HP Fortran Programmer's Guide (B3908-90031; September 2011)

Calling C routines from HP Fortran
C strings
Chapter 8196
Note that both C and Fortran both pass strings by reference. This means that, if Fortran passes only string
arguments to C, you need not use the %VAL and %REF built-in functions to indicate how the arguments are to
be passed. For information about these functions, see “Argument-passing conventions” on page 188.
Passing a string
The example program in this section illustrates how to pass a string—which, in Fortran, is a character
variable or constant—to a C function. It also illustrates how to process a C string so that it can be
manipulated in Fortran.
The program consists of two source files:
• The Fortran source file, which consists of a main program unit that declares two initialized character
variables and passes them to a C function.
• The C source code, which consists of two functions:
❏ get_string: receives the two character array arguments from Fortran and overwrites the strings
in the arrays with new strings
❏ fix_string_for_f90: processes the string in its character array argument to replace the
null-terminating character with a blank character and to blank-fill the remaining characters. This
processing is necessary so that Fortran can manipulate the character variable.
The get_string function has two additional arguments in its argument list, which pick up the hidden
string length arguments that Fortran implicitly passes with each string argument.
The following are example C and Fortran programs.
Example 8-7 pass_chars.f90
PROGRAM main
! This program passes to character variables to a C routine,
! which overwrites them. This program displays the
! character variables before and after the call.
! Initialize the character variables and append null
! characters so that C can process them.
CHARACTER(LEN=10) :: first_name = "Pete"//CHAR(0)
CHARACTER(LEN=15) :: last_name = "Seeger"//CHAR(0)
! Note that character variables, like arrays, are passed by
! reference in both languages. There’s no need to use the
! %REF built-in function, so long as the C routine
! provides an extra argument for the "hidden" length
! parameter. To suppress passing that parameter, use %REF.
CALL get_string(first_name, last_name)
PRINT 20, first_name, last_name