Fortran-Dynamic C Interface - PLEASE HELP!

[bwilt]

I have c libraries that unpack GRIB2 data that I receive from NCEP/NWS. The data looks correct INSIDE my c routines, but I am at a loss to accessing the c structure from fortran. The c library creates and dynamically allocates the a structure called 'gribfield' as follows:

struct

gribfield {

g2int version,discipline;

g2int *idsect;

g2int idsectlen;

unsigned char *local;

g2int locallen;

g2int ifldnum;

g2int griddef,ngrdpts;

g2int numoct_opt,interp_opt,num_opt;

g2int *list_opt;

g2int igdtnum,igdtlen;

g2int *igdtmpl;

g2int ipdtnum,ipdtlen;

g2int *ipdtmpl;

g2int num_coord;

g2float *coord_list;

g2int ndpts,idrtnum,idrtlen;

g2int *idrtmpl;

g2int unpacked;

g2int expanded;

g2int ibmap;

g2int *bmap;

g2float *fld;

};

typedef

struct gribfield gribfield;

The C routine I call simply returns a pointer to this structure as follows:

g2int g2_getfld(

unsigned char *cgrib,g2int ifldnum,g2int unpack,g2int expand,

gribfield **gfld)

I have tried everything trying to access this data from IVF 10.025 with no luck. My interface code is as follows:

interface function g2_getfld (mgrib,ifldnum,unpack,expand,gfld) !DEC$ ATTRIBUTES C, DECORATE, ALIAS:'g2_getfld' :: g2_getfld integer (kind=int_ptr_kind()) gfld type gribfield sequence integer*4 version integer*4 discipline integer*4, allocatable :: idsect(:) integer*4 idsectlen character, allocatable :: local(:) integer*4 locallen integer*4 ifldnum integer*4 griddef,ngrdpts integer*4 numoct_opt,interp_opt,num_opt integer*4, allocatable :: list_opt(:) integer*4 igdtnum,igdtlen integer*4, allocatable :: igdtmpl(:) integer*4 ipdtnum,ipdtlen integer*4, allocatable :: ipdtmpl(:) integer*4 num_coord real, allocatable :: coord_list(:) integer*4 ndpts,idrtnum,idrtlen integer*4, allocatable :: idrtmpl(:) integer*4 unpacked integer*4 expanded integer*4 ibmap integer*4, allocatable :: bmap(:) real, allocatable :: fld2(:) end type pointer (gfld, version) character*1 mgrib(131040) integer*4 ifldnum,unpack,expand end function g2_getfld end interface

How can I access the data from the gribfield structure in Fortran? For example, I want to print the parameter 'version'?

Thanks for any help you can provide!

Brett A. Wilt

[Jugoslav_Dujic]

No, you cannot map a Fortran POINTER or ALLOCATABLE array to a C pointer. What you can do is use one of the following:

* Integer (or Cray) pointers on Fortran side, or, better
* ISO_C_BINDING module

The former is a non-standard (but common) extension. The latter is the new standard, so I recommend its use:

In short, each g2int* member will map to a TYPE(C_PTR) in Fortran. To dereference it into an array, you have to use a temporary POINTER array:

integer, pointer:: pidsect(:)

...

call C_F_POINTER(gfld%idsect, pidsect, (/idsectlen/) )

write(*,*) pidsect(1)

In the spirit of ISO_C_BINDING, you may also want to change !DEC$ATTRIBUTES into BIND(C, NAME="g2_getfld"). Don't forget to USE, INTRINSIC:: ISO_C_BINDING on the relevant places.

[bwilt]

Thanks for the reply!

Do I use the type structure 'gribfield' in my fortran INTERFACE code to match the c structure? Is my code correct for this or do I not use the TYPE structure along with the allocatable array declarations?

[Jugoslav_Dujic]

Ah, I missed that.

INTERFACE is somewhat peculiar in Fortran, as the declaration of TYPE gribfiled will not be "visible" outside the INTERFACE (unless there's IMPORT statement supported in IVF10 -- I didn't check -- or the SEQUENCE attribute in both type definitions). Instead, it's best that you write it like this:

MODULE m_gribfield

USE, INTRINSIC:: ISO_C_BINDING

INTEGER, PARAMETER:: g2int =4

TYPE gribfield

 INTEGER(g2int):: version, discipline

 TYPE(C_PTR):: idsect

 INTEGER(g2int):: idsectlen

 ...

