directive. The %REFfunction tells Fortran that its argument is to be passed by reference (as when passing an array or pointer in C), and the %VALfunction 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 would be declared as follows:

INTEGER :: ptr, i

INTEGER, DIMENSION(100) :: iarray

The call from Fortran to the C function (using the %VAL and %REFbuilt-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 %VALand %REFbuilt-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; see “Case sensitivity” (page 114), which includes an example program that uses the $HP$ ALIAS directive and the%VAL and %REFbuilt-in functions to call a C function. For other examples, see “Complex numbers” (page 112)and “File handling” (page 120). Note that the example Fortran program in “Arrays” (page 116)does not require the built-in functions because both Fortran and C pass arrays by reference.

For detailed information about the $HP$ ALIAS directive and the and %REF built-in functions, see the HP Fortran Programmer's Reference.

Case sensitivity

Unlike HP Fortran, C is a case-sensitive language. HP Fortran converts all external names to lowercase, and it disregards the case of internal names. Thus, for example, the names fooand FOOare the same in Fortran. C, however, is a case-sensitive language: fooand FOOare different in C. If an HP Fortran program is linked to a C object file and references a C function that uses uppercase characters in its name, the linker will not be able to resolve the reference.

If case sensitivity is an issue when calling a C function from an HP Fortran program, you have two choices:

•Compile the Fortran program with the +uppercaseoption, which forces Fortran to use uppercase for external names.

•Use the $HP$ ALIAS directive to specify the case that Fortran should use when calling an external name.

It is unusual that all names in the C source file would be uppercase, which would be the only case justifying the use of the +uppercase option. Therefore, we recommend using the $HP$ ALIAS directive. This directive enables you to associate an external name with an external name, even if the external name uses uppercase characters.

114 Calling C routines from HP Fortran