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.TI F77/LINK_TO_C "Sep. 4, 1985"
Linking to C Procedures

C is a commonly used programming language in UNIX.  "A Portable
Fortran 77 Compiler" gives complete specification of the C/Fortran
interface.  This help file summarizes the basics for simple cases and
gives a simple example.

The following table from that paper lists corresponding Fortran and C 
declarations:

.nf
	Fortran			C
	-----------		---------
	integer*2 x		short int x;
	integer x		long int x;
	logical x		long int x;
	real x			float x;
	double precision x	double x;
	complex x		struct { float r, i; } x;
	double complex x	struct { double dr, di; } x;
	character*6 x		char x[6];
.fi


The following Fortran statements calls a procedure fill() that
sets the first 10 elements of ix() to the value 20:

.nf
		integer ix(100)
	c
		call fill(ix, 10,  20)
.fi

In creating external symbols for subprogram and common block names,
f77 adds an underscore before and after external names,
while the C compiler only adds an underscore before the name.
Thus the C name fill_() corresponds to the f77 name fill();
and the corresponding C procedure is:

.nf
	fill_( vec, nvec, val)
	long *vec, *nvec, *val;
	{
		long nv = *nvec;
	
		while (nv-- > 0) *vec++ = *val;
	}
.fi

Since Fortran passes variables by address, the C procedure treats the
parameters as pointers.  This also means that a temporary must be used.
If a temporary is not used, as in:

	while ((*nvec)-- > 0) *vec++ = *val;

the value of the third parameter in the call would be destroyed.

When mixing C and Fortran, if I/O is done in both languages and
the main program is in C, then call 'f_init()' in the
Fortran I/O library to initialize units 0, 5, and 6 to standard error,
standard input and standard output and to line-buffer standard error.

C stores arrays in row-major order with subscripts starting at 0
while Fortran stores arrays in column-major order with subscripts
starting (by default) at 1.

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