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• Of ctors and dtors

  18 février 2011, par Multimedia Mike — Programming, Sega Dreamcast

  I haven’t given up on the Sega Dreamcast programming. I was able to compile a bunch of homebrew code for the DC many years ago and I can’t make it work anymore. Again, I was working with a purpose-built, open source RTOS named KallistiOS (or KOS). I can make the programs compile but not run. I had ELF files left over from years ago which still executed. But when I tried to build new ELF files, no luck— the programs crashed before even reaching my main() function.

  I found the problem : ELF files are comprised of a number of sections and 2 of these sections are named ’.ctors’ and ’.dtors’ which stand for constructors and destructors. The KOS RTOS performs a manual traversal of .ctors section during program initialization and this is where things go bad. The traversal code doesn’t seem to account for a .ctors section that only contains a single entry. I commented out the function that does the traversal and programs started to work, at least until it was time to exit the program and return control to the program loader. That’s when the counterpart .dtors section traversal code ran and demonstrated the same problem. I’ll exhibit the problematic code at the end of this post.

  So I’m finally tinkering with Sega Dreamcast programming once again and with a slightly better grasp of software engineering than the first time I did this.

  Portable and Compatible C ?
  If nothing else, this low-level embedded stuff exposes you to some serious toolchain arcana, the likes of which you will likely never see working strictly in the desktop arena.

  Still, this exercise makes me wonder why C code from a decade ago doesn’t compile reliably now. Part of it is because gcc has gotten stricter about the syntax it will accept. In the case of this specific crashing problem, I suspect it comes down to a difference in the way the linker generates the final ELF file. I’ve written a list of items I have had to modify in the KOS codebase in order to get it to compile on more recent gcc versions. I wonder if it would be worth publishing the specifics, or if anyone would ever find the information useful ? Oh, who am I kidding ? Of course I’ll write it up, perhaps publish a new version of the code, if only because that’s the best chance I have of finding my own work again some years down the road.

  Problematic C Code
  See if this code makes any sense to you. It somehow traverse a list of 32-bit function pointers (in different directions, depending on constructors or destructors), executing each in turn. However, it appears to fall over if the list of pointers consists of a single entry.
  

  PLAIN TEXT

  C :

  1. typedef void (*fptr)(void) ;
  2.  
  3. static fptr ctor_list[1] __attribute__((section(".ctors"))) = { (fptr) -1 } ;
  4. static fptr dtor_list[1] __attribute__((section(".dtors"))) = { (fptr) -1 } ;
  5.  
  6. /* Call this to execute all ctors */
  7. void arch_ctors() {
  8.     fptr *fpp ;
  9.  
  10.     /* Run up to the end of the list (defined by crtend) */
  11.     for (fpp=ctor_list + 1 ; *fpp != 0 ; ++fpp)
  12.          ;
  13.  
  14.     /* Now run the ctors backwards */
  15.     while (—fpp> ctor_list)
  16.         (**fpp)() ;
  17. }
  18.  
  19. /* Call this to execute all dtors */
  20. void arch_dtors() {
  21.     fptr *fpp ;
  22.  
  23.     /* Do the dtors forwards */
  24.     for (fpp=dtor_list + 1 ; *fpp != 0 ; ++fpp )
  25.         (**fpp)() ;
  26. }

• MPEG-DASH - Multiplexed Representations Issue

  26 avril 2017, par Mike

  I’m trying to learn ffmpeg, MP4Box, and MPEG-DASH, but I’m running into an issue with the .mp4 I’m using. I’m using ffmpeg to demux the mp4 with this command :

  ffmpeg -i test.mp4 -c:v copy -g 72 -an video.mp4 -c:a copy audio.mp4

  Once the two files are created, I use MP4Box to segment the files for the dash player using this command :

  MP4Box -dash 4000 -frag 1000 -rap -segment-name segment_ output.mp4

  Which does create all the files I think I need. Then I point the player to the output_dash.mpd and nothing happens except a ton of messages in the console :

  [8] EME detected on this user agent! (ProtectionModel_21Jan2015) 
  
  [11] Playback Initialized 
  
  [21] [dash.js 2.3.0] MediaPlayer has been initialized 
  
  [64] Parsing complete: ( xml2json: 3.42ms, objectiron: 2.61ms, total: 0.00603s) 
  
  [65] Manifest has been refreshed at Wed Apr 12 2017 12:16:52 GMT-0600 (MDT)[1492021012.196]  
  
  [72] MediaSource attached to element.  Waiting on open... 
  
  [77] MediaSource is open! 
  
  [77] Duration successfully set to: 148.34 
  
  [78] Added 0 inline events 
  
  [78] No video data. 
  
  [79] No audio data. 
  
  [79] No text data. 
  
  [79] No fragmentedText data. 
  
  [79] No embeddedText data. 
  
  [80] Multiplexed representations are intentionally not supported, as they are not compliant with the DASH-AVC/264 guidelines 
  
  [81] No streams to play.

