[FFmpeg-devel] [PATCH] Notify user when encode a CBR mp3 file without "-write_xing false". This may result in mp3 file duration incorect on ios safari browser and webview. I try to fix it but it can’t be done as mp3 muxer don’t know it is a CBR file until encode process finished. And it’s hard to remove the first frame at that time. Only the mp3 encoder(such as libmp3lame) know it is CBR or not before encoding start.

[sharpbai at gmail.com]

From: sharpbai <sharpbai at gmail.com>

Bug example:

ffmpeg -i a.mp3 -c:a mp3 -ab 32k -ar 44100 -ac 1 b.mp3

The duration of the generated file b.mp3 is wrong on ios safari browser from ios7 to ios10.
---
 libavformat/mp3enc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libavformat/mp3enc.c b/libavformat/mp3enc.c
index 3cbf7bd..a55dbf4 100644
--- a/libavformat/mp3enc.c
+++ b/libavformat/mp3enc.c
@@ -395,8 +395,13 @@ static void mp3_update_xing(AVFormatContext *s)
     int i, rg_size;
 
     /* replace "Xing" identification string with "Info" for CBR files. */
-    if (!mp3->has_variable_bitrate)
+    if (!mp3->has_variable_bitrate) {
         AV_WL32(mp3->xing_frame + mp3->xing_offset, MKTAG('I', 'n', 'f', 'o'));
+        av_log(s, AV_LOG_WARNING,
+            "This is a CBR mp3 file. But its first frame header might be wrong, "
+            "which result in file duration incorrect on ios browser. "
+            "Please add \"-write_xing false\" to avoid this problem.\n");
+    }
 
     AV_WB32(mp3->xing_frame + mp3->xing_offset + 8,  mp3->frames);
     AV_WB32(mp3->xing_frame + mp3->xing_offset + 12, mp3->size);
-- 
2.2.1