1 files changed, 67 insertions, 0 deletions
diff --git a/libc/misc/time/difftime.c b/libc/misc/time/difftime.c
new file mode 100644
index 000000000..6a9b708ea
--- /dev/null
+++ b/libc/misc/time/difftime.c
@@ -0,0 +1,67 @@
+/* Copyright (C) 1991, 1994, 1996 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., 59 Temple Place - Suite 330,
+   Boston, MA 02111-1307, USA.  */
+
+#include <time.h>
+#include <limits.h>
+#include <sys/types.h>
+
+
+/* Return the difference between TIME1 and TIME0.  */
+double
+difftime (time1, time0)
+     time_t time1;
+     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 = (~ (time_t) 0) << ((sizeof (time_t) * CHAR_BIT) - 1);
+
+  /* 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;
+}