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diff --git a/sys/gio/gcancel.x b/sys/gio/gcancel.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<gio.h>
+
+# GCANCEL -- Cancel any buffered graphics output (as far as possible).  Should
+# be called by interrupt handlers to avoid leaving GIO in a funny state
+# following an interrupt.
+#
+# As far as possible, GKI instructions are built up in internal storage and
+# written to the output file in a single write.  This decreases the likliehood
+# of leaving a botched instruction in the output stream in response to an
+# interrupt.  Do not call FSETI to cancel the file output because that will
+# almost certainly guarantee a botched instruction.  Instead, we discard any
+# partially built polylines still in GPL storage and append the GKI_CANCEL to
+# the output instruction stream.  The cancel instruction is passed on to the
+# graphics kernel which eventually calls FSETI to cancel its output file
+# buffer (containing device instructions).  If a metacode reader does detect
+# a botched instruction it will scan forward for the next BOI to try to resync
+# the instruction stream.
+
+procedure gcancel (gp)
+
+pointer	gp			# graphics descriptor
+int	and()
+
+begin
+	if (and (GP_GFLAGS(gp), GF_WSOPEN) != 0) {
+	    call gki_cancel (GP_FD(gp))
+	    call gki_fflush (GP_FD(gp))
+	}
+	call gfrinit (gp)
+end