in reply to Creating log files for errors and warnings
Just to take a different approach, you could also do this on the command line without any changes to your code. Take that with a pinch of salt though as you didn't give us any code to go on.
If you want all output from your program to go into a file:./yourprog > combined_file.txt 2>&1
If you want regular data in one file, error data in another:./yourprog 1> regular_data.txt 2> error_data.txt