[PATCH 3/5] gettime-res: new module

[Paul Eggert]

* lib/gettime-res.c, modules/gettime-res: New files.
* lib/timespec.h (gettime_res): New decl.
* m4/clock_time.m4 (gl_CLOCK_TIME): Also check for clock_getres.
* m4/gettime.m4 (gl_GETTIME_RES): New macro.
---
 ChangeLog                             |  6 +++
 doc/posix-functions/clock_getres.texi |  3 ++
 lib/gettime-res.c                     | 78 +++++++++++++++++++++++++++
 lib/timespec.h                        |  1 +
 m4/clock_time.m4                      |  7 +--
 m4/gettime.m4                         | 10 +++-
 modules/gettime-res                   | 30 +++++++++++
 7 files changed, 131 insertions(+), 4 deletions(-)
 create mode 100644 lib/gettime-res.c
 create mode 100644 modules/gettime-res

diff --git a/ChangeLog b/ChangeLog
index 23c87e8e1..8b25c542b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2021-12-28  Paul Eggert  <eggert@cs.ucla.edu>
 
+       gettime-res: new module
+       * lib/gettime-res.c, modules/gettime-res: New files.
+       * lib/timespec.h (gettime_res): New decl.
+       * m4/clock_time.m4 (gl_CLOCK_TIME): Also check for clock_getres.
+       * m4/gettime.m4 (gl_GETTIME_RES): New macro.
+
        gettime: port better to non-POSIX C2x
        C2x requires a function timespec_get; prefer that to
        gettimeofday, since it has better resolution.
diff --git a/doc/posix-functions/clock_getres.texi 
b/doc/posix-functions/clock_getres.texi
index 09536ff3b..1381c4649 100644
--- a/doc/posix-functions/clock_getres.texi
+++ b/doc/posix-functions/clock_getres.texi
@@ -16,3 +16,6 @@ Portability problems not fixed by Gnulib:
 This function is missing on some platforms:
 Mac OS X 10.11, Minix 3.1.8, mingw, MSVC 14.
 @end itemize
+
+The Gnulib module @code{gettime-res} is a partial substitute; it implements
+the @code{CLOCK_REALTIME} functionality of @code{clock_getres}.
diff --git a/lib/gettime-res.c b/lib/gettime-res.c
new file mode 100644
index 000000000..0d7f1cfea
--- /dev/null
+++ b/lib/gettime-res.c
@@ -0,0 +1,78 @@
+/* Get the system clock resolution.
+
+   Copyright 2021 Free Software Foundation, Inc.
+
+   This file 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 3 of the
+   License, or (at your option) any later version.
+
+   This file 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 this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+#include <config.h>
+
+#include "timespec.h"
+
+static long int
+gcd (long int a, long int b)
+{
+  while (b != 0)
+    {
+      long int r = a % b;
+      a = b;
+      b = r;
+    }
+  return a;
+}
+
+/* Return the system time resolution in nanoseconds.  */
+
+long int
+gettime_res (void)
+{
+  struct timespec res;
+#if defined CLOCK_REALTIME && HAVE_CLOCK_GETRES
+  clock_getres (CLOCK_REALTIME, &res);
+#elif defined HAVE_TIMESPEC_GETRES
+  timespec_getres (&res, TIME_UTC);
+#else
+  /* Guess high and let the later code deduce better.  */
+  res.tv_sec = 1;
+  res.tv_nsec = 0;
+#endif
+
+  /* On all Gnulib platforms the following calculations do not overflow.  */
+
+  long int hz = TIMESPEC_HZ;
+  long int r = hz * res.tv_sec + res.tv_nsec;
+
+  /* On some platforms, clock_getres (CLOCK_REALTIME, ...) yields a
+     too-large resolution, under the mistaken theory that it should
+     return the timer interval.  For example, on AIX 7.2 POWER8
+     clock_getres yields 1 cs even though clock_gettime yields 1 µs
+     resolution.  Work around the problem with high probability by
+     trying clock_gettime several times and observing the resulting
+     bounds on resolution.  */
+  for (int i = 0; 1 < r && i < 32; i++)
+    {
+      struct timespec now = current_timespec ();
+      r = gcd (r, now.tv_nsec ? now.tv_nsec : hz * now.tv_sec);
+    }
+
+  return r;
+}
+
+/*
+ * Hey Emacs!
+ * Local Variables:
+ * coding: utf-8
+ * End:
+ */
diff --git a/lib/timespec.h b/lib/timespec.h
index 94a5db751..68da20268 100644
--- a/lib/timespec.h
+++ b/lib/timespec.h
@@ -88,6 +88,7 @@ timespectod (struct timespec a)
   return a.tv_sec + a.tv_nsec / 1e9;
 }
 
