How to use txt file as a parameter to roslaunch file?

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Hello All,

Background:

My idea is to receive not constant number of topics, to which I want to subscribe in my C code. I decided to do it in the next way: I will prepare a text file with the number and the name of the topics ---> convert it using launch file syntax to a string ---> parse the string in my C code ---> decide to which topics to subscribe.

In such a way I will only have to change the text file, while the launch file and the C code will not be changed.

Question:

I have written the next line of code in my launch file:

<param name="topics_list" textfile="$(find package_name)/file_name.txt"/>

And I have placed my text file in the appropriate folder. In my C file I do the next:

if ( nh.getParam("topics_list", topicsList) )
{
   ROS_INFO ("The contents of the file \"file_name.txt\" as a string are: %s\n",   
   topicsList); 
}
else
{
   ROS_INFO ("The topics list was not loaded succesfully. Aborting...\n");
   ros::shutdown();
}

It doesn´t work.

1. Is my way correct?
2. Why the above doesn´t work properly?

Thank you all in advance, Felix.