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                  J3/02-229

Date:     July 15 2002 
To:       J3 
From:     Aleksandar Donev Subject:
Enhanced C_LOC: Pointer Arguments 
Reference: Paper J3/02-230
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Summary
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I propose a modification of the specification of C_LOC from ISO_C_BINDING
to allow associated scalar pointers of interoperable type and type
parameters as an argument. This corrects for the (unjustifiable) lack of
this functionality in the current draft.

I hope the modification will be accepted at meeting 162, which I will
attend.

I would like to thank and acknowledge the help of Richard Maine and John
Reid.

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Motivation
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Fortran pointers are not interoperable with C and thus not allowed as
arguments to C_LOC. However, allocated allocatables are, in which case
C_LOC returns the C address of the storage associated with the allocatable
array. It only seems reasonable to allow the same for associated scalar
pointers, i.e. C_LOC would return the C address of the target of the
pointer. Array pointers should be excluded because the variables they are
associated to might not be interoperable with C (for example, be
noncontiguous array sections).

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Possible alternatives
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Note that it is possible for a user to make a wrapper around C_LOC to do
what I propose in a perfectly conforming way for scalar pointers (but not
for array pointers since assumed-shape arrays are not interoperable and
cannot be arguments to C_LOC):

TYPE(C_PTR) :: c_address INTEGER, POINTER :: variable

ALLOCATE(variable) c_address=C_LOC_Integer(variable)
   ! We cannot do c_address=C_LOC(variable) directly
   
CONTAINS

   FUNCTION C_LOC_Integer(variable) RESULT(c_address)
      USE ISO_C_BINDING INTEGER, INTENT(IN), TARGET :: variable
         ! Must be TARGET
      TYPE(C_PTR) :: c_address ! C_LOC(variable)

      c_address=C_LOC(variable)
   END FUNCTION C_LOC_Integer
   
The big disadvantage of this is that we are forcing the user to write a
separate wrapper for each TK combination he needs for no reason. This would
become particularly painful when interoperable derived types are used,
since one cannot make a generic wrapper of C_LOC for all interoperable
derived types one might use.

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Edits:
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These are just excerpts from the accompanying paper number 02-229 which
gives more elaborate edits with explanations in the form of self-notes.
Please understand that this is my first-ever attempt at edits! I do not see
any other changes other then in the description of C_LOC as the proposed
modification is really simple.

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382: 20-22 Replace with: Argument. X shall be a procedure that is
interoperable, a procedure pointer associated with an interoperable
procedure, a variable that has the TARGET attribute and is interoperable,
an associated scalar pointer that has interoperable type and type
parameters, or an allocated allocatable variable that has the TARGET
attribute and has interoperable type and type parameters.
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382: 24-25 Replace with: Result Value: If the argument X is an
interoperable procedure or an interoperable variable, the result is the
value that the target C processor returns as the result of applying the
unary "&" operator to X, as defined in the C standard, 6.5.3.2.  If the
argument X is a procedure pointer or a scalar pointer, the result is as if
the target of the pointer had been passed instead of X.  [See self-note 6
in J3/02-230] If the argument is an allocated allocatable array, the result
is the the value that the target C processor returns as the result of
applying the unary "&" operator to the first element of the array in array
element order.
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