+long int gettime_res (void);
 struct timespec current_timespec (void);
 void gettime (struct timespec *) _GL_ARG_NONNULL ((1));
 int settime (struct timespec const *) _GL_ARG_NONNULL ((1));
diff --git a/m4/clock_time.m4 b/m4/clock_time.m4
index c3b9e7ea4..543099c98 100644
--- a/m4/clock_time.m4
+++ b/m4/clock_time.m4
@@ -1,10 +1,11 @@
-# clock_time.m4 serial 10
+# clock_time.m4 serial 11
 dnl Copyright (C) 2002-2006, 2009-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
 
-# Check for clock_gettime and clock_settime, and set LIB_CLOCK_GETTIME.
+# Check for clock_getres, clock_gettime and clock_settime,
+# and set LIB_CLOCK_GETTIME.
 # For a program named, say foo, you should add a line like the following
 # in the corresponding Makefile.am file:
 # foo_LDADD = $(LDADD) $(LIB_CLOCK_GETTIME)
@@ -26,6 +27,6 @@ AC_DEFUN([gl_CLOCK_TIME],
     AC_SEARCH_LIBS([clock_gettime], [rt posix4],
                    [test "$ac_cv_search_clock_gettime" = "none required" ||
                     LIB_CLOCK_GETTIME=$ac_cv_search_clock_gettime])
-    AC_CHECK_FUNCS([clock_gettime clock_settime])
+    AC_CHECK_FUNCS([clock_getres clock_gettime clock_settime])
   LIBS=$gl_saved_libs
 ])
diff --git a/m4/gettime.m4 b/m4/gettime.m4
index cb58bc8eb..43447f28e 100644
--- a/m4/gettime.m4
+++ b/m4/gettime.m4
@@ -1,4 +1,4 @@
-# gettime.m4 serial 10
+# gettime.m4 serial 11
 dnl Copyright (C) 2002, 2004-2006, 2009-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -11,3 +11,11 @@ AC_DEFUN([gl_GETTIME],
   AC_REQUIRE([gl_TIMESPEC])
   AC_CHECK_FUNCS_ONCE([timespec_get])
 ])
+
+AC_DEFUN([gl_GETTIME_RES],
+[
+  dnl Prerequisites of lib/gettime-res.c.
+  AC_REQUIRE([gl_CLOCK_TIME])
+  AC_REQUIRE([gl_TIMESPEC])
+  AC_CHECK_FUNCS_ONCE([timespec_getres])
+])
diff --git a/modules/gettime-res b/modules/gettime-res
new file mode 100644
index 000000000..90f3e81d5
--- /dev/null
+++ b/modules/gettime-res
@@ -0,0 +1,30 @@
+Description:
+Return current time resolution
+
+Files:
+lib/gettime-res.c
+m4/gettime.m4
+
+Depends-on:
+clock-time
+gettime
+timespec
+extensions
+
+configure.ac:
+gl_GETTIME_RES
+
+Makefile.am:
+lib_SOURCES += gettime-res.c
+
+Include:
+"timespec.h"
+
+Link:
+$(LIB_CLOCK_GETTIME)
+
+License:
+LGPL
+
+Maintainer:
+Paul Eggert, Jim Meyering
-- 
2.33.1