initialization but also for execing into a
 user-level service manager like s6 or `systemd --user`.
 
-You'll have to explicitly set `XDG_RUNTIME_DIR` if your system doesn't do it for
-you, e.g. `export XDG_RUNTIME_DIR=/tmp/xdg-runtime-$USER && mkdir -p
-$XDG_RUNTIME_DIR`.
-
 More/less verbose output can be requested with flags as well:
 
 * `-q`: quiet (log level WLR_SILENT)
 * `-v`: verbose (log level WLR_INFO)
 * `-d`: debug (log level WLR_DEBUG)
 
+Note: Wayland requires a valid `XDG_RUNTIME_DIR`, which is usually set up by a
+session manager such as `elogind` or `systemd-logind`.  If your system doesn't
+do this automatically, you will need to configure it prior to launching `dwl`,
+e.g.:
+
+    export XDG_RUNTIME_DIR=/run/user/$(id -u)
+    mkdir -p $XDG_RUNTIME_DIR
+
 
 ## Known limitations and issues
 

dwl.c

@@ -1527,6 +1527,13 @@ main(int argc, char *argv[])
 		goto usage;
 	wlr_log_init(loglevel, NULL);
 
+	// Wayland requires XDG_RUNTIME_DIR for creating its communications
+	// socket
+	if (!getenv("XDG_RUNTIME_DIR")) {
+		fprintf(stderr, "XDG_RUNTIME_DIR must be set\n");
+		exit(EXIT_FAILURE);
+	}
+
 	/* The Wayland display is managed by libwayland. It handles accepting
 	 * clients from the Unix socket, manging Wayland globals, and so on. */
 	dpy = wl_display_create();