The USEROPEN specifier indicates a user-written external function that controls the opening of the file. It takes the following form:
The function must be declared in a previous EXTERNAL statement; if it is typed, it must be of type INTEGER(4) (INTEGER*4).
The USEROPEN specifier lets experienced users use additional features of the operating system that are not normally available in Fortran.
For More Information:
For details on user-supplied functions to use with USEROPEN, including examples, see your user manual.