END TYPE gribfield

END MODULE m_gribfield

INTERFACE

 function g2_getfld (mgrib,ifldnum,unpack,expand,gfld) bind(C, NAME="g2_getfld")

 USE m_gribfield

 INTEGER(g2int):: g2_getfld

 INTEGER(g2int), VALUE:: ifldnum

 INTEGER(g2int), VALUE:: unpack

 INTEGER(g2int), VALUE:: expand

 TYPE(C_PTR):: gfld

 END FUNCTION

END INTERFACE

TYPE(g2_getfld), POINTER:: gfld

TYPE(C_PTR):: p_gfld

...

iret = g2_getfld(...p_gfld)

CALL C_F_POINTER(p_gfld, gfld)

I'm not positive what is mgrib/cgrib, so I left it untranslated -- you may wish to retain the character, or do it through type(C_PTR) as well.

[Steven_L_Intel1]

IMPORT is supported.

[bwilt]

Ireally appreciate your time and help with this. I am notextensively familiar with c, so this is learning for me. Here is where I am now:

module m_gribfield use, intrinsic:: iso_c_binding integer, parameter:: g2int=4 type gribfield integer(g2int):: version integer(g2int):: discipline type(c_ptr):: idsect integer(g2int):: idsectlen type(c_ptr):: local integer(g2int):: locallen integer(g2int):: ifldnum integer(g2int):: griddef integer(g2int):: ngrdpts integer(g2int):: numoct_opt integer(g2int):: interp_opt integer(g2int):: num_opt type(c_ptr):: list_opt integer(g2int):: igdtnum integer(g2int):: igdtlen type(c_ptr):: igdtmpl integer(g2int):: ipdtnum integer(g 2int):: ipdtlen type(c_ptr):: ipdtmpl integer(g2int):: num_coord type(c_ptr):: coord_list integer(g2int):: ndpts integer(g2int):: idrtnum integer(g2int):: idrtlen type(c_ptr):: idrtmpl integer(g2int):: unpacked integer(g2int):: expanded integer(g2int):: ibmap type(c_ptr):: bmap type(c_ptr):: fld2 end type gribfield end module m_gribfield

subroutine readgrib

use m_gribfield use, intrinsic:: iso_c_binding

type

(g2_getfld), pointer:: gfld

type

(c_ptr):: p_gfld interface function g2_getfld (mgrib,ifldnum,unpack,expand,gfld) !dec$ attributes c, decorate, alias:'g2_getfld' :: g2_getfld use m_gribfield character*1 mgrib(131040) integer(g2int):: g2_getfld integer(g2int), value:: ifldnum integer(g2int), value:: unpack integer(g2int), value:: expand type(c_ptr):: gfld end function g2_getfld end interface ierr=g2_getfld(%ref(mgrib),1,1,0,%ref(gfld)) call c_f_pointer(p_gfld, gfld) print*,gfld%version

return

end

Here are my compiler error messages:

Error1 Error: This is not a derived type name. [G2_GETFLD]C:DevelopmentModel_x32_VS2005Sourcegribread.f1370

This occurs when the line

type(g2_getfld), pointer:: gfld

is added to the subroutione as indicated above.

Error2 Error: This variable or component must be of a derived or structure type [GFLD]C:DevelopmentModel_x32_VS2005Sourcegribread.f1476
Error3 Error: This is not a field name that is defined in the encompassing structure. [VERSION]C:DevelopmentModel_x32_VS2005Sourcegribread.f1476

These occur when I try and print the value of version with:

print*,gfld%version

Can you show me how to print the value of version? Is it incorrect above?

[g_f_thomas]

What if you BIND(C,NAME="g2_getfld" )? but keep the !DEC$ meta command as it acts as a workaround for the compiler's failure to prepend a _ to the symbol.

Gerry

[Steven_L_Intel1]

Don't use that ATTRIBUTES line at the same time as BIND(C). At best you may get odd results, at worst a compile-time error (a future update will complain).

[Steven_L_Intel1]

First problem - the type you declared is gribfield, not m2_getfield. The other errors derive from that.

Your ATTRIBUTES line looks fine to me as it is.

[g_f_thomas]

He could throw out the Alias and retain the Decorate and all would be fine unless Decorate's behavior changes in the future. Now you know why the HP guy suggestedthat one needs to experiment from release to release.

Gerry

[Steven_L_Intel1]

Gerry, you keep bringing up that HP article which was wrong. It is also not relevant here.