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From: bobm@hpfcmgw.HP.COM (Bob Montgomery)
Newsgroups: comp.sys.hp
Subject: Re: C calls to Fortran subroutines on HP9000/300
Message-ID: <1080053@hpfcmgw.HP.COM>
Date: 25 May 89 18:54:24 GMT
References: <436a0d6e.d498@mets.engin.umich.edu>
Organization: HP Fort Collins, CO
Lines: 71

> I am trying to call Fortran subroutines from a C main 
> program, but don't know how to set up the call.  I can't find
> any preprocessor commands like ALIAS which you can use when calling
> C from a Fortran program.  The Fortran routines I want
> to call are part of a general environment graphics package. 

   C has a lot more flexibility than FORTRAN, so the functionality of
ALIAS is not usually required.  Specifically, C has pointers so you can
pass the addresses of variables to FORTRAN routines, since FORTRAN
expects parameters to be passed by reference.  C has case-sensitive
names, so you can match the name requirements of the FORTRAN routines,
(usually all lower-case, unless the -U (uppercase) option was used to
compile the FORTRAN code; running nm(1) on the FORTRAN objects will tell
you which it is.)  The Series 300 uses identical naming rules for FORTRAN
and C (i.e.  no trailing underscore for FORTRAN names).

   As long as you are only dealing with integer and real scalars and
array parameters, just remember to pass pointers from C and it's no big
deal.  If you need to pass or accept returned character strings, please
post an example of what is required by your library.  If the FORTRAN
application library expects to do its own FORTRAN IO, you may need to
start your program as a FORTRAN main that immediately calls your C main
code so the FORTRAN IO library can be initialized correctly.

   Other details are included in a manual called the HP-UX Portability
Guide (HP Part Number 98794-90046).  There are sections on calling C
from FORTRAN and on calling FORTRAN from C.  You should read both
sections, since some of the material that applies in both directions
does not appear in the FORTRAN from C section.

Bob Montgomery
Workstations, Support, and Big Deals
HP


P.S. An example:

cmain.c:

float arr[100];

main()
{
   int i;
   float sum, sumv();
   for (i=0; i<100; i++) 
      arr[i] = (float)i;
   i = 100;
   /* pass address of i instead of i */
   sum = sumv(arr,&i);
   printf("sum: %f\n", sum);
}
   
---------------------------------------------
fsub.f:

	real*4 function sumv(v, n)
	integer i, n
	real*4 v(n)
	real*4 sum
	sum = 0.0
	do i=1,n
	   sum = sum + v(i)
	end do
	sumv = sum
	end

---------------------------------------------
Output:

sum: 4950.000000