xprop-sink

commit 04aa8891cd43b293af8a26e484707c3a7a04201a
Author: Jul <jul@9o.is>
Date:   Sat,  7 Feb 2026 03:48:16 -0500

init

Diffstat:
A
.gitignore
 | 
2
++
A
Makefile
 | 
30
++++++++++++++++++++++++++++++
A
README.md
 | 
10
++++++++++
A
contrib/user.WindowStatus
 | 
47
+++++++++++++++++++++++++++++++++++++++++++++++
A
xprop-sink.1
 | 
23
+++++++++++++++++++++++
A
xprop-sink.c
 | 
92
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

6 files changed, 204 insertions(+), 0 deletions(-)
diff --git a/.gitignore b/.gitignore
@@ -0,0 +1 @@
+/xprop-sink
+\ No newline at end of file
diff --git a/Makefile b/Makefile
@@ -0,0 +1,29 @@
+.POSIX:
+.SUFFIXES:
+
+CC = gcc
+CFLAGS = -std=c99 -pedantic -Wall -Wextra -O2
+LIBS = -lX11
+TARGET = xprop-sink
+PREFIX = /usr/local
+MANPREFIX = $(PREFIX)/share/man
+
+all: $(TARGET)
+
+$(TARGET): xprop-sink.c
+	$(CC) $(CFLAGS) xprop-sink.c -o $(TARGET) $(LIBS)
+
+install: $(TARGET)
+	mkdir -p $(DESTDIR)$(PREFIX)/bin
+	mkdir -p $(DESTDIR)$(MANPREFIX)/man1/
+	install -m 0755 $(TARGET) $(DESTDIR)$(PREFIX)/bin/
+	install -m 0644 xprop-sink.1 $(DESTDIR)$(MANPREFIX)/man1/
+
+uninstall:
+	rm -f $(DESTDIR)$(PREFIX)/bin/$(TARGET) \
+		$(DESTDIR)$(MANPREFIX)/man1/xprop-sink.1
+
+clean:
+	rm -f $(TARGET)
+
+.PHONY: all install uninstall clean
+\ No newline at end of file
diff --git a/README.md b/README.md
@@ -0,0 +1,10 @@
+# xprop-sink
+
+A lightweight, streaming X11 property updater. It's useful for updating a rapidly changing property. I currently use it to set a custom, per-window status in my window manager bar. If you happen to be using Qubes like I am, check out the qrexec script `contrib/user.WindowStatus` as an example.
+
+## Installation
+
+```sh
+make
+sudo make install
+```
diff --git a/contrib/user.WindowStatus b/contrib/user.WindowStatus
@@ -0,0 +1,47 @@
+#!/usr/bin/env sh
+set -e
+STATUS_PROP=_MY_WIN_STATUS
+##
+## Allows an AppVM to update an X11 property on its own window via `qrexec`
+## without compromising the security of other windows. The map_window function checks
+## if the window belongs to the appvm making the request. The window status prop is
+## hardcoded in the STATUS_PROP variable.
+##
+## Usage:
+##     while true; do
+##         echo "Counter: $(date +%S)"
+##         sleep 0.1
+##     done | qrexec-client-vm @default user.WindowStatus+$WINDOWID
+##
+## Policy (if using sys-gui):
+##     user.WindowStatus * @tag:guivm-sys-gui @default allow target=sys-gui
+##
+
+map_window() {
+	local target_vm="$1"
+	local target_xid="$2"
+
+	for win in $(xprop -root _NET_CLIENT_LIST | cut -d'#' -f2 | tr ',' ' '); do
+		vm=$(xprop -id "$win" _QUBES_VMNAME 2>/dev/null | cut -d'"' -f2)
+		vmid=$(xprop -id "$win" _QUBES_VMWINDOWID 2>/dev/null | awk -F' # ' '{print $2}')
+
+		if [ "$vm" = "$target_vm" ] && [ "$vmid" = "$target_xid" ]; then
+			echo "$win"
+			return 0
+		fi
+	done
+}
+
+if [ -z "$1" ]; then
+	echo "Error: Missing window ID argument" >&2
+	exit 1
+fi
+
+XID=$(map_window "$QREXEC_REMOTE_DOMAIN" "$(printf '0x%x' $1)")
