Recherche avancée

Sur d’autres sites (13795)

• Pointer peril

  18 octobre 2011, par Mans — Bugs, Optimisation

  Use of pointers in the C programming language is subject to a number of constraints, violation of which results in the dreaded undefined behaviour. If a situation with undefined behaviour occurs, anything is permitted to happen. The program may produce unexpected results, crash, or demons may fly out of the user’s nose.

  Some of these rules concern pointer arithmetic, addition and subtraction in which one or both operands are pointers. The C99 specification spells it out in section 6.5.6 :

  When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. […] If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow ; otherwise, the behavior is undefined. […]

  When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object ; the result is the difference of the subscripts of the two array elements.

  In simpler, if less accurate, terms, operands and results of pointer arithmetic must be within the same array object. If not, anything can happen.
  
  To see some of this undefined behaviour in action, consider the following example.

  #include <stdio.h>
  int foo(void)
  
  
  
      int a, b ;
  
      int d = &b - &a ; /* undefined */
  
      int *p = &a ;
  
      b = 0 ;
  
      p[d] = 1 ;        /* undefined */
  
      return b ;
  
  
  int main(void)
  
  
  
      printf("%d\n", foo()) ;
  
      return 0 ;
  
  
  

  This program breaks the above rules twice. Firstly, the &a - &b calculation is undefined because the pointers being subtracted do not point to elements of the same array. Most compilers will nonetheless evaluate this to the distance between the two variables on the stack. Secondly, accessing p[d] is undefined because p and p + d do not point to elements of the same array (unless the result of the first undefined expression happened to be zero).

  It might be tempting to assume that on a modern system with a single, flat address space, these operations would result in the intuitively obvious outcomes, ultimately setting b to the value 1 and returning this same value. However, undefined is undefined, and the compiler is free to do whatever it wants :

  $ gcc -O undef.c
  $ ./a.out
  0

  Even on a perfectly normal system, compiled with optimisation enabled the program behaves as though the write to p[d] were ignored. In fact, this is exactly what happened, as this test shows :

  $ gcc -O -fno-tree-pta undef.c
  $ ./a.out
  1

  Disabling the tree-pta optimisation in gcc gives us back the intuitive behaviour. PTA stands for points-to analysis, which means the compiler analyses which objects any pointers can validly access. In the example, the pointer p, having been set to &a cannot be used in a valid access to the variable b, a and b not being part of the same array. Between the assignment b = 0 and the return statement, no valid access to b takes place, whence the return value is derived to be zero. The entire function is, in fact, reduced to the assembly equivalent of a simple return 0 statement, all because we decided to violate a couple of language rules.

  While this example is obviously contrived for clarity, bugs rooted in these rules occur in real programs from time to time. My most recent encounter with one was in PARI/GP, where a somewhat more complicated incarnation of the example above can be found. Unfortunately, the maintainers of this program are not responsive to reports of such bad practices in their code :

  Undefined according to what rule ? The code is only requiring the adress space to be flat which is true on all supported platforms.

  The rule in question is, of course, the one quoted above. Since the standard makes no exception for flat address spaces, no such exception exists. Although the behaviour could be logically defined in this case, it is not, and all programs must still follow the rules. Filing bug reports against the compiler will not make them go away. As of this writing, the issue remains unresolved.

• Multimedia Exploration Journal : The Past Doesn’t Die

  12 juillet 2011, par Multimedia Mike — Game Hacking

  New haul of games, new (old) multimedia formats.

  Lords of Midnight
  Check out the box copy scan for Lords of Midnight in MobyGames. In particular, I’d like to call your attention to this little blurb :

  
  
  

  Ahem, "Journey through an immense world — the equivalent of 8 CD-ROMs." Yet, when I procured the game, it only came on a single CD-ROM. It’s definitely a CD-ROM (says so on the disc) and, coming from 1995, certainly predates the earliest DVD-ROMs (which can easily store 8 CD-ROMs on a disc). Thus, I wanted to jump in a see if they were using some phenomenal compression in order to squeeze so much info into 600 or so megabytes.

  I was surprised to see the contents of the disc clocking in at just under 40 megabytes. An intro movie and an outro movie account for 75% of that. Format ? None other than that curious ASCII anomaly, ARMovie/RPL with Escape 122 codec data.

  Cyclemania

  
  
  

  Cyclemania is one of those FMV backdrop action games, but with a motorcycle theme. I had a good feeling I would find some odd multimedia artifacts here and the game didn’t disappoint. The videos are apparently handled using 3-4 discrete files per animation. I’ve documented my cursory guesses and linked some samples at the new MultimediaWiki page.

  Interplay ACMP
  This is unrelated to this particular acquistion, but I was contacted today about audio files harvested from the 1993 DOS game Star Trek : Judgment Rites. The files begin with the ASCII signature "Interplay ACMP Data". This reminds me of Interplay MVE files which begin with the similar string "Interplay MVE File". My theory is that these files use the ACOMP compression format, though I’m still trying to make it fit.

  Wiki and samples are available as usual if you’d like to add your own research.

• Convert mp3 to AAC with mpeg-2 container (FFMPEG)

  18 mars 2016, par jsurf

  I’m trying to convert an mp3 audio file to an AAC file with FFMPEG, and I need the audio to be wrapped in an MPEG-2 container.
  The resulting AAC file needs to be AAC-LC (Low Complexity), 1-channel, CBR mode, 44100 sample rate, and 48kb/s bitrate, so I use this command :

  ffmpeg -y -i input.mp3 -ar 44100 -ab 48k -acodec libfdk_aac -ac 1 output.aac

  But when I examine the ADTS headers, the audio file is always being wrapped in an MPEG-4 container. I have tried all the codecs listed here but I still end up with an mpeg-4 container wrapped around the audio : http://trac.ffmpeg.org/wiki/AACEncodingGuide.

  Here are the headers I get when examining the AAC output file :

  mpeg_type : ’MPEG4’,
  profile : 2,
  profile_name : ’AAC LC’,
  sample_freq : 44100,
  channel_config : 1,
  channels : 1,
  frame_length : 139,
  buffer_fullness : 157,
  number_of_frames : 1,
  frames_per_sec : 43.06640625

  Any ideas as to why ffmpeg wraps an mp4 container around the audio ? Can I get around this somehow ? Are there any other encoders I can try aside from FFMPEG ? I was giving FAAC encoder a shot and it gives me the proper encoding and ADTS headers, but alas it does not support mp3, only WAV.