Fope() in keil

I am trying to generate a log file by invoking the function "fopen()" in keil for samsung S3FS92G target though i m getting the result in serial window but finally needs to log in txt or xml file.
Even though I am not getting any compilation error (like fopen() missing) but the log file is not created at the expected system path.
any help on generating a txt file using fopen

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