Original from Rudolf Polzer in dcou.x11.

Added timestamps, flushing of stdout,
and usleep to reduce CPU usage.

xkeylog/GNUmakefile

@@ -0,0 +1,8 @@
+all: xkeylog
+xkeylog: xkeylog.c
+	gcc -O xkeylog.c -L/usr/X11R6/lib -lX11 -o xkeylog
+
+install: /usr/local/bin/xkeylog
+
+/usr/local/bin/xkeylog: xkeylog
+	cp $^ $@

xkeylog/xkeylog.c

@@ -0,0 +1,47 @@
+#include <X11/Xlib.h>
+#include <sys/time.h>
+#include <stdio.h>
+#include <unistd.h>
+
+typedef char Keymap[32];
+
+void CompareKeymaps(Display *dpy, const Keymap k1, const Keymap k2)
+{
+    int i, j;
+    struct timeval tv;
+    gettimeofday(&tv, NULL);
+    for(i = 0; i != 32; ++i)
+    {
+        char d = k1[i] &~ k2[i];
+        char p = k2[i] &~ k1[i];
+	int k;
+        for(j = 0, k = 1; j != 8; ++j, k <<= 1)
+        {
+	    if (p & k) {
+		printf("%d.%06d:p:%d\n", tv.tv_sec, tv.tv_usec, 8*i+j);
+	    }
+	    if (d & k) {
+		printf("%d.%06d:r:%d\n", tv.tv_sec, tv.tv_usec, 8*i+j);
+	    }
+        }
+    }
+    fflush(stdout);
+}
+
+int main()
+{
+    Keymap k1 = {0};
+    Keymap k2 = {0};
+    Display *dpy = XOpenDisplay(0);
+    if(!dpy)
+        err(1, "don't have a display, cu\n");
+    for(;;)
+    {
+	usleep(1000);
+        XQueryKeymap(dpy, k1);
+        CompareKeymaps(dpy, k2, k1);
+	usleep(1000);
+        XQueryKeymap(dpy, k2);
+        CompareKeymaps(dpy, k1, k2);
+    }
+}