diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 2c90e21ec9..63e1e83f4f 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -128,7 +128,8 @@ typedef struct AVFormatParameters {
     enum CodecID audio_codec_id;
 } AVFormatParameters;
 
-#define AVFMT_NOFILE        0x0001 /* no file should be opened */
+//! demuxer will use url_fopen, no opened file should be provided by the caller
+#define AVFMT_NOFILE        0x0001
 #define AVFMT_NEEDNUMBER    0x0002 /* needs '%d' in filename */
 #define AVFMT_SHOW_IDS      0x0008 /* show format stream IDs numbers */
 #define AVFMT_RAWPICTURE    0x0020 /* format wants AVPicture structure for
diff --git a/libavformat/utils.c b/libavformat/utils.c
index 07a483199d..7e25478cd3 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -270,6 +270,9 @@ int av_filename_number_test(const char *filename)
 
 /**
  * Guess file format.
+ *
+ * @param is_opened whether the file is already opened, determines whether
+ *                  demuxers with or without AVFMT_NOFILE are probed
  */
 AVInputFormat *av_probe_input_format(AVProbeData *pd, int is_opened)
 {