Function skip

src/od.c:1127–1230

   advance IN_STREAM.  */

static bool
skip (intmax_t n_skip)
{
  bool ok = true;
  int in_errno = 0;

  if (n_skip == 0)
    return true;

  while (in_stream != NULL)	/* EOF.  */
    {
      struct stat file_stats;

      /* First try seeking.  For large offsets, this extra work is
         worthwhile.  If the offset is below some threshold it may be
         more efficient to move the pointer by reading.  There are two
         issues when trying to seek:
           - the file must be seekable.
           - before seeking to the specified position, make sure
             that the new position is in the current file.
             Try to do that by getting file's size using fstat.
             But that will work only for regular files.  */

      if (fstat (fileno (in_stream), &file_stats) == 0)
        {
          /* The st_size field is valid for regular files.
             If the number of bytes left to skip is larger than
             the size of the current file, we can decrement n_skip
             and go on to the next file.  Skip this optimization also
             when st_size is no greater than the block size, because
             some kernels report nonsense small file sizes for
             proc-like file systems.  */
          if (S_ISREG (file_stats.st_mode)
              && STP_BLKSIZE (&file_stats) < file_stats.st_size)
            {
              if (file_stats.st_size < n_skip)
                n_skip -= file_stats.st_size;
              else
                {
                  if (fseeko (in_stream, n_skip, SEEK_CUR) != 0)
                    {
                      in_errno = errno;
                      ok = false;
                    }
                  n_skip = 0;
                }
            }

          else if (! (S_ISREG (file_stats.st_mode)
                      || S_TYPEISSHM (&file_stats)
                      || S_TYPEISTMO (&file_stats))
                   && fseeko (in_stream, n_skip, SEEK_CUR) == 0)
            n_skip = 0;

          /* If it's neither a seekable file with unusable size, nor a
             regular file so large it can't be in a proc-like file system,
             position the file pointer by reading.  */