Save map before exiting launch file

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I have successfully implement it using some trick. I compiled a c program with the following fraction of codes

system(roslaunch slam_gmapping.launch &); //In this launch file, put slam_gmapping node so that required='true'

system("rosbag play bagfile.bag");

system("rosrun map_server map_saver");

system("rosnode kill slam_gmapping");

Firstly, the program launch slam_gmapping.launch. This launch file was set such that required='true' for slam_gmapping node. Then, play the rosbag. After the rosbag play finished, the map is saved (at this time, the slam_gmapping.launch is still running and the map will definitely be available). Then, the slam_gmapping node is killed and thus killing the launch file safely (since the required='true' is set to the node).