HP Fortran Programmer Guide (766160-001, March 2014)

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 FNUMand FSTREAMintrinsics.
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” (page 108)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 writesystem 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 WRITEstatement, the program uses theALIASdirective to avoid the conflict by
referring to write as IWRITE.)
Example 29 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
! a = the character array to write
! 10 = the number of elements (bytes) to write
! the return value, status, is the number of bytes actually
written; if the write was successful, it should be 10
status=IWRITE(fd, a, 10)
CLOSE (1, STATUS=’KEEP’)
! open the file for reading; we want to see if the write was
! successful
OPEN (1, FILE='file1', STATUS='UNKNOWN')
READ (1, 4) (a(i), i = 1, 10)
4 FORMAT (10A1)
CLOSE (1, STATUS='DELETE')
DO i = 1, 10
! if we find anything other than x's, the write failed
IF (a(i) .NE. 'x') STOP 'FNUM_TEST failed'
END DO
! check write's return value; it should be 10
IF (status .EQ. 10) PRINT *, 'FNUM_TEST passed'
END
Below are the command lines to compile, link, and execute the program, followed by the output
from a sample run.
$ f90 fnum_test.f90
$ a.out
FNUM_TEST passed
The HP Fortran Programmer's Referencedescribes theFNUM and FNUMintrinsics and the
ALIASdirective. For information about the write system routine, see the write((2))man page.
File handling 121