1 files changed, 48 insertions, 0 deletions
diff --git a/sysdeps/ieee754/logb.c b/sysdeps/ieee754/logb.c
new file mode 100644
index 0000000000..918de9ebad
--- /dev/null
+++ b/sysdeps/ieee754/logb.c
@@ -0,0 +1,48 @@
+/* Copyright (C) 1992, 1995 Free Software Foundation, Inc.
+This file is part of the GNU C Library.
+
+The GNU C Library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Library General Public License as
+published by the Free Software Foundation; either version 2 of the
+License, or (at your option) any later version.
+
+The GNU C Library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Library General Public License for more details.
+
+You should have received a copy of the GNU Library General Public
+License along with the GNU C Library; see the file COPYING.LIB.  If
+not, write to the Free Software Foundation, Inc., 675 Mass Ave,
+Cambridge, MA 02139, USA.  */
+
+#include <ansidecl.h>
+#include <math.h>
+#include <float.h>
+#include "ieee754.h"
+
+/* Return the base 2 signed integral exponent of X.  */
+double
+DEFUN(__logb, (x), double x)
+{
+  union ieee754_double u;
+
+  if (__isnan (x))
+    return x;
+  else if (__isinf (x))
+    return HUGE_VAL;
+  else if (x == 0.0)
+    return - HUGE_VAL;
+
+  u.d = x;
+
+  if (u.ieee.exponent == 0)
+    /* A denormalized number.
+       Multiplying by 2 ** DBL_MANT_DIG normalizes it;
+       we then subtract the DBL_MANT_DIG we added to the exponent.  */
+    return (__logb (x * ldexp (1.0, DBL_MANT_DIG)) - DBL_MANT_DIG);
+
+  return (int) u.ieee.exponent - (DBL_MAX_EXP - 1);
+}
+
+weak_alias (__logb, logb)