dos2unix modifies binary files - why

Question

By default it is not supposed to affect binary files.

I tested it in a folder with images and although most images were not affected, a few were. If dos2unix cannot tell a binary file from a text file, must I resort to specifically including and/or excluding certain file extensions for it to work properly?

NOTE: when I run file image.jpg on any of the jpgs, whether it got modified or not, the result is:

JPEG image data, JFIF standard 1.01

Answer 1

This is a relevant part of the source code of dos2unix program:

if ((ipFlag->Force == 0) &&
      (TempChar < 32) &&
      (TempChar != 0x0a) &&  /* Not an LF */
      (TempChar != 0x0d) &&  /* Not a CR */
      (TempChar != 0x09) &&  /* Not a TAB */
      (TempChar != 0x0c)) {  /* Not a form feed */
        RetVal = -1; 
        ipFlag->status |= BINARY_FILE ;
        if (ipFlag->verbose) {
          if ((ipFlag->stdio_mode) && (!ipFlag->error)) ipFlag->error = 1;
          d2u_fprintf(stderr, "%s: ", progname);
          d2u_fprintf(stderr, _("Binary symbol 0x00%02X found at line %u\n"),TempChar, line_nr);
        }
        break;
      } 

It seems that if the file has other control character it is considered as a binary file and is skipped, otherwise it is processed as a text file. So if the binary file (e.g. an image) doesn't contain these characters, it will be corrupted.