+
+if [ -n "$XID" ]; then
+	xprop-sink "$XID" "$STATUS_PROP"
+else
+	echo "Error: Window mapping failed" >&2
+	exit 1
+fi
diff --git a/xprop-sink.1 b/xprop-sink.1
@@ -0,0 +1,22 @@
+.TH XPROP-SINK 1 "FEBRUARY 2026" "Linux" "User Commands"
+.SH NAME
+xprop-sink \- Stream stdin updates to an X11 window property
+.SH SYNOPSIS
+.B xprop-sink
+\fIWINDOW_ID\fR \fIPROPERTY_NAME\fR
+.SH DESCRIPTION
+.B xprop-sink
+reads lines from standard input and sets the specified X11 property on the given window. It is optimized for high-frequency updates, such as status bar indicators or keyboard state tracking.
+.PP
+The program is line-buffered. If an input line exceeds the internal 1024-byte buffer, the remainder of the line is drained and ignored to prevent property fragmentation.
+.SH ARGUMENTS
+.TP
+.I WINDOW_ID
+The X11 window ID (decimal or hex) to target.
+.TP
+.I PROPERTY_NAME
+The name of the X11 atom (property) to update (e.g., _MY_WIN_STATUS).
+.SH EXIT STATUS
+The program exits if standard input is closed or if the target window becomes invalid (e.g., the window is closed by the user).
+.SH SEE ALSO
+.BR xprop (1)
+\ No newline at end of file
diff --git a/xprop-sink.c b/xprop-sink.c
@@ -0,0 +1,92 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <X11/Xlib.h>
+#include <X11/Xatom.h>
+
+static int running = 1;
+
+void
+usage(const char *prog)
+{
+	fprintf(stderr, "Usage: %s <window_id> <property_name>\n", prog);
+}
+
+int
+handle_x_errors(Display *dpy, XErrorEvent *ev)
+{
+	(void)dpy;
+	if (ev->error_code == BadWindow)
+		running = 0;
+	return 0;
+}
+
+int
+main(int argc, char **argv)
+{
+	Display *dpy;
+	Window win;
+	XWindowAttributes wa;
+	Atom atom;
+	char *endptr, buffer[1024];
+	size_t len;
+	int c;
+
+	if (argc != 3) {
+		usage(argv[0]);
+		return 1;
+	}
+
+	win = (Window) strtol(argv[1], &endptr, 0);
+	if (*endptr != '\0' || win == 0) {
+		fprintf(stderr, "Error: Invalid Window ID\n");
+		usage(argv[0]);
+		return 1;
+	}
+	
+	dpy = XOpenDisplay(NULL);
+	if (!dpy) {
+		fprintf(stderr, "Error: Cannot open X display\n");
+		return 1;
+	}
+
+	XSetErrorHandler(handle_x_errors);
+
+	atom = XInternAtom(dpy, argv[2], False);
+	if (atom == None) {
+		fprintf(stderr, "Error: Cannot create X atom\n");
+		return 1;
+	}
+
+	if (!XGetWindowAttributes(dpy, win, &wa)) {
+		fprintf(stderr, "Error: Window not found\n");
+		return 1;
+	}
+
+	setvbuf(stdin, NULL, _IONBF, 0);
+
+	while (running && fgets(buffer, sizeof(buffer), stdin)) {
+		len = strlen(buffer);
+
+		// If line exceeds buffer size, the remainder drained and ignored
+		if (len == sizeof(buffer) - 1 && buffer[len-1] != '\n') {
+			while ((c = getchar()) != '\n' && c != EOF);
+			continue;
+		}
+
+		len = strlen(buffer);
+		buffer[strcspn(buffer, "\r\n")] = 0;
+
+		if (!XGetWindowAttributes(dpy, win, &wa))
+			break;
+
+		if (len > 0) {
+			XChangeProperty(dpy, win, atom, XA_STRING, 8,
+					PropModeReplace, (unsigned char *)buffer, (int)len);
+			XFlush(dpy);
+		}
+	}
+
+	XCloseDisplay(dpy);
+	return 0;
+}