diff -uNr libX11-1.6.8/src/XlibInt.c libX11-1.6.8.mod/src/XlibInt.c
--- libX11-1.6.8/src/XlibInt.c	2019-06-17 17:47:35.000000000 +0300
+++ libX11-1.6.8.mod/src/XlibInt.c	2019-06-29 22:13:06.054460413 +0300
@@ -1263,7 +1263,7 @@
     WSASetLastError(last_error);
 #else
     last_error = errno;
-    ioctl(ConnectionNumber(dpy), FIONREAD, &bytes);
+    ioctl(ConnectionNumber(dpy), /* FIONREAD */ 0x541B, &bytes);
     errno = last_error;
 #endif
     return bytes;