nRF9160 How to store a text file, and read and write to it while running?

I'm trying to figure out how I can store a text file on the device and access it while the device is running.  I'm having a hard time finding documentation on the best way to go about this.  Are there libraries for reading/writing?  Or do I need to access flash directly? 

I noticed fopen is not available to me.  And if I need to go the flash route and accessing memory directly, I'm not sure what flash addresses are available to me for storing persistent items.

Any help is appreciated!