11-105
To: J3
From: Nick Maclaren
Subject: Interop TR: Initialisation using C facilities
Date: 2011 January 19
Reference: 10-251.pdf



10-251 initalises descriptors almost entirely by calling functions that
update a previously declared descriptor.  However, C99 added the
facility to initialise structures by descriptor.  I realised after
writing 10-232r2 that I had forgotten to check that aspect of the C
standard and it was then too late to do so.

I have now done so, and have reminded myself how problematic that area
of the C standard is.  In particular, it is most unclear whether a
structure containing a flexible member is a complete or incomplete type
(or both or neither or what), which is critical to 6.2.7 paragraph 1
(compatibility of structure types) and hence this issue.

[ Aside: if anyone looks at it, the condition "declared in different
translation units" doesn't mean what it seems to. ]

There are two reasonable possibilities.  One is to forbid the use of C
initializers, which seems a bit harsh, and the other to spell out what
is required, copying (and slightly simplifying) the terminology from
that.

Option 1:
---------

Edits to 10-251:

[11:16] Append to the end of the paragraph: "A variable declared using
CFI_DESC_T shall not have an initializer."

Option 2:
---------

[11:16] Append to the end of the paragraph: "The type that CFI_DESC_T
evaluates to is a structure type, with members that correspond
one-to-one with those of CFI_desc_t, with corresponding members declared
with the same types and names, except that the last is an array of type
CFI_dim_t of length the evaluated argument and not a flexible array."