json_object: Introduce json_object_new_string_noalloc()

On some low memory systems, calling malloc (by the use of strdup)
may cause memory fragmentation, which eventually results in out of
memory situations.

Using the json_object_new_string_noalloc() function can be a help
to solve such issues.

json_object.c

@@ -1158,6 +1158,14 @@ struct json_object *json_object_new_string_ext(const char *s, const int len,
 		dstbuf = jso->o.c_string.str.data;
 		memcpy(dstbuf, s, jso->o.c_string.len);
 		dstbuf[len] = '\0';
+		if (flags & JSON_C_NEW_STRING_NO_STRDUP)
+		{
+			/* a little hack to get rid of constness */
+			memcpy(&dstbuf, &s, sizeof(char *));
+			/* string buffer must be free'd here
+			   to avoid memory leaks */
+			free(dstbuf);
+		}
 		return jso;
 	}
 
@@ -1172,6 +1180,9 @@ struct json_object *json_object_new_string_ext(const char *s, const int len,
 			dstbuf[len] = '\0';
 		}
 	}
+	else if (flags & JSON_C_NEW_STRING_NO_STRDUP)
+		/* a little hack to get rid of constness */
+		memcpy(&dstbuf, &s, sizeof(char *));
 
 	/* cannot be reached if o.c_string.str.data is used */
 	if (!dstbuf)
@@ -1195,6 +1206,11 @@ struct json_object *json_object_new_string_len(const char *s, const int len)
 	return json_object_new_string_ext(s, len, JSON_C_NEW_STRING_LENGTH);
 }
 
+struct json_object *json_object_new_string_noalloc(const char *s)
+{
+	return json_object_new_string_ext(s, strlen(s), JSON_C_NEW_STRING_NO_STRDUP);
+}
+
 const char *json_object_get_string(struct json_object *jso)
 {
 	if (!jso)

json_object.h

@@ -54,6 +54,11 @@ extern "C" {
  * memory is managed by the json_object.
  */
 #define JSON_C_NEW_STRING_LENGTH (1 << 1)
+/**
+ * No copy of the string is made, the given string buffer must be heap-allocated,
+ * and not be free'd, as its memory will be managed by the json_object then.
+ */
+#define JSON_C_NEW_STRING_NO_STRDUP (1 << 2)
 
 /**
  * A flag for the json_object_to_json_string_ext() and
@@ -913,6 +918,21 @@ JSON_EXPORT struct json_object *json_object_new_string(const char *s);
  */
 JSON_EXPORT struct json_object *json_object_new_string_len(const char *s, const int len);
 
+/** Create a new empty json_object of type json_type_string without allocating
+ * new memory.
+ *
+ * Create a new json_object of type json_type_string and use the given string
+ * buffer as is, without allocating new memory nor duplication. The ownership
+ * of the string buffer will be transferred to the newly created json_object.
+ * The buffer will be freed when the last instance of this object will be
+ * destroyed by calling json_object_put().
+ *
+ * @param s the string buffer, must be heap alloacted and must not be freed.
+ * @returns a json_object of type json_type_string
+ * @see json_object_new_string_ext()
+ */
+JSON_EXPORT struct json_object *json_object_new_string_noalloc(const char *s);
+
 /** Get the string value of a json_object
  *
  * If the passed object is of type json_type_null (i.e. obj == NULL),