1 files changed, 66 insertions, 0 deletions
diff --git a/time/difftime.c b/time/difftime.c
new file mode 100644
index 0000000000..49c5bfbc5c
--- /dev/null
+++ b/time/difftime.c
@@ -0,0 +1,66 @@
+/* Copyright (C) 1991, 1994 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 <time.h>
+
+
+/* Return the difference between TIME1 and TIME0.  */
+double
+DEFUN(difftime, (time1, time0), time_t time1 AND time_t time0)
+{
+  /* Algorithm courtesy Paul Eggert (eggert@twinsun.com).  */
+
+  time_t delta, hibit;
+
+  if (sizeof (time_t) < sizeof (double))
+    return (double) time1 - (double) time0;
+  if (sizeof (time_t) < sizeof (LONG_DOUBLE))
+    return (LONG_DOUBLE) time1 - (LONG_DOUBLE) time0;
+
+  if (time1 < time0)
+    return - difftime (time0, time1);
+
+  /* As much as possible, avoid loss of precision by computing the
+    difference before converting to double.  */
+  delta = time1 - time0;
+  if (delta >= 0)
+    return delta;
+
+  /* Repair delta overflow.  */
+  hibit = 1;
+  while ((hibit <<= 1) > 0)
+    continue;
+
+  /* The following expression rounds twice, which means the result may not
+     be the closest to the true answer.  For example, suppose time_t is
+     64-bit signed int, long_double is IEEE 754 double with default
+     rounding, time1 = 9223372036854775807 and time0 = -1536.  Then the
+     true difference is 9223372036854777343, which rounds to
+     9223372036854777856 with a total error of 513.  But delta overflows to
+     -9223372036854774273, which rounds to -9223372036854774784, and
+     correcting this by subtracting 2 * (long_double) hibit (i.e. by adding
+     2**64 = 18446744073709551616) yields 9223372036854776832, which rounds
+     to 9223372036854775808 with a total error of 1535 instead.  This
+     problem occurs only with very large differences.  It's too painful to
+     fix this portably.  We are not alone in this problem; many C compilers
+     round twice when converting large unsigned types to small floating
+     types, so if time_t is unsigned the "return delta" above has the same
+     double-rounding problem.  */
+  return delta - 2 * (LONG_DOUBLE) hibit;
+}