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.

time/Makefile

@@ -36,7 +36,7 @@ routines := offtime asctime clock ctime ctime_r difftime \
 	    stime dysize timegm ftime			 \
 	    getdate strptime strptime_l			 \
 	    strftime wcsftime strftime_l wcsftime_l	 \
-	    timespec_get				 \
+	    timespec_get timespec_getres		 \
 	    clock_getcpuclockid clock_getres		 \
 	    clock_gettime clock_settime clock_nanosleep
 
@@ -50,7 +50,7 @@ tests	:= test_time clocktest tst-posixtz tst-strptime tst_wcsftime \
 	   tst-clock tst-clock2 tst-clock_nanosleep tst-cpuclock1 \
 	   tst-adjtime tst-ctime tst-difftime tst-mktime4 tst-clock_settime \
 	   tst-settimeofday tst-itimer tst-gmtime tst-timegm \
-	   tst-timespec_get
+	   tst-timespec_get tst-timespec_getres
 
 include ../Rules
 

time/Versions

@@ -74,6 +74,9 @@ libc {
     clock_getres; clock_gettime; clock_settime; clock_getcpuclockid;
     clock_nanosleep;
   }
+  GLIBC_2.34 {
+    timespec_getres;
+  }
   GLIBC_PRIVATE {
     # same as clock_gettime; used in other libraries
     __clock_gettime;

time/clock_getres.c

@@ -27,6 +27,7 @@ __clock_getres (clockid_t clock_id, struct timespec *res)
   __set_errno (ENOSYS);
   return -1;
 }
+libc_hidden_def (__clock_getres)
 
 versioned_symbol (libc, __clock_getres, clock_getres, GLIBC_2_17);
 /* clock_getres moved to libc in version 2.17;

time/time.h

@@ -259,6 +259,13 @@ extern int timespec_get (struct timespec *__ts, int __base)
 #endif
 
 
+#if __GLIBC_USE (ISOC2X)
+/* Set TS to resolution of time base BASE.  */
+extern int timespec_getres (struct timespec *__ts, int __base)
+     __THROW;
+#endif
+
+
 #ifdef __USE_XOPEN_EXTENDED
 /* Set to one of the following values to indicate an error.
      1  the DATEMSK environment variable is null or undefined,

time/timespec_getres.c

@@ -0,0 +1,32 @@
+/* Get resolution of a time base.
+   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.
+
+   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>
+
+
+/* Set TS to resolution of time base BASE.  */
+int
+timespec_getres (struct timespec *ts, int base)
+{
+  if (base == TIME_UTC)
+    {
+      __clock_getres (CLOCK_REALTIME, ts);
+      return base;
+    }
+  return 0;
+}

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>