Add method 'json_object_to_json_string_length'

This new method allows to also
get the length of the generated string.

Fix #165

Change-Id: Iea91404027f143ca3d29a4c58d7c07ae53556110
Signed-off-by: José Bollo <jose.bollo@iot.bzh>

json_object.c

@@ -295,20 +295,35 @@ void json_object_set_serializer(json_object *jso,
 
 /* extended conversion to string */
 
-const char* json_object_to_json_string_ext(struct json_object *jso, int flags)
+const char* json_object_to_json_string_length(struct json_object *jso, int flags, size_t *length)
 {
-	if (!jso)
-		return "null";
+	const char *r = NULL;
+	size_t s = 0;
 
-	if ((!jso->_pb) && !(jso->_pb = printbuf_new()))
-		return NULL;
+	if (!jso)
+	{
+		s = 4;
+		r = "null";
+	}
+	else if ((jso->_pb) || (jso->_pb = printbuf_new()))
+	{
+		printbuf_reset(jso->_pb);
 
-	printbuf_reset(jso->_pb);
+		if(jso->_to_json_string(jso, jso->_pb, 0, flags) >= 0)
+		{
+			s = (size_t)jso->_pb->bpos;
+			r = jso->_pb->buf;
+		}
+	}
 
-	if(jso->_to_json_string(jso, jso->_pb, 0, flags) < 0)
-		return NULL;
+	if (length)
+		*length = s;
+	return r;
+}
 
-	return jso->_pb->buf;
+const char* json_object_to_json_string_ext(struct json_object *jso, int flags)
+{
+	return json_object_to_json_string_length(jso, flags, NULL);
 }
 
 /* backwards-compatible conversion to string */

json_object.h

@@ -223,6 +223,16 @@ extern const char* json_object_to_json_string(struct json_object *obj);
 extern const char* json_object_to_json_string_ext(struct json_object *obj, int
 flags);
 
+/** Stringify object to json format
+ * @see json_object_to_json_string() for details on how to free string.
+ * @param obj the json_object instance
+ * @param flags formatting options, see JSON_C_TO_STRING_PRETTY and other constants
+ * @param length a pointer where, if not NULL, the length (without null) is stored
+ * @returns a string in JSON format and the length if not NULL
+ */
+extern const char* json_object_to_json_string_length(struct json_object *obj, int
+flags, size_t *length);
+
 /**
  * Returns the userdata set by json_object_set_userdata() or
  * json_object_set_serializer()