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diff --git a/src/pj_math.c b/src/pj_math.c
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+/******************************************************************************
+ * Project:  PROJ
+ * Purpose:  Make C99 math functions available on C89 systems
+ * Author:   Kristian Evers
+ *
+ ******************************************************************************
+ * Copyright (c) 2018, Kristian Evers
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *****************************************************************************/
+
+#include "proj_math.h"
+
+#if !(defined(HAVE_C99_MATH) && HAVE_C99_MATH)
+
+/* Compute hypotenuse */
+double pj_hypot(double x, double y) {
+    x = fabs(x);
+    y = fabs(y);
+    if ( x < y ) {
+        x /= y;
+        return ( y * sqrt( 1. + x * x ) );
+    } else {
+        y /= (x != 0.0 ? x : 1.0);
+        return ( x * sqrt( 1. + y * y ) );
+    }
+}
+
+/* Compute log(1+x) accurately */
+double pj_log1p(double x) {
+    volatile double
+      y = 1 + x,
+      z = y - 1;
+    /* Here's the explanation for this magic: y = 1 + z, exactly, and z
+     * approx x, thus log(y)/z (which is nearly constant near z = 0) returns
+     * a good approximation to the true log(1 + x)/x.  The multiplication x *
+     * (log(y)/z) introduces little additional error. */
+    return z == 0 ? x : x * log(y) / z;
+}
+
+/* Compute asinh(x) accurately */
+double pj_asinh(double x) {
+    double y = fabs(x); /* Enforce odd parity */
+    y = log1p(y * (1 + y/(hypot(1.0, y) + 1)));
+    return x > 0 ? y : (x < 0 ? -y : x);
+}
+
+/* Returns 0 if not a NaN and non-zero if val is a NaN */
+int pj_isnan (double x) {
+    /* cppcheck-suppress duplicateExpression */
+    return x != x;
+}
+
+
+#endif /* !(defined(HAVE_C99_MATH) && HAVE_C99_MATH) */