HP Fortran Programmer's Guide (September 2007)

Calling C routines from HP Fortran
Data types
Chapter 8 177
• Unsigned integers
• Logicals
• Complex numbers
• Derived types
Unsigned integers
Unlike Fortran, C allows integer data types (char, int, short, and long) to be declared as
either signed or unsigned. If a Fortran program passes a signed integer to a C function that
expects an unsigned integer argument, C will interpret the bit pattern as an unsigned value.
An unsigned integer in C can represent twice the number of positive values as the same-sized
integer in HP Fortran. If an HP Fortran program calls a C function that returns an unsigned
integer and the return value is greater than can be represented in a signed integer,
HP Fortran will interpret the bit pattern as a negative number.
Logicals
C uses integers for logical types. In HP Fortran, a 2-byte LOGICAL is equivalent to a C short,
and a 4-byte LOGICAL is equivalent to a long or int. In C and HP Fortran, zero is false and
any nonzero value is true. HP Fortran sets the value 1 for true.
Complex numbers
C has no complex numbers, but they are easy to simulate. To illustrate this, create a struct
type containing two floating-point members of the correct size — two floats for the complex
type, and two doubles for the double complex type. The following creates the typedef
COMPLEX:
typedef struct
{
float real;
float imag;
} COMPLEX;
Consider a program that consists of two source files:
• The Fortran source file, which defines the main program unit
• The C source file, which defines a function sqr_complex, having the following prototype
declaration:
COMPLEX sqr_complex(COMPLEX cmx_val);