net: lwm2m: plain text: fix float formatting

Formatting a float32/64 value for plain text is broken.
Example for 32bit: val1=0 and val2=500000 is equivalent to 0.5

Current formatter was using %d.%d (%lld.%lld for 64bit) so
exported value was 0.500000 (or 0.5)

To fix this, for val2 use a zero-padded formatter for the maximum
length of each bit length (6 for 32bit and 9 for 64bit), and then
remove the zero characters at the end of the string.

Notes re: handling of val1/val2 signs:
- eliminate potential negative sign when converting val2 to avoid:
  a value like: 0.-5
- use negative val2 when val1 is 0 to fix small negative handling
  such as -0.5

Fixes: #16154

Signed-off-by: Michael Scott <[email protected]>

Author: mike-scott

subsys/net/lib/lwm2m/lwm2m_rw_plain_text.c

@@ -125,16 +125,50 @@ static size_t put_float32fix(struct lwm2m_output_context *out,
 			     struct lwm2m_obj_path *path,
 			     float32_value_t *value)
 {
-	return plain_text_put_format(out, "%d.%d",
-				     value->val1, value->val2);
+	size_t len;
+	char buf[sizeof("000000")];
+
+	/* value of 123 -> "000123" -- ignore sign */
+	len = snprintf(buf, sizeof(buf), "%06d", abs(value->val2));
+	if (len != 6U) {
+		strcpy(buf, "0");
+	} else {
+		/* clear ending zeroes, but leave 1 if needed */
+		while (len > 1U && buf[len - 1] == '0') {
+			buf[--len] = '\0';
+		}
+	}
+
+	return plain_text_put_format(out, "%s%d.%s",
+				     /* handle negative val2 when val1 is 0 */
+				     (value->val1 == 0 && value->val2 < 0) ?
+						"-" : "",
+				     value->val1, buf);
 }
 
 static size_t put_float64fix(struct lwm2m_output_context *out,
 			     struct lwm2m_obj_path *path,
 			     float64_value_t *value)
 {
-	return plain_text_put_format(out, "%lld.%lld",
-				     value->val1, value->val2);
+	size_t len;
+	char buf[sizeof("000000000")];
+
+	/* value of 123 -> "000000123" -- ignore sign */
+	len = snprintf(buf, sizeof(buf), "%09lld", abs(value->val2));
+	if (len != 9U) {
+		strcpy(buf, "0");
+	} else {
+		/* clear ending zeroes, but leave 1 if needed */
+		while (len > 1U && buf[len - 1] == '0') {
+			buf[--len] = '\0';
+		}
+	}
+
+	return plain_text_put_format(out, "%s%lld.%s",
+				     /* handle negative val2 when val1 is 0 */
+				     (value->val1 == 0 && value->val2 < 0) ?
+						"-" : "",
+				     value->val1, buf);
 }
 
 static size_t put_string(struct lwm2m_output_context *out,