[Bug libc/15767] New: C++ ABI inconsistency for fpos_t on 64-bit archs

["bugdal at aerifal dot cx" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15767

            Bug ID: 15767
           Summary: C++ ABI inconsistency for fpos_t on 64-bit archs
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: bugdal at aerifal dot cx
                CC: drepper.fsp at gmail dot com

On 64-bit archs, -D_FILE_OFFSET_BITS=64 should not affect the C++ ABI (name
mangling). Regardless of whether there's an official policy to this effect, it
makes sense, since some applications will always compile with
-D_FILE_OFFSET_BITS=64 and others will attempt to detect whether they need it
before adding it.

For most types that change definition according to _FILE_OFFSET_BITS, there is
an underlying type name independent of the typedef name, and no issue arises.
However, fpos_t is defined to a structure with no tag, and thus the original
typedef name of either _G_fpos_t or _G_fpos64_t gets used in name mangling.
Thus if a C++ library uses fpos_t as part of its public interface, and the
library and application are compiled with different values of
_FILE_OFFSET_BITS, the ABI will gratuitously mismatch. On 32-bit archs it
should be expected to mismatch, and the library would have a policy that you
have to build with 64-bit off_t. But on 64-bit archs, it should not matter
whether _FILE_OFFSET_BITS was defined. This issue would be even more of a
problem if multiple libraries disagreed on whether it was supposed to be set,
and an application needed to use both libraries.

With that said, I'm not sure why features.h doesn't make _FILE_OFFSET_BITS a
complete no-op on 64-bit archs. That would make the issue go away entirely.

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