HP Fortran Programmer's Guide (March 2010)

Calling C routines from HP Fortran
Argument-passing conventions
Chapter 8 195
Argument-passing conventions
The important difference between the argument-passing conventions of HP C and HP Fortran
is that Fortran passes arguments by reference — that is, it passes the address of the
argument — whereas C passes non-array and non-pointer arguments by value — that is, it
passes a copy of the argument. This difference affects calls not only to user-written routines
in C but also to all HP-UX system calls and subroutines, which are accessed as C functions.
HP Fortran provides two built-in functions, %VAL and %REF, to override Fortran’s default
argument-passing conventions to conform to C. These functions are applied to the actual
arguments you want to pass, either in the argument list of the routine you are calling or with
the $HP$ ALIAS directive. The %REF function tells Fortran that its argument is to be passed by
reference (as when passing an array or pointer in C), and the %VAL function tells Fortran that
its argument is to be passed by value (the default case in C).
Consider a C function having the following prototype declaration:
void foo(int *ptr, int iarray[100], int i);
In Fortran, the actual arguments to be passed to foo would be declared as follows:
INTEGER :: ptr, i
INTEGER, DIMENSION(100) :: iarray
The call from Fortran to the C function (using the %VAL and %REF built-in functions) would be
as follows:
CALL foo(%REF(ptr), %REF(iarray), %VAL(i))
If the Fortran program were to make numerous calls to foo at different call sites, you might
find it more convenient to use the $HP$ ALIAS directive with the %VAL and %REF built-in
functions. Using the $HP$ ALIAS directive allows you to establish the argument-passing
modes for each parameter in a particular routine once and for all, without having to use %VAL
and %REF at each call site. Here is the $HP$ ALIAS directive for the Fortran program that calls
foo:
!$HP$ ALIAS foo(%REF, %REF, %VAL)
Note that the functions are used here without arguments; their positions in the argument list
indicate the parameters to which each applies.
You can also use the $HP$ ALIAS directive to handle case-sensitivity difference between C and
HP Fortran; “Case sensitivity” on page 197, which includes an example program that uses the
$HP$ ALIAS directive and the %VAL and %REF built-in functions to call a C function. For other
examples, see “Complex numbers” on page 191 and “File handling” on page 207. Note that the
example Fortran program in “Arrays” on page 200 does not require the built-in functions
because both Fortran and C pass arrays by reference.