Fortran 95 Language

The PLplot Plotting Library: Programmer's Reference Manual
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# Chapter 8. Fortran 95 Language

As discussed in the preceding section, PLplot's integer representation is a PLINT and its floating point representation is a PLFLT. To the Fortran 95 user, this most commonly translates to a type `integer` and type `real`, respectively. This is somewhat system dependent (and up to the installer of the package) so you should check the release notes to be sure, or just try it and see what happens.

Because the PLplot kernel is written in C, standard C syntax is used in the description of each PLplot function. Thus to understand this manual it is helpful to know a little about C, but fortunately the translation is very easy and can be summarized here. As an example, the routine `plline` call from C would look like:

    	plline(n,x,y);

The argument `n` is the number of points that make up the line and the arguments `x` and `y` are arrays of floating-point numbers containing the x- and y-coordinates of the points.

In C you need to specify the array dimensions explicitly, whereas in Fortran 95 the array dimension can be implicit, which leads to less mistakes. The interface to `plline` would ideally look like this:

    	interface
    		subroutine plline(x,y)
    		real, dimension(:) :: x, y
    		end subroutine plline
    	end interface

This is the way of calling PLplot routines in Fortran 95 - it is less error-prone than the Fortran 77 way (see the chapter on Fortran 77). [1]

There is one slight complication: PLplot can be compiled with either single-precision reals or double-precision reals. It is very important to keep the variables that are passed to PLplot in the same precision. Fortunately, Fortran 95 provides the `KIND` mechanism for this.

The actual interface to `plline` therefore looks like:

    	interface
    		subroutine plline(x,y)
    		real(kind=plflt), dimension(:) :: x, y
    		end subroutine plline
    	end interface

The parameter `plflt` is defined in the `PLplot` module and should be used consistently with all real variables that you pass to PLplot routines.

Here is a short overview of how C data types correspond to Fortran 95 data types:

PLFLT | real(kind=plflt)
PLINT | integer
char * | character
PLFLT * | real(kind=plflt) or real(kind=plflt), dimension(:)
PLFLT ** | real(kind=plflt), dimension(:,:)
`"string"` | `'string'`
array[0] | array(1)

In C there are two ways to pass a variable --- by value (the default) or by reference (pointer), whereas only the latter is used by Fortran 95. Therefore when you see references in the text to either an ordinary argument or a pointer argument (e.g. `*data`), you simply use an ordinary Fortran 95 variable or array name (the interfacing routines take care of any transformations that may be necessary).

The PLplot library comes with a set of Fortran 95 interface routines that allow the same call semantics (usually) regardless of whether calling from C or Fortran 95. In some cases, the Fortran 95 interface uses implicit array dimensions, so that it has fewer arguments than the C counterpart.

These "stub" routines handle transforming the data from the normal Fortran 95 representation to that typically used in C. This includes:

- Variables passed by value instead of by reference.

  Fortran 95 passes all subroutine arguments by reference, i.e., a pointer to the argument value is pushed on the stack. In C all values, except for arrays (including char arrays), are passed by value, i.e., the argument value itself is pushed on the stack. The stub routine converts the Fortran 95 call by reference to a call by value. As an example, here is how the plpoin stub routine works. In your Fortran 95 program you might have a call to plpoin that looks something like

            real(kind=pllft), dimension(6) :: x, y
            x = ...
            y = ...
            call plpoin(x,y,9)

  where x and y are arrays with 6 elements and you want to plot symbol 9. The routine `plpoin` calls the underlying routine `plpoinf77`:

            subroutine plpoin( x, y, code )
               integer                        :: code
               real(kind=plflt), dimension(:) :: x, y

               call plpoinf77( size(x), x, y, code )
            end subroutine plpoin

  This takes care of the size of the arrays - it is not possible to transfer this information to C in an implicit way.

  The routine plpoinf77 is implemented in C to take care of the question pass by value or pass by reference: [2]

      #include "plplot/plstubs.h"

      void
      PLPOIN(n, x, y, code)
      PLINT *n, *code;
      PLFLT *x, *y;
      {
          c_plpoin(*n, x, y, *code);
      }

  All this stub routine does is convert the number of points (`*n` and the symbol `*code` to call by value (i.e. pushes their value on the stack) and then calls the C plpoin library routine.

- Get mapping between Fortran 95 and C namespace right (system dependent).

  The external symbols (i.e. function and subroutine names) as you see them in your program often appear differently to the linker. For example, the Fortran 95 routine names may be converted to uppercase or lowercase, and/or have an underscore appended or prepended. This translation is handled entirely via redefinition of the stub routine names, which are macros. During the build process, the properties of the build environment are detected and the correct compiler options are used.

  Once the name translation is established during installation, name translation is completely transparent to the user.

- Translation of character string format from Fortran 95 to C.

  Fortran 95 character strings are passed differently than other quantities, in that a string descriptor is pushed on the stack along with the string address. C doesn't want the descriptor, it wants a NULL terminated string. For routines that handle strings two stub routines are necessary, one written in Fortran 95 and one written in C. Your Fortran 95 program calls the Fortran 95 stub routine first. This stub converts the character string to a null terminated integer array and then calls the C stub routine. The C stub routine converts the integer array (type `long`) to the usual C string representation (which may be different, depending on whether your machine uses a big endian or little endian byte ordering; in any case the way it is done in PLplot is portable). See the `plmtex` stubs for an example of this.

  Note that the portion of a Fortran 95 character string that exceeds 299 characters will not be plotted by the text routines (`plmtex` and `plptex`).

- Multidimensional array arguments are changed from row-dominant to column-dominant ordering through use of a temporary array.

  In Fortran 95, arrays are always stored so that the first index increases most rapidly as one steps through memory. This is called "row-dominant" storage. In C, on the other hand, the first index increases least rapidly, i.e. "column-dominant" ordering. Thus, two dimensional arrays (e.g. as passed to the contour or surface plotting routines) passed into PLplot must be transposed in order to get the proper two-dimensional relationship to the world coordinates. This is handled in the C stub routines by dynamic memory allocation of a temporary array. This is then set equal to the transpose of the passed in array and passed to the appropriate PLplot routine. The overhead associated with this is normally not important but could be a factor if you are using very large 2d arrays.

This all seems a little messy, but is very user friendly. Fortran 95 and C programmers can use the same basic interface to the library, which is a powerful plus for this method. The fact that stub routines are being used is completely transparent to the Fortran 95 programmer.

For more information on calling PLplot from Fortran 95, please see the example Fortran 95 programs (`/examples/f95/x??f.f`) distributed with PLplot.

### Notes

[1] | The Fortran 77 way is still available: you can call the routine `pllinef77` that has the same argument list as the Fortran 77 routine `plline`. This is not documented, however, other than by this note.
[2] | `PLPOIN` is a macro that get translated into the correct name for this routine - various Fortran compilers use different conventions, such as adding an underscore or translating the name into capitals.

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