How to use posix_spawn in ROS and Qt?

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Hello

I would like to use the system call command posix_spawn in my Qt GUI. I would like to have button that start a bag file and a launch file (with different nodes) where can select which node is active and can run. So for example I have a start button that play the bag file and run the ROS node 1. So from that node extract some parameters and if the selected parameters is for example bigger or smaller than some values gives as an output some number.

I know that have to be something like this:

System call ("rosbag play bagfile.bag")
posix_spawn(..

I dont know how to implement system command posix_spawn in Qt ROS. Any help?