How do you choose which types of API's to use in COBOL program??

Hello everyone!
 
I have a question - I would like to know how to determine which api's to
use on the iSeries in COBOL?  HFS, IFS, etc?  I will be reading/writing
to application log files (stream) and I will need to create the file(s)
internally within the COBOL program - Also I will need to recognize the
delivery/existence of files.  All of this will be occurring in x time
intervals using the sleep API.  Currently I am testing with the API's in
the C++ manual - but the write is giving me strange results.  It does
write a record to the end of the file, but it is not the fixed record
size as in the rest of the file and it is garbage ridden beyond the end
of the record.  
 
Thanks,
 
Adrienne