To specify a virtual file handler, use the /ap (application program) option with the /f option in the database reference file to specify the program to which I/O requests for a particular data file are passed by the Fileshare Server.
fs /d dbase.ref /f user1.dat /ap myprog
This adds entries to the database reference file, dbase.ref which specify that the Fileshare Server should pass all I/O requests for the data file user1.dat to the program myprog.
On UNIX, your program should be compiled to intermediate or generated code, or to a callable shared object, and must be callable from the Fileshare process.
You also need to use the fcd3 copybook instead of the standard fcd layout.