don't have the linux docs up north here;
from the ifort windows documentation installed with 12.1.1:
User-Supplied OPEN Procedures: USEROPEN Specifier
You can use the USEROPEN specifier in an
Intel Fortran OPEN statement to pass control to a routine that directly opens a
file. The called routine can use system calls or library routines to open the
file and establish special context that changes the effect of subsequent Intel
Fortran I/O statements.
The Intel Fortran RTL I/O
support routines call the USEROPEN function in place of the system calls usually
used when the file is first opened for I/O. The USEROPEN specifier in an OPEN
statement specifies the name of a function to receive control. The called
function must open the file (or pipe) and return the file descriptor of the file
when it returns control to the RTL.
When opening the file, the
called function usually specifies options different from those provided by a
normal OPEN statement.
You can obtain the file
descriptor from the Intel Fortran RTL for a specific unit number with the getfd routine.
