HP Fortran Programmer's Guide (March 2010)

Calling C routines from HP Fortran
Sharing data
Chapter 8210
Note that the extern specifier references the name of the common block, globals, not the
name of the array. From C’s point of view, the common block is treated as though it were the
array.
The common block to be shared with a C function can contain more than one data item. To do
so, the C source file must declare a structure whose members match the data items in
common. Any C function needing access to an item in common uses the extern statement to
declare a variable of the structure type. The name of the variable is that of the common block.
To access an individual data item, the function uses the C notation for referencing members of
a structure.
HP Fortran uses the same packing and alignment rules when laying out common blocks in
memory that HP C uses for structures. However, the programmer must be sure to declare the
number, types, and sizes of the structure members in the same order as they appear in the
common block. Refer to Table on page 189 for the data type correspondences for both
languages.
The following example program consists of two source files that contain the Fortran main
program unit and a C function called from Fortran. The main program unit specifies a
common block having two double-precision variables. It writes to one of the variables and
calls the C function. The C function reads the variable written by Fortran and writes to the
other one. After the call returns, Fortran reads both variables.
The following are examples of Fortran and C source files.
Example 8-10 shared_common.f90
PROGRAM main
! This program uses the common block to share data with
! the C function get_nlog. C uses a structure type to
! declare the same items in common.
REAL(KIND=8) :: num, nlog_of_num
COMMON /globals/num, nlog_of_num
! a header for the table that is printed by the following
! DO loop
PRINT *, 'Number Natural Log of Number'
PRINT *, '-------+-----------------------'
! At each iteration, write a value to the common block
! variable num, call the C function get_nlog, and
! print the contents of both common block variables
! to the screen.
DO num = 2.0, 10.0
CALL get_nlog()
PRINT 10, num, ‘|’, nlog_of_num
END DO