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At least on the VAX under 4.3 BSD, f77 COMMON blocks are represented in
exactly the same way as C externals.  The following two files, one
Fortran and one C, demonstrate the correspondence:

------------------------------------------------------------------------
	integer a
	real b(4)
	double complex c
	character*4 d
	integer*2 e(2)
	complex f
	common /z/ a, b, c, d, e, f

	a = 1
	b(1) = 2.
	c = (3., 4.)
	d = 'a'
	e(2) = 5
	f = (6., 7.)

	call croutine
	stop
	end
------------------------------------------------------------------------
struct complex { float r, i; };
struct double_complex { double dr, di; };
struct z {
	int a;
	float b[4];
	struct double_complex c;
	char d[4];
	short e[2];
	struct complex f;
} z_;

void
croutine_()
{
	printf("%d %f %f %.4s %hd %f\n",
		z_.a,
		z_.b[0],
		z_.c.di,
		z_.d,
		z_.e[1],
		z_.f.r);
}
------------------------------------------------------------------------

Notice the difference in the origin for arrays...  A properly aligned
f77 COMMON block should produce a properly aligned C structure.

Donn Seeley    University of Utah CS Dept    donn@utah-cs.arpa
40 46' 6"N 111 50' 34"W    (801) 581-5668    decvax!utah-cs!donn