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• 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.

• C# File.Move creates an empty file and IO exception

  30 novembre 2020, par Maddie

  My code processes a video file (using ffmpeg) and creates different qualities (360p, 480p, etc) and formats (mp4 and HLS) of that. After creating these files, I move all of them to another drive (a network location).

  


  my code looks Like this :

  


  var files = Directory.GetFiles(srcFolder);
string filename, destFile = string.Empty, srcFile = string.Empty;
try
{
    for (int i = 0; i < files.Length; i++)
    {
        srcFile = files[i];
        filename = Path.GetFileName(srcFile);
        destFile = Path.Combine(destFolder, filename);
        File.Move(srcFile, destFile);
    }
}
catch
{
    _logger.LogError("Error in moving file. srcFile: {0}, destFile: {1}", destFile, srcFile);
    throw;
}


  


  This process works fine most of the time, but for some files, I get an IO exception every time I run this process.

  


  


  System.IO.IOException : The file exists. at System.IO.FileSystem.MoveFile(String sourceFullPath, String destFullPath, Boolean overwrite)

  


  


  I made sure that destFolder does not exist before, nor did the destFile.

  


  After logging the error and finding the path of source and target files, I downloaded both of them. The source file is a .ts file with a size of 1,048 KB, and the target file is an empty file with the same name (0KB).

  


  This error happened multiple times with the same video, so I assume that it has something to do with the file itself. But I cannot figure it out.

  


• youtube-dl (ffmpeg) blank error on embedding subtitles

  22 mars 2021, par Human1

  I am new to using youtube-dl, and therefore is quite unfamiliar with how it works. I configured it to download auto subs and english subs (if there were any), and embed them in the video. It worked for me for quite a good while (1 or 2 months), and it suddenly stopped working with blank errors starting to pop up, and I have no idea why that happened. I have checked that I am using the most updated version of youtube-dl and ffmpeg. I am currently using a m1 mac , and I don't know if that caused the problem. Below is the output I got with the -v command, as well as some background information.

  


  [debug] System config: []
[debug] User config: ['-o', '~/Documents/playlists/%(playlist)s/%(playlist_index)-i%(title)s.%(ext)s', '-f', 'mp4', '--write-sub', '--sub-lang', 'en', '--write-auto-sub', '--embed-subs']
[debug] Custom config: []
[debug] Command-line args: ['https://youtube.com/playlist?list=PL4cUxeGkcC9i3FXJSUfmsNOx8E7u6UuhG', '-v']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, pref UTF-8
[debug] youtube-dl version 2021.03.14
[debug] Git HEAD: 59ada80ca
[debug] Python version 3.9.2 (CPython) - macOS-11.2.3-arm64-arm-64bit
[debug] exe versions: ffmpeg present, ffprobe present, rtmpdump 2.4
[debug] Proxy map: {}
[youtube:tab] PL4cUxeGkcC9i3FXJSUfmsNOx8E7u6UuhG: Downloading webpage
[download] Downloading playlist: CSS Flexbox Tutorial
[youtube:tab] playlist CSS Flexbox Tutorial: Downloading 12 videos
[download] Downloading video 1 of 12
[youtube] Y8zMYaD1bz0: Downloading webpage
[info] Writing video subtitles to: /Users/{username}/Documents/playlists/CSS Flexbox Tutorial/1CSS Flexbox Tutorial #1 - Introduction.en.vtt
[debug] Invoking downloader on 'https://r2---sn-45gpjx-3x4e.googlevideo.com/videoplayback?expire=1616392628&ei=VN1XYKikA9eo1gai6KegBg&ip=45.133.192.196&id=o-APtlxrUI2wU4tPFiSxBi5MzLnuR5EFYL3-TE-BpiPEYG&itag=22&source=youtube&requiressl=yes&mh=U3&mm=31%2C29&mn=sn-45gpjx-3x4e%2Csn-5hne6nlr&ms=au%2Crdu&mv=m&mvi=2&pl=24&initcwndbps=1155000&vprv=1&mime=video%2Fmp4&ns=r0wuXA2lBVMGhop9uJeEHHUF&ratebypass=yes&dur=317.161&lmt=1607169749865434&mt=1616370759&fvip=3&fexp=24001373%2C24007246&c=WEB&txp=5432434&n=_jK4PI5-7sa9Q0X2ye&sparams=expire%2Cei%2Cip%2Cid%2Citag%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cns%2Cratebypass%2Cdur%2Clmt&sig=AOq0QJ8wRAIgR0wbVc18mWDheT_QVT5Jsbmd647wKJNyVfzGRJd_oOkCIArV-vvKQmD3D7W90dw1l9b3zaJQNzltitp3BJPt2fqA&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRgIhAI9I7LQw5zTAldgCydhe3yzicBgvxYfxeHxI8lCxh3sLAiEAlT8Dpf93fiPbdMI3rHOebH-nFkToCZr1VfWaoX0HVeg%3D'
[download] /Users/{username}/Documents/playlists/CSS Flexbox Tutorial/1CSS Flexbox Tutorial #1 - Introduction.mp4 has already been downloaded
[download] 100% of 16.50MiB
[ffmpeg] Embedding subtitles in '/Users/{username}/Documents/playlists/CSS Flexbox Tutorial/1CSS Flexbox Tutorial #1 - Introduction.mp4'
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:/Users/{username}/Documents/playlists/CSS Flexbox Tutorial/1CSS Flexbox Tutorial #1 - Introduction.mp4' -i 'file:/Users/{username}/Documents/playlists/CSS Flexbox Tutorial/1CSS Flexbox Tutorial #1 - Introduction.en.vtt' -map 0 -c copy -map -0:s -map -0:d -c:s mov_text -map 1:0 -metadata:s:s:0 language=eng 'file:/Users/{username}/Documents/playlists/CSS Flexbox Tutorial/1CSS Flexbox Tutorial #1 - Introduction.temp.mp4'
ERROR: 
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/youtube-dl/2021.3.14/libexec/lib/python3.9/site-packages/youtube_dl/YoutubeDL.py", line 2106, in post_process
    files_to_delete, info = pp.run(info)
  File "/opt/homebrew/Cellar/youtube-dl/2021.3.14/libexec/lib/python3.9/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 426, in run
    self.run_ffmpeg_multiple_files(input_files, temp_filename, opts)
  File "/opt/homebrew/Cellar/youtube-dl/2021.3.14/libexec/lib/python3.9/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 235, in run_ffmpeg_multiple_files
    raise FFmpegPostProcessorError(msg)
youtube_dl.postprocessor.ffmpeg.FFmpegPostProcessorError


  


  I would be very grateful if anyone can tell me what happened and what should I do next. Thanks ! ;)