fseek得到fileSize 和 fread得到filesize的区别

/*!
 ***********************************************************************
 * /brief
 *    allocates memory buf, opens file Filename in f, reads contents into
 *    buf and returns buf
 * /param Filename
 *    name of config file
 * /return
 *    if successfull, content of config file
 *    NULL in case of error. Error message will be set in errortext
 ***********************************************************************
 */
char *GetConfigFileContent (char *Filename)
{
  long FileSize;
  FILE *f;
  char *buf;

  if (NULL == (f = fopen (Filename, "r")))
  {
      snprintf (errortext, ET_SIZE, "Cannot open configuration file %s.", Filename);
      return NULL;
  }

  if (0 != fseek (f, 0, SEEK_END))
  {
    snprintf (errortext, ET_SIZE, "Cannot fseek in configuration file %s.", Filename);
    return NULL;
  }

  FileSize = ftell (f);
  if (FileSize < 0 || FileSize > 100000)
  {
    snprintf (errortext, ET_SIZE, "Unreasonable Filesize %ld reported by ftell for configuration file %s.", FileSize, Filename);
    return NULL;
  }
  if (0 != fseek (f, 0, SEEK_SET))
  {
    snprintf (errortext, ET_SIZE, "Cannot fseek in configuration file %s.", Filename);
    return NULL;
  }

  if ((buf = malloc (FileSize + 1))==NULL) no_mem_exit("GetConfigFileContent: buf");

  // Note that ftell() gives us the file size as the file system sees it.  The actual file size,
  // as reported by fread() below will be often smaller due to CR/LF to CR conversion and/or
  // control characters after the dos EOF marker in the file.

  FileSize = (long) fread (buf, 1, FileSize, f);
  buf[FileSize] = '/0';

  fclose (f);
  return buf;
}