tst: Provide test for difftime

This change adds new test to assess difftime's functionality by
adding some arbitrary offsets to current time_t value (read via
time).

If 64 bit time_t is supported, the same procedure is applied around
the threshold of Y2038 time overflow.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

2 files changed, 57 insertions, 1 deletions

diff --git a/time/Makefile b/time/Makefile
index 2dd70111b5..7caec2b271 100644
--- a/time/Makefile
+++ b/time/Makefile
@@ -48,7 +48,7 @@ tests	:= test_time clocktest tst-posixtz tst-strptime tst_wcsftime \
 	   tst-strptime3 bug-getdate1 tst-strptime-whitespace tst-ftime \
 	   tst-tzname tst-y2039 bug-mktime4 tst-strftime2 tst-strftime3 \
 	   tst-clock tst-clock2 tst-clock_nanosleep tst-cpuclock1 \
-	   tst-adjtime tst-ctime
+	   tst-adjtime tst-ctime tst-difftime
 
 include ../Rules
 
diff --git a/time/tst-difftime.c b/time/tst-difftime.c
new file mode 100644
index 0000000000..262f3d1db7
--- /dev/null
+++ b/time/tst-difftime.c
@@ -0,0 +1,56 @@
+/* Test for difftime
+   Copyright (C) 2021 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 Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <time.h>
+#include <support/check.h>
+
+static void
+test_difftime_helper (time_t t1, time_t t0, double exp_val)
+{
+  double sub = difftime (t1, t0);
+  if (sub != exp_val)
+    FAIL_EXIT1 ("*** Difftime returned %f (expected %f)\n", sub, exp_val);
+}
+
+static int
+do_test (void)
+{
+  time_t t = 1383791700; /* Provide reproductible start value.  */
+
+  /* Check if difftime works with current time.  */
+  test_difftime_helper (t + 1800, t - 1800, 3600.0);
+  test_difftime_helper (t - 1800, t + 1800, -3600.0);
+
+  t = 0x7FFFFFFF;
+  /* Check if we run on port with 32 bit time_t size */
+  time_t tov;
+  if (__builtin_add_overflow (t, 1, &tov))
+    return 0;
+
+  /* Check if the time is converted after 32 bit time_t overflow.  */
+  test_difftime_helper (t + 1800, t - 1800, 3600.0);
+  test_difftime_helper (t - 1800, t + 1800, -3600.0);
+
+  t = tov;
+  test_difftime_helper (t + 1800, t - 1800, 3600.0);
+  test_difftime_helper (t - 1800, t + 1800, -3600.0);
+
+  return 0;
+}
+
+#include <support/test-driver.c>