  Here is the MP4Box -info on the video I’m using :

  * Movie Info *
  
      Timescale 1000 - Duration 00:02:28.336
  
      Fragmented File no - 2 track(s)
  
      File suitable for progressive download (moov before mdat)
  
      File Brand mp42 - version 512
  
      Created: GMT Wed Feb  6 06:28:16 2036
  
  
  
  File has root IOD (9 bytes)
  
  Scene PL 0xff - Graphics PL 0xff - OD PL 0xff
  
  Visual PL: Not part of MPEG-4 Visual profiles (0xfe)
  
  Audio PL: Not part of MPEG-4 audio profiles (0xfe)
  
  No streams included in root OD
  
  
  
  iTunes Info:
  
      Name: Rogue One - A Star Wars Story
  
      Artist: Lucasfilm
  
      Genre: Trailer
  
      Created: 2016
  
      Encoder Software: HandBrake 0.10.2 2015060900
  
      Cover Art: JPEG File
  
  
  
  Track # 1 Info - TrackID 1 - TimeScale 90000 - Duration 00:02:28.335
  
  Media Info: Language "Undetermined" - Type "vide:avc1" - 3552 samples
  
  Visual Track layout: x=0 y=0 width=1920 height=816
  
  MPEG-4 Config: Visual Stream - ObjectTypeIndication 0x21
  
  AVC/H264 Video - Visual Size 1920 x 816
  
      AVC Info: 1 SPS - 1 PPS - Profile High @ Level 4.1
  
      NAL Unit length bits: 32
  
      Pixel Aspect Ratio 1:1 - Indicated track size 1920 x 816
  
  Self-synchronized
  
  
  
  Track # 2 Info - TrackID 2 - TimeScale 44100 - Duration 00:02:28.305
  
  Media Info: Language "English" - Type "soun:mp4a" - 6387 samples
  
  MPEG-4 Config: Audio Stream - ObjectTypeIndication 0x40
  
  MPEG-4 Audio MPEG-4 Audio AAC LC - 2 Channel(s) - SampleRate 44100
  
  Synchronized on stream 1
  
  Alternate Group ID 1

  I know I need to separate the video and audio and I think that’s where my issue is. The command I’m using probably isn’t doing the right thing.

  Is there a better command to demux my mp4 ?
  Is the MP4Box command I’m using best for segmenting the files ?
  If I use different files, will they always need to be demuxed ?

  One thing to mention, if I use the following commands everything works fine, but there is no audio because of the -an which means it’s only video :

  ffmpeg -i test.mp4 -c:v copy -g 72 -an output.mp4
  
  
  
  MP4Box -dash 4000 -frag 1000 -rap -segment-name segment_ output.mp4

  UPDATE

  I noticed that the video had no audio stream, but the audio had the video stream which is why I got the mux error. I thought that might be an issue so I ran this command to keep the unwanted streams out of the outputs :

  ffmpeg -i test.mp4 -c:v copy -g 72 -an video.mp4 -c:a copy -vn audio.mp4

  then I run :

  MP4Box -dash 4000 -frag 1000 -rap -segment-name segment_ video.mp4 audio.mp4

  now I no longer get the Multiplexed representations are intentionally not supported... message, but now I get :

  [122] Video Element Error: MEDIA_ERR_SRC_NOT_SUPPORTED 
  
  [123] [object MediaError] 
  
  [125] Schedule controller stopping for audio 
  
  [126] Caught pending play exception - continuing (NotSupportedError: Failed to load because no supported source was found.)

  I tried playing the video and audio independently through Chrome and they both work, just not through the dash player. Ugh, this is painful to learn, but I feel like I’m making progress.

• How to make HLS start from the begining

  15 septembre 2020, par Ariel Argañaraz

  I need some help with HLS streaming, I'm trying to create a playlist for streaming,

  



  I'm using ffmpeg to generate the files and the .m3u8 files. And for playing i'm using a web page with videojs player

  



  The idea is to simulate a live streaming using files already created.

  



  The problem is that in some version of iphone works and in another does not work.
The problem is that when I start play the video in the Safari browser of my phone it freezes for a while and then download and play the last segment.

  



  for example if the video is split in 4 differents .ts files.
It starts playing from the 4th .ts file and then stop.

  



  The problem is that I have 2 iphone mobile phone, one of them works perfectly but the other one not

  



  The not working cell phone is a iphone 6 (version 9.2.1)

  



  Here is my m3u8 file. Note that I'm using the #EXT-X-PLAYLIST-TYPE:EVENT and I remove the #EXT-X-ENDLIST from the botton. So it should be played as an live streaming.

  



  It works for another phones, it begins from the 0 second when I set the #EXT-X-START:TIME-OFFSET=0

  



  but in this version (9.2.1) the video freezes and then jumps to the last segment (webinar-3.ts) plays that segment and finally stop.

  



  #EXTM3U
#EXT-X-VERSION:4
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-ALLOW-CACHE:NO
#EXT-X-PLAYLIST-TYPE:EVENT
#EXT-X-START:TIME-OFFSET=0
#EXT-X-TARGETDURATION:6
#EXTINF:5.046444,
webinar-0.ts
#EXTINF:5,
webinar-1.ts
#EXTINF:5,
webinar-2.ts
#EXTINF:5,
webinar-3.ts


  



  Have anybody got an idea of what i'm doing wrong or how to get a m3u8 config that works for this iphone version ?