diff --git a/sys/src/libbio/blethal.c b/sys/src/libbio/blethal.c
new file mode 100644
index 000000000..11e1eedee
--- /dev/null
+++ b/sys/src/libbio/blethal.c
@@ -0,0 +1,33 @@
+#include	<u.h>
+#include	<libc.h>
+#include	<bio.h>
+
+void
+Berror(Biobufhdr *bp, char *fmt, ...)
+{
+	va_list va;
+	char buf[ERRMAX];
+
+	if(bp->errorf == nil)
+		return;
+	
+	va_start(va, fmt);
+	vsnprint(buf, ERRMAX, fmt, va);
+	va_end(va);
+	bp->errorf(buf);
+}
+
+static void
+Bpanic(char *s)
+{
+	sysfatal("%s", s);
+}
+
+void
+Blethal(Biobufhdr *bp, void (*errorf)(char *))
+{
+	if(errorf == nil)
+		errorf = Bpanic;
+
+	bp->errorf = errorf;
+}