(j3.2006) Reading C descriptors from different vendors

[Rafik Zurob]

Hello

As you know, TS29113 does not specify the offsets of the rank, type, 
attribute, or dim fields in the C descriptor.  C descriptors are also 
allowed to contain a vendor-specific area of unspecified length in the 
middle of the descriptor.  The descriptors also don't contain any field to 
identify the vendor.  This makes it impossible to convert between 
descriptors from different vendors at run time.  I raised this issue once 
before when TS29113 was under development, but couldn't provide a strong 
case for binary compatibility.  As was pointed out to me back then, the TS 
only provides source portability.  I think I now have an example that 
would benefit from better binary compatibility:  Third party libraries 
like OpenCoarrays have APIs that take descriptors.  OpenCoarrays currently 
takes gfortran descriptors but plans to move to C descriptors when 
gfortran moves as well.  For other vendors to be able to call the 
OpenCoarray API, there needs to be a way of letting the API understand the 
vendors' C descriptors.  I think it's undesirable to hardcode for each API 
what C descriptor format needs to be generated to call the API.

I have several ideas on how to address this:

1. Add new functions to get the offsets of the fields whose offsets are 
not specified by TS29113:
        int CFI_rank_offset() that returns the offset in bytes of the rank 
field relative to the start of the C descriptor.
        int CFI_type_offset() that returns the offset in bytes of the type 
field relative to the start of the C descriptor.
        int CFI_attribute_offset() that returns the offset in bytes of the 
attribute field relative to the start of the C descriptor.
        int CFI_dim_offset() that returns the offset in bytes of the dim 
array relative to the start of the C descriptor.

2. Add new functions to return pointers to the fields whose offsets are 
not specified by TS29113:
        CFI_rank_t *CFI_rank(CFI_desc_t *) that returns a pointer to the 
rank field of a given C descriptor
        CFI_type_t *CFI_type(CFI_desc_t *) that returns a pointer to the 
type field of a given C descriptor
        CFI_attribute_t *CFI_attribute(CFI_desc_t *) that returns a 
pointer to the attribute field of a given C descriptor
        CFI_dim_t *CFI_dim(CFI_desc_t *) that returns a pointer to the dim 
field of a given C descriptor

3. Define a typedef CFI_min_cdesc_t that maps to a C descriptor with no 
vendor-specific area.  Provide a conversion function to convert from 
CFI_cdesc_t to CFI_min_cdesc_t:
        int CFI_get_min_cdesct_t(CFI_min_cdesc_t *, const CFI_cdesc_t *);

4. Go for broke and redesign the C descriptor:  Specify the offsets of the 
rank, type, and attribute fields.  Add a new field (with known offset) 
containing the size of the vendor-specific section.  If the 
vendor-specific section size is non-zero, the first 4 bytes of it must be 
a four-character vendor identifier.  Define a macro (with known value) 
CFI_VERSION_F2015.  C descriptors that follow this format must have a 
version field with a value such that ((version & CFI_VERSION_F2015) == 
CFI_VERSION_F2015).

5. Other ideas?

I believe we should address this in Fortran 2015 since it's the first 
version to incorporate TS29113.

Regards

Rafik