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

Calling C routines from HP Fortran
Arrays
Chapter 8192
Arrays
There are two differences between HP Fortran and C to consider when passing arrays from Fortran to C:
• In HP Fortran, array subscripts start by default at 1, whereas in C they always start at 0
• In HP Fortran, multi-dimensional arrays are laid out differently in memory than they are in C.
The difference in subscript-numbering does not result in any size discrepancies: an array of 10 elements in
Fortran has 10 elements in C, too. But the subscripts in Fortran will be numbered 1-10, whereas in C
they will be numbered 0-9. This difference should not require any change to the normal coding practice
for C or for Fortran.
The difference in the way multi-dimensional arrays are laid out is well-known but more significant: Fortran
lays out multi-dimensional arrays in column-major order, so that the leftmost dimension varies fastest;
whereas C lays out multi-dimensional arrays in row-major order, so that the rightmost dimension varies
fastest.
Figure 8-1 shows the Fortran and C declarations for a two-dimensional array of integers, each having the
same number of rows and columns. The boxes under each array declaration represents the memory locations
where each element of the array is stored. As shown, each language represents the six elements in a different
order: the value stored at the first row and second column is not the same for Fortran as for C.
Figure 8-1 Memory layout of a two-dimensional array in Fortran and C
To compensate for this difference, the dimensions of the array in either the C or Fortran code should be
declared in the reverse order of the other. For example, if the array is declared in Fortran as follows:
INTEGER, DIMENSION(3,6) :: my_array
then the array should be declared in C as follows:
int my_array[6][3];
int a[2][3];
INTEGER, DIMENSION(2,3) :: a
a[0][0] a[0][1] a[0][2] a[1][0] a[1][1] a[1][2]
a(1,1)
a(2,1) a(1,2) a(2,2) a(1,3) a(2,3)