diff -Nru /n/sources/plan9/sys/src/libthread/debug.c /sys/src/libthread/debug.c
--- /n/sources/plan9/sys/src/libthread/debug.c	Thu Feb 28 20:29:17 2002
+++ /sys/src/libthread/debug.c	Sun Nov  6 00:00:00 2016
@@ -42,7 +42,7 @@
 
 	p = _threadgetproc();
 	if(p && p->thread)
-		n = sprint(buf, "%d.%d ", p->pid, p->thread->id);
+		n = snprint(buf, sizeof buf, "%d.%d ", p->pid, p->thread->id);
 	else
 		n = 0;
 	snprint(buf+n, sizeof(buf)-n, "%s: assertion failed\n", s);