Add CONFIG_CLOCK_MONTONIC

ChangeLog

@@ -7095,3 +7095,6 @@
 	* libc/stdio/lib_ftell.c:  Fix a logic error in ftell().  It was
 	  simply using the file offset and did not take into account data
 	  buffered in memory.  From Macs N (2013-3-31).
+	* Add CONFIG_CLOCK_MONOTONIC that case used to disable CLOCK_MONOTONIC
+	  for a smaller footprint (2013-3-31).
+

include/time.h

@@ -99,7 +99,9 @@
  * system time-of-day clock.
  */
 
-#define CLOCK_MONOTONIC    2
+#ifdef CLOCK_MONOTONIC
+#  define CLOCK_MONOTONIC  2
+#endif
 
 /* This is a flag that may be passed to the timer_settime() function */
 

sched/Kconfig

@@ -53,6 +53,20 @@ config SYSTEM_TIME64
 		and/or if a very long "uptime" is required, then this option can be
 		selected to support a 64-bit wide timer.
 
+config CLOCK_MONOTONIC
+	bool "Support CLOCK_MONOTONIC"
+	default n
+	---help---
+		CLOCK_MONOTONIC is an optional standard POSIX clock.  Unlike
+		CLOCK_REALTIME which can move forward and backward when the
+		time-of-day changes, CLOCK_MONOTONIC is the elapsed time since some
+		arbitrary point in the post (the system start-up time for NuttX)
+		and, hence, is always monotonically increasing.  CLOCK_MONOTONIC
+		is, hence, the more appropriate clock for determining time
+		differences.
+
+		The value of the CLOCK_MONOTONIC clock cannot be set via clock_settime().
+
 config RR_INTERVAL
 	int "Round robin timeslice (MSEC)"
 	default 0

sched/clock_gettime.c

@@ -107,6 +107,7 @@ int clock_gettime(clockid_t clock_id, struct timespec *tp)
   sdbg("clock_id=%d\n", clock_id);
   DEBUGASSERT(tp != NULL);
 
+#ifdef CLOCK_MONOTONIC
   /* CLOCK_MONOTONIC is an optional under POSIX: "If the Monotonic Clock
    * option is supported, all implementations shall support a clock_id
    * of CLOCK_MONOTONIC defined in <time.h>. This clock represents the
@@ -139,6 +140,8 @@ int clock_gettime(clockid_t clock_id, struct timespec *tp)
       tp->tv_sec  = (time_t)secs;
       tp->tv_nsec = (long)nsecs;
     }
+  else
+#endif
 
   /* CLOCK_REALTIME - POSIX demands this to be present.  CLOCK_REALTIME
    * represents the machine's best-guess as to the current wall-clock,
@@ -152,9 +155,9 @@ int clock_gettime(clockid_t clock_id, struct timespec *tp)
    */
 
 #ifdef CONFIG_RTC
-  else if (clock_id == CLOCK_REALTIME || clock_id == CLOCK_ACTIVETIME)
+  if (clock_id == CLOCK_REALTIME || clock_id == CLOCK_ACTIVETIME)
 #else
-  else if (clock_id == CLOCK_REALTIME)
+  if (clock_id == CLOCK_REALTIME)
 #endif
     {
       /* Do we have a high-resolution RTC that can provide us with the time? */