Frequently Asked Questions - Fortran 90 Xlib Interface

   Fortran 90 Xlib Interface FAQ - created 12/20/95.
                                 - last updated 04/09/99.

   This Fortran90 interface was created by Garnatz and Grovender, Inc.
   email: gginc@gginc.biz
   URL:   http://gginc.biz

   Please send additions or corrections to the above email address.

   Necessary pieces: xlib.f90 xlibf90.c
   Optional pieces:  ast.f90  xlib-faq.txt(this file)

   A new feature, 10/11/96: A limited version of the interface revised
   for the "F" language from Imagine1 Inc.
   files: ast.F xlib.F  (xlibf90.c and xlib-faq.txt are the same)

   A long pointer enhancement, for the DEC Alpha architecture, 11/11/96:
   compile with -DLONG_PTR


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Q1)
   > How do you handle C pointers in Fortran 90.

     I treat them as default integers.  I cast them to (int) passing
     back from C, and use an indirect index to the pointer when calling C.
     See more information below. (Q7)

Q2)
   > How about C Structures to and from f90 derived data types

     There is no guarantee in Fortran 90 that derived data types will
     match up with C structs;  so to pass them safely I copy all 
     components to individual variable, pass them as arguments, and
     then copy them back to structure/type components.

Q3) 
   > How about C strings to and from f90 characters

     Similar to structures above, the only safe and portable way I know of, 
     is to copy to an array of integers, then back again.
 
Q4)   
   > I tried to compile xlibf90.c using gcc version 2.6.3 on Linux but I got
   > a huge amount of errors:
   > 
   > gcc -c xlibf90.c 
   > xlibf90.c:77: parse error before `*'
   > xlibf90.c:80: `idisp' undeclared here (not in a function)
   > xlibf90.c:80: warning: data definition has no type or storage class
   > xlibf90.c:81: parse error before `return'
   > xlibf90.c:98: parse error before `*'
   > <snip>
   > 
   > Do these messages make sense to you??? 
   > (I'm not familiar with C)
   > 
   > Best Regards,
   > 
   > Jens B. Helmers
   > jenhel@marin.unit.no
   
     Short answer: try  cc -DSPARC -c xlibf90.c
   
     Long answer: 
       when Fortran compilers set up entry points for external calls there are
       3 popular styles: 
       ' call fred' becomes a call to 
          1) fred_  trailing underscore, popular on Sun SPARC and other
                    UNIX systems
          2) FRED   all upper case, used by Cray Research PVP systems
          3) _fred  leading underscore, used by others - I don't know any 
                    specifically (implemented 10/11/96.)
   
     In order to write C routines that are callable by Fortran 90 on any system
     I have to handle thses possible function names in the C code, so I have 
     put in a lot of conditional compilation tests (#ifdef) to try and test for
     the kind of system you are using, and compile the right version of the
     source code.  If your specific system is not in the test sequence, then
     you need to specify an option on the command line with -D.
   
     Note: I fixed it to make trailing underscore default.

Q5)   
   > When will all the call interfaces be complete? 

     I don't know, this is a part time "fun" project.  Without funding
     it could take years. (and may never be complete)

Q6)   
   > When can you do Xt and Motif (Xm) call interfaces? 

     See Q5, and don't hold your breath.  Are you willing to send money?
 
Q7)
   > I am trying to compile on a DEC AXP system, and I get errors:

     xlibf90.c: In function `defaultgc_':
     xlibf90.c:204: warning: cast from pointer to integer of different size
     xlibf90.c: In function `defaultgcofscreen_':
     xlibf90.c:219: warning: cast from pointer to integer of different size
     xlibf90.c: In function `defaultscreenofdisplay_':
     xlibf90.c:264: warning: cast from pointer to integer of different size
     xlibf90.c: In function `defaultvisual_':
     (SNIP)

     The DEC alpha chip uses 40 bit addresses, too long to fit into
     a default integer either in f90 or C.  I wiil need to convert
     to long int in the C code, and selected_int_kind( ? ) of something
     that will hold the whole address (pointer).

     This behavior may occur on other systems as well. In general the
     following inequality must be satisfied:

     Size_of_Fortran_integer >= Size_of_C_integer >= Size_of_C_pointer
     
     By using a long C integer to hold the pointer, and a long f90 
     varaible this wil work correctly.  Implemented and tested for
     the DEC Alpha machines. 11/11/96.
     Size_of_Fortran_integer(kind=8) >= Size_of_C_long >= Size_of_C_pointer
     DEC Alpha:     64                =       64       >  40

     To determine the sizes of the various integer types run the
     following C and Fortran 90 programs.
========== sizes.c =============

#include <stdio.h>
main() {
short sval;
int ival;
long lval;
/* does your compiler accept long long? */
int *p;

ival = sizeof(sval);
printf(" size of a short is %d \n",ival);
ival = sizeof(ival);
printf(" size of an int  is %d \n",ival);
ival = sizeof(lval);
printf(" size of a long  is %d \n",ival);
ival = sizeof(p);
printf(" size of a ptr   is %d \n",ival);
}
========== end of sizes.c =============

========== sizes.f90 ============
program sizes
!
  integer :: itype, jtype, i
!
  jtype = 0
  do i=1,30
    itype = selected_int_kind(i)
    if(itype .ne. jtype) then
       write(unit=*,fmt=*) " new kind ", itype, " at ",i
       jtype = itype
    endif
    if (itype <= 0) then
       exit
    endif
  enddo
!
  itype = kind(jtype)
  write(unit=*,fmt=*) " default kind ", itype
  inquire (iolength=jtype) itype
  itype = bit_size(jtype)
  write(unit=*,fmt=*) " size (for I/O) of default integer ", &
        jtype," = ",itype," bits "
  stop
end program sizes
========== end of sizes.f90 ============