diff --git a/libavformat/avio.h b/libavformat/avio.h
index 9b9b65e218..cd40f78e5e 100644
--- a/libavformat/avio.h
+++ b/libavformat/avio.h
@@ -171,6 +171,7 @@ int avio_check(const char *url, int flags);
  * @param opaque An opaque pointer to user-specific data.
  * @param read_packet  A function for refilling the buffer, may be NULL.
  * @param write_packet A function for writing the buffer contents, may be NULL.
+ *        The function may not change the input buffers content.
  * @param seek A function for seeking to specified byte position, may be NULL.
  *
  * @return Allocated AVIOContext or NULL on failure.