diff --git a/libm/isinf.c b/libm/isinf.c
new file mode 100644
index 0000000..2da8f58
--- /dev/null
+++ b/libm/isinf.c
@@ -0,0 +1,69 @@
+/*-
+ * Copyright (c) 2004 David Schultz <das@FreeBSD.ORG>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#include <math.h>
+#include <sys/cdefs.h>
+#include "src/fpmath.h"
+
+/*
+ * XXX These routines belong in libm, but they must remain in libc for
+ *     binary compat until we can bump libm's major version number.
+ */
+
+int
+__isinf(double d)
+{
+	union IEEEd2bits u;
+
+	u.d = d;
+	return (u.bits.exp == 2047 && u.bits.manl == 0 && u.bits.manh == 0);
+}
+
+int
+__isinff(float f)
+{
+	union IEEEf2bits u;
+
+	u.f = f;
+	return (u.bits.exp == 255 && u.bits.man == 0);
+}
+
+int
+__isinfl(long double e)
+{
+	union IEEEl2bits u;
+
+	u.e = e;
+	mask_nbit_l(u);
+#ifndef __alpha__
+	return (u.bits.exp == 32767 && u.bits.manl == 0 && u.bits.manh == 0);
+#else
+	return (u.bits.exp == 2047 && u.bits.manl == 0 && u.bits.manh == 0);
+#endif
+}
+