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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def exec_command(self, cmd):
 """Executes the given command. This method executes a command by calling the DLL's exec method. Direct API methods should be prioritized over calling this method. Args: self (JLink):
 the ``JLink`` instance cmd (str):
 the command to run Returns: The return code of running the command. Raises: JLinkException: if the command is invalid or fails. See Also: For a full list of the supported commands, please see the SEGGER J-Link documentation, `UM08001 <https://www.segger.com/downloads/jlink>`__. """

        err_buf = (ctypes.c_char * self.MAX_BUF_SIZE)()
        res = self._dll.JLINKARM_ExecCommand(cmd.encode(), err_buf, self.MAX_BUF_SIZE)
        err_buf = ctypes.string_at(err_buf).decode()

        if len(err_buf) > 0:
            # This is how they check for error in the documentation, so check
            # this way as well.
            raise errors.JLinkException(err_buf.strip())

        return res