docs: def pid_max(self):
          """
          Get the maximum PID value.
  
          On Linux, the value is read from the `/proc/sys/kernel/pid_max` file.
  
          From `man 5 proc`:
          The default value for this file, 32768, results in the same range of
          PIDs as on earlier kernels. On 32-bit platfroms, 32768 is the maximum
          value for pid_max. On 64-bit systems, pid_max can be set to any value
          up to 2^22 (PID_MAX_LIMIT, approximately 4 million).
  
          If the file is unreadable or not available for whatever reason,
          returns None.
  
          Some other OSes:
          - On FreeBSD and macOS the maximum is 99999.
          - On OpenBSD >= 6.0 the maximum is 99999 (was 32766).
          - On NetBSD the maximum is 30000.
  
          :returns: int or None
          """
          if LINUX:
              # XXX: waiting for https://github.com/giampaolo/psutil/issues/720
              try:
                  with open('/proc/sys/kernel/pid_max', 'rb') as f:
                      return int(f.read())
              except (OSError, IOError):
                  return None
          else:
              return None