It is good style to use typedef names to map the Fortran data
types to their C counterparts.
Up to now all platforms are satisfied by a single set of
definitions:
typedef int INTEGER;
typedef INTEGER LOGICAL;
typedef float REAL;
typedef double DBLPREC;
typedef struct { REAL re; REAL im; } COMPLEX;
but, for example, a Fortran compiler with AUTODBL option for
generation 64-bit code would require different mappings.