HP Fortran Programmer's Guide (September 2007)

Using Fortran directives
Using HP Fortran directives
Chapter 9 205
Examples
The $HP$ ALIAS directive is especially useful when calling a routine in a language that uses
different conventions than Fortran. The following examples illustrate how to use the
$HP$ ALIAS directive to resolve differences with:
• Case sensitivity
• Argument-passing conventions
• Strings
Case sensitivity
Names in HP Fortran are not case sensitive; that is, the compiler converts all names to
lowercase. This means that if you reference a routine in a language that is case sensitive and
the routine name contains uppercase letters, a call to that routine in HP Fortran will result in
an unresolved reference—unless you use the $HP$ ALIAS directive to redefine the name in all
lowercase letters, as in the following example:
!$HP$ ALIAS printnames = 'PrintNames'
Argument-passing conventions
By default, HP Fortran assumes that all parameters in a subroutine or function call are
passed by reference; that is, the call passes the addresses of the parameters, not their values.
On the other hand, C code assumes that parameters are passed by value; that is, the current
value of the actual parameter is passed to the called routine. Without the $HP$ ALIAS
directive, it would be difficult to call a C routine from a Fortran program.
For example, suppose you want to call the system routine calloc (see the malloc(3C) man
page) to obtain dynamic memory. The man page describes the calling sequence as:
char *calloc(unsigned nelem, unsigned elsize);
It would be difficult, using standard Fortran constructs, to provide actual parameters
corresponding to nelem and elsize because HP Fortran always passes addresses. The
$HP$ ALIAS directive can solve this problem by directing the compiler to generate
call-by-value actual parameters:
!$HP$ ALIAS calloc(%VAL, %VAL)
Strings
Programs written in C expect strings to be terminated with the null character ('\0'). But HP
Fortran programs pass a hidden length parameter to indicate the end of a string argument.
Thus, if you want to pass a string from HP Fortran to a C language function, you must
explicitly append the null to the string and suppress the hidden length parameter. The
$HP$ ALIAS directive enables you to pass the string from Fortran to C. For example, consider
the following routine: