Re: libstdc++/8610: std::streamoff type is 32-bit in GCC 3.2whereas it was 64-bit in GCC 2.96

[Andreas Jaeger <aj at suse dot de>]

The following reply was made to PR libstdc++/8610; it has been noted by GNATS.

From: Andreas Jaeger <aj@suse.de>
To: Paolo Carlini <pcarlini@unitus.it>
Cc: paolo@gcc.gnu.org, davido@csse.uwa.edu.au, gcc-bugs@gcc.gnu.org,
	gcc-prs@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Subject: Re: libstdc++/8610: std::streamoff type  is 32-bit  in GCC 3.2
 whereas it was 64-bit in GCC 2.96
Date: Sun, 09 Feb 2003 11:21:38 +0100

 Paolo Carlini <pcarlini@unitus.it> writes:
 
 > Andreas Jaeger wrote:
 >
 >>Paolo, I need some help here since I don't know the internals of
 >>libstc++.
 >>
 > Hi Andreas and thanks for your _very_ quick reply!
 > My impression, in general, is that enabling LFS for libstdc++ is a
 > new project! I would appreciate having some general guidelines from
 > you and trying to work on it in the next few months...
 > So:
 >
 >>/opt/gcc/3.4-devel/include/c++/3.4/i686-pc-linux-gnu/bits/c++io.h
 >>(where does this file come from?) indeed defines:
 >> namespace std {
 >>// for fpos.h
 >>  typedef long          streamoff;
 >>  typedef ptrdiff_t     streamsize; // Signed integral type
 >>
 > It comes from libstdc++-v3/config/io/c_io_stdio.h. It's wrong, right?
 > What should be, instead, off64_t?
 
 It really depends on the functions you use - if you use fseek/ftell,
 it's right.  But if you compile with _FILE_OFFSET_BITS=64 and use
 ftello/fseeko, you need off_t which is in this case a 64-bit variable.
 Or use fseeko64/ftello64 with off64_t, or fsetpos64/fgetpos64 with
 fpos64_t.
 
 > Is this the correct type, which becomes a real 64 bit type as soon
 > as _FILE_OFFSET_BITS=64 is defined?
 
 off_t becomes 64-bit if you use that define.
 
 > Also, why _two_ different defines are needed, both _FILE_OFFSET_BITS=64
 > and _LARGEFILE_SOURCE ?? Could you please explain a bit?
 
 _FILE_OFFSET_BITS=64 basically renames the types and functions so that
 instead of fsetpos, fsetpos64 is called.
 
 _LARGEFILE_SOURCE gives only the declaration for 64-bit types and
 functions without the renaming, so that your program can use both
 fsetpos and fsetpos64.
 
 >>And libstdc++/config/io/c_io_libio.h has:
 >>
 > Disregard this file: it was only used by the old libio.
 >
 >>But the problem is even more subitle.  config/io/basic_file_stdio.cc
 >>uses fseek/ftell which use a long int and therefore - under 32-bit -
 >>allow only 2 GB. You should use fseeko/ftello or fsetpos/fgetpos.
 >>
 > I see... Isn't possible to have a single name for both 32 and 64 bit
 > which transparently adapts according to the value of the defines??
 > Is it fseeko/ftello?
 
 Yes - or fsetpos/fgetpos.
 
 > I read in the glibc docs that open64 for instance can be avoided and
 > only open used transaparently...
 
 With _FILE_OFFSET_BITS=64
 
 > In your opinion, which amount of transparency can be achieved?
 > I mean, it will ever be possible to have one single libstdc++, which,
 > without configure magic, becomes LFS only by setting
 > _FILE_OFFSET_BITS=64 and _LARGEFILE_SOURCE when it's compiled?
 
 Using _FILE_OFFSET_BITS=64 would work if we use off_t and avoid
 fseek/ftell.  But it might give an incompatible lib depending which
 define is use.
 
 I therefore advise to use _LARGEFILE_SOURCE only and call the 64-bit
 functions directly.
 
 > Thanks for your past and future help,
 
 ;-)
 
 Btw. What shall we do with this PR?
 Andreas
 -- 
  Andreas Jaeger
   SuSE Labs aj@suse.de
    private aj@arthur.inka.de
     http://www.suse.de/~aj