J3/00-265

Date:    2000/08/18
To:      J3
From:    Matthijs van Waveren
Subject: C pointer comparison
Ref:     ISO/IEC JTC1/SC22/WG5 N1402a
Ref:     Issue 252 on page 386

Edits and references are to J3/00-007r2.

1. Analysis

Fortran 95 has the functionality of finding out whether a pointer is
associated with its target or not. This is implemented with the
intrinsic function ASSOCIATED.

This functionality can be implemented for objects of type C_PTR in two
ways:

1. Specify an intrinsic procedure, which compares two objects of type
C_PTR, and returns the value .TRUE. if the two objects are the same,
and .FALSE., if the two objects are different. Specify that the
C_NULL_PTR can be used as argument in the intrinsic procedure.

2. Allow comparison with .EQ. of objects of type C_PTR, and define a
constant of type C_PTR, which corresponds to the (null) pointer (for
instance with name C_NULL_PTR).

The following straw votes were taken at the WG5 Oulu meeting.

Straw Vote #1:

What should be the requirement of the intrinsic function:

Test against C_NULL_PTR only			1
Test against pointer equality			13
Undecided						1

Straw Vote #2:

Which language specification should J3 use:

Specify an intrinsic procedure		14
Extend the .EQ. operator			0
Undecided						1

2. Syntax

Thus WG5 has a preference for an intrinsic procedure which tests
against procedure equality. The name of the function has not yet been
determined. The example below uses C_PTR_COMPARE as name.

3. Syntax Example

USE ISO_C_BINDING
INTERFACE
  SUBROUTINE FOO(GAMMA), BIND(C)
	USE ISO_C_BINDING
	TYPE (C_PTR), VALUE :: GAMMA
  END SUBROUTINE FOO
END INTERFACE
REAL (C_FLOAT), TARGET, DIMENSION(100) :: ALPHA
TYPE (C_PTR) :: BETA
...
IF(C_PTR_COMPARE(BETA, C_NULL_PTR)) THEN
BETA = C_LOC(ALPHA)
ENDIF
...
CALL FOO(BETA)
...

4. Edits to 00-007r2

[386:26-44] Delete J3 note

[389:31] Change the section title to:

Interoperability inquiry functions

[389:33] Add the following text at end of sentence:

The C_PTR_COMPARE function is provided so that Fortran applications
can compare "C addresses".

[389:42+] Add the following text:

"C_PTR_COMPARE(C_PTR_1, C_PTR_2)

Description. Inquires whether C_PTR_1 compares equal to C_PTR_2.

Class. Inquiry function

Arguments.
C_PTR_1	shall be of type C_PTR.

C_PTR_2 	shall be of type C_PTR.

Result Characteristics. Default logical scalar

Result Value. The result is true if C_PTR_1 compares equal to C_PTR_2
in the sense of section 6.3.2.3 of the C standard, and false if it
compares unequal."

[390:198-30] Delete Note

[390:17+] Add the following note:

"The following example illustrates the use of C_LOC and C_PTR_COMPARE.

USE ISO_C_BINDING
INTERFACE
  SUBROUTINE FOO(GAMMA), BIND(C)
	USE ISO_C_BINDING
	TYPE (C_PTR), VALUE :: GAMMA
  END SUBROUTINE FOO
END INTERFACE
REAL (C_FLOAT), TARGET, DIMENSION(100) :: ALPHA
TYPE (C_PTR) :: BETA
...
IF(C_PTR_COMPARE(BETA, C_NULL_PTR)) THEN
BETA = C_LOC(ALPHA)
ENDIF
...
CALL FOO(BETA)
..."