libs/libc/stdio: Fix the %f floating point output format.

libs/libc/stdio/Kconfig

@@ -121,9 +121,8 @@ config NANO_PRINTF
 	depends on !LIBC_LONG_LONG
 	---help---
 		Replace printf code with version from newlib-nano. This version
-		provides improved floating point output support, including 'g' mode
-		as well as making the default 'f' format include digits past the
-		decimal point. However, it does not include 'long long' support.
+		provides adds support for the 'g' format.  However, it does not
+		include 'long long' support.
 
 config NANO_PRINTLEVEL
 	int "Nano printf support level"

libs/libc/stdio/lib_libdtoa.c

@@ -46,6 +46,7 @@
 
 #include <nuttx/config.h>
 
+#include <stdbool.h>
 #include <math.h>
 #include <assert.h>
 
@@ -67,6 +68,17 @@
 #  define MAX(a,b) (a > b ? a : b)
 #endif
 
+/* Use (almost) the maximim precision with %f format if no precision is
+ * specified.  We do not use the full precision beause the least significant
+ * digits are probably garbage.
+ *
+ * REVISIT:  This should be smarter.  15 digits is the maximum size of the
+ * number.  The maximum precision is really 15 minus the number of digits
+ * in the integer part.
+ */
+
+#define DOUBLE_PRECISON_MAX 13 /* vs 15 which is the maximum */
+
 /****************************************************************************
  * Private Functions
  ****************************************************************************/
@@ -132,6 +144,7 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
 {
   FAR char *digits;     /* String returned by __dtoa */
   FAR char *rve;        /* Points to the end of the return value */
+  bool hasdot;          /* True: precision specified */
   int  expt;            /* Integer value of exponent */
   int  numlen;          /* Actual number of digits returned by cvt */
   int  nchars;          /* Number of characters to print */
@@ -149,6 +162,14 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
   DEBUGASSERT(up_interrupt_context() == false);
 #endif
 
+  /* Set to default precision if none specified */
+
+  hasdot = IS_HASDOT(flags);
+  if (!hasdot && prec == 0)
+    {
+      prec = DOUBLE_PRECISON_MAX;
+    }
+
   /* Special handling for NaN and Infinity */
 
   if (isnan(value))
@@ -178,8 +199,8 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
 
   /* Perform the conversion */
 
-  digits   = __dtoa(value, 3, prec, &expt, &dsgn, &rve);
-  numlen   = rve - digits;
+  digits = __dtoa(value, 3, prec, &expt, &dsgn, &rve);
+  numlen = rve - digits;
 
   /* Avoid precision error from missing trailing zeroes */
 
@@ -198,7 +219,7 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
    * the print precision.
    */
 
-  if (value == 0 || expt < -prec)
+  if (value == 0 || (expt < (hasdot ? -prec : 0)))
     {
       /* kludge for __dtoa irregularity */
 
@@ -208,13 +229,20 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
        * particular precision is requested.
        */
 
-      if (prec > 0 || IS_ALTFORM(flags))
+      if ((prec > 0 && hasdot) || IS_ALTFORM(flags))
         {
           obj->put(obj, '.');
 
           /* Always print at least one digit to the right of the decimal point. */
 
-          prec = MAX(1, prec);
+          if (hasdot)
+            {
+              prec = MAX(1, prec);
+            }
+          else
+            {
+              prec = MAX(1, numlen);
+            }
         }
     }
 
@@ -222,7 +250,6 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
 
   else
     {
-
       /* Handle the case where the value is less than 1.0 (in magnitude) and
        * will need a leading zero.
        */
@@ -239,7 +266,7 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
 
           /* Print any leading zeros to the right of the decimal point */
 
-          if (expt < 0)
+          if (expt < 0 || hasdot)
             {
               nchars = MIN(-expt, prec);
               zeroes(obj, nchars);
@@ -277,7 +304,8 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
            * requested.
            */
 
-          if (numlen > 0 || prec > 0 || IS_ALTFORM(flags))
+          if (numlen > 0 || (prec > 0 && hasdot) ||
+              IS_ALTFORM(flags))
             {
               /* Print the decimal point */
 
@@ -287,7 +315,14 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
                * point.
                */
 
-              prec = MAX(1, prec);
+              if (hasdot)
+                {
+                  prec = MAX(1, prec);
+                }
+              else
+                {
+                  prec = MAX(1, numlen);
+                }
             }
         }
 
@@ -319,7 +354,10 @@ static void lib_dtoa(FAR struct lib_outstream_s *obj, int fmt, int prec,
 
   /* Finally, print any trailing zeroes */
 
-  zeroes(obj, prec);
+  if (hasdot)
+    {
+      zeroes(obj, prec);
+    }
 }
 
 /****************************************************************************

libs/libc/stdio/lib_libvsprintf.c

@@ -118,10 +118,6 @@
 #  define FMT_PREV     src--                        /* Backup to the previous character */
 #endif
 
-/* Default precision to use with %f format if no precision is specified. */
-
-#define FLOAT_PRECISION_DEFAULT 6
-
 /****************************************************************************
  * Private Type Declarations
  ****************************************************************************/
@@ -1559,13 +1555,6 @@ int lib_vsprintf(FAR struct lib_outstream_s *obj, FAR const IPTR char *src,
           double dblval = va_arg(ap, double);
           int dblsize;
 
-          /* Set to default precision if none specified */
-
-          if (!IS_HASDOT(flags) && trunc == 0)
-            {
-              trunc = FLOAT_PRECISION_DEFAULT;
-            }
-
           /* Get the width of the output */
 
           dblsize = getdblsize(FMT_CHAR, trunc, flags, dblval);