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

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