Add C2X timespec_getres

ISO C2X adds a timespec_getres function alongside the C11
timespec_get, with functionality similar to that of POSIX clock_getres
(including allowing a NULL pointer to be passed to the function).
Implement this function for glibc, similarly to the implementation of
timespec_get.

This includes a basic test like that of timespec_get, but no
documentation in the manual, given that TIME_UTC and timespec_get
aren't documented in the manual at all.  The handling of 64-bit time
follows that in timespec_get; people maintaining patch series for
64-bit time will need to update them accordingly (to export
__timespec_getres64, redirect calls in time.h and run the test for
_TIME_BITS=64).

Tested for x86_64 and x86, and (previous version; only testcase
differs) with build-many-glibcs.py.

1 files changed, 51 insertions, 0 deletions

diff --git a/time/tst-timespec_getres.c b/time/tst-timespec_getres.c
new file mode 100644
index 0000000000..e15824c0ed
--- /dev/null
+++ b/time/tst-timespec_getres.c
@@ -0,0 +1,51 @@
+/* Basic tests for timespec_getres.
+   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 int
+do_test (void)
+{
+  {
+    struct timespec ts;
+    TEST_COMPARE (timespec_getres (&ts, 0), 0);
+    TEST_COMPARE (timespec_getres (NULL, 0), 0);
+  }
+
+  {
+    struct timespec ts;
+    TEST_COMPARE (timespec_getres (&ts, TIME_UTC), TIME_UTC);
+    /* Expect all supported systems to support TIME_UTC with
+       resolution better than one second.  */
+    TEST_VERIFY (ts.tv_sec == 0);
+    TEST_VERIFY (ts.tv_nsec > 0);
+    TEST_VERIFY (ts.tv_nsec < 1000000000);
+    TEST_COMPARE (timespec_getres (NULL, TIME_UTC), TIME_UTC);
+    /* Expect the resolution to be the same as that reported for
+       CLOCK_REALTIME with clock_getres.  */
+    struct timespec cts;
+    TEST_COMPARE (clock_getres (CLOCK_REALTIME, &cts), 0);
+    TEST_COMPARE (ts.tv_sec, cts.tv_sec);
+    TEST_COMPARE (ts.tv_nsec, cts.tv_nsec);
+  }
+
+  return 0;
+}
+
+#include <support/test-driver.c>