J3/04-230

Date:      02 February 2004
To:        J3
From:      Aleksandar Donev
Subject:   TYPEDEF facility


Title: TYPEDEF facility

Submitted by: J3

Status: For Consideration

References: J3/03-256

Basic Functionality:

Bring back the previously removed TYPEALIAS facility as a TYPEDEF
facility to be used in C Interop. This facility would allow one to give
alias names to other types. This facility may be restricted to
interoperable types only if so desired. A proper and better designed
Fortran TYPEALIAS facility may then be developed separately, not
compromising the very urgent C Interop void created by the removal of
TYPEALIAS.

Rationale:

C has a typedef facility which allows one to give aliases to existing C
types. These are used very widely, and therefore we need to provide a
way to easily interface with libraries that use them. Header files are
central to "portability" in C, and they usually mainly consist of
preprocessing directives like #define's, function prototypes and
typedef's. The first seem outside the scope of Interop and can be
emulated with PARAMETERs, the second are
handled already, and now we also need to be able to handle typedef's.
The goal is to be able to write a Fortran module that emulates a header
file provided  by the vendor of a library, and that someday it may even
be possible to do an automatic .h->.f90 conversion (at least an initial
stage thereof). Here is a short piece of the include file GL/gl.h which
is the central include file for the OpenGL library:

...
typedef unsigned int    GLenum;
typedef unsigned char   GLboolean;
typedef unsigned int    GLbitfield;
typedef void            GLvoid;
...
#define GL_BYTE                                 0x1400
#define GL_UNSIGNED_BYTE                        0x1401
...
GLAPI void GLAPIENTRY glTexCoord1dv( const GLdouble *v );
GLAPI void GLAPIENTRY glTexCoord1fv( const GLfloat *v );

for which one should be able to write an equivalent module:

MODULE GL
   USE ISO_C_INTEROP

   TYPEDEF :: GLenum=>INTEGER(C_INT), &
              GLboolean=>CHARACTER(C_CHAR), ...

   TYPE(GLenum), PARAMETER :: GL_BYTE=...

   INTERFACE
      SUBROUTINE glTexCoord1dv(v)
         TYPE(GLdouble), DIMENSION(*), INTENT(IN) :: v
      END SUBROUTINE
      ...
   END INTERFACE

END MODULE GL

As another example, consider writing a Fortran interface to the MPICH
implementation of MPI. Typically, this will be a module that contains
various constants, type parameters, and interfaces. MPI uses many
typealiases, which are needed when writing interfaces. For example,
MPI_Datatype is typically an alias for int. But one cannot assume this,
nor that it is indeed an integer. One cannot get away with our untyped
C_PTR, since arguments of type MPI_Datatype are passed by value, not by
reference. It is necessary for any kind of portability that one be able
to write:

TYPEDEF :: MPI_Datatype=>INTEGER(KIND=C_INT)

in the module for the interface to MPICH.

Estimated Impact:

The complication is mostly syntactic. The same issues we had with
TYPEALIAS remain. If we restrict this facility only to interoperable
types some of the problems may go away (for example, no more
parameterized types).

Detailed Specification:

Same as previous TYPEALIAS, but called TYPEDEF, possibly restricted
only to interoperable types.

History: