Can't print in a file using "fprintf" when running the node through roslaunch

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Hello, I have a node that uses printf, File and fprintf functions. If I run it using rosrun it works perfectly:

1. it shows output on the screen
2. It creates a file and prints data inside it.

However, if I run it through roslaunch:

1. Nothing is showed in the screen
2. It creates a file. However, the file is always empty; nothing is stored inside it.

I used output=screen in roslaunch put still can't see anything on the screen and nothing is stored in the file.

Could you please help.