HP Fortran Programmer's Guide (September 2007)

Calling C routines from HP Fortran
File handling
Chapter 8 193
File handling
A Fortran unit number cannot be passed to a C routine to perform I/O on the associated file;
nor can a C file pointer be used by a Fortran routine. However, a file created by a program
written in either language can be used by a program in the other language if the file is
declared and opened within the program that uses it.
C accesses files using HP-UX I/O subroutines and intrinsics. This method of file access can
also be used from Fortran instead of Fortran I/O.
You can pass file units and file pointers from Fortran to C with the FNUM and FSTREAM
intrinsics. FNUM returns the HP-UX file descriptor corresponding to a Fortran unit, which
must be supplied as an argument; see “Establishing a connection to a file” on page 171 for
information about file descriptors. FSTREAM returns C's file pointer for a Fortran unit number,
which must also be supplied as an argument.
The following Fortran program calls the write system routine to perform I/O on a file, passing
in a file descriptor returned by FNUM. (Because of the name conflict between the write system
routine and the Fortran WRITE statement, the program uses the ALIAS directive to avoid the
conflict by referring to write as IWRITE.)
Example 8-9 fnum_test.f90
PROGRAM fnum_test
! Use the ALIAS directive to rename the "write" system routine.
! The built-in functions %VAL and %REF indicate how the
! arguments are to be passed.
!$HP$ ALIAS IWRITE = 'write' (%VAL, %REF, %VAL)
CHARACTER*1 :: a(10)
INTEGER :: i, fd, status
! fill the array with x's
a = 'x'
! open the file for writing
OPEN(1, FILE='file1', STATUS='UNKNOWN')
! pass in the unit number and get back a file descriptor
fd = FNUM(1)
! call IWRITE (the alias for the "write" system routine),
! passing in three arguments:
! fd = the file descriptor returned by FNUM