[buytenh@gnu.org: Re: [uml-devel] allow >2GB files to be used for ubd's]

[Lennert B. <bu...@gn...>]

Sorry.. getting used to my new mailer.


----- Forwarded message from Lennert Buytenhek <bu...@gn...> -----

Date: Sun, 4 Mar 2001 17:52:28 -0500
From: Lennert Buytenhek <bu...@gn...>
To: Jeff Dike <jd...@ka...>
Subject: Re: [uml-devel] allow >2GB files to be used for ubd's
User-Agent: Mutt/1.2.5i
In-Reply-To: <200...@cc...>; from jd...@ka... on Sun, Mar 04, 2001 at 06:43:24PM -0500



On Sun, Mar 04, 2001 at 06:43:24PM -0500, Jeff Dike wrote:

> What's the -D_FILE_OFFSET_BITS=64 for?  Does that turn on LFS in libc?

Yup. /me quoteth from the GNU libc manual:

<quote>
 - Macro: _FILE_OFFSET_BITS
     This macro determines which file system interface shall be used,
     one replacing the other.  While `_LARGEFILE64_SOURCE' makes the
     64 bit interface available as an additional interface
     `_FILE_OFFSET_BITS' allows the 64 bit interface to replace the old
     interface.

     If `_FILE_OFFSET_BITS' is undefined, or if it is defined to the
     value `32', nothing changes.  The 32 bit interface is used and
     types like `off_t' have a size of 32 bits on 32 bit systems.
     value `32', nothing changes.  The 32 bit interface is used and
     types like `off_t' have a size of 32 bits on 32 bit systems.

     If the macro is defined to the value `64', the large file interface
     replaces the old interface.  I.e., the functions are not made
     available under different names as `_LARGEFILE64_SOURCE' does.
     Instead the old function names now reference the new functions,
     e.g., a call to `fseeko' now indeed calls `fseeko64'.

     This macro should only be selected if the system provides
     mechanisms for handling large files.  On 64 bit systems this macro
     has no effect since the `*64' functions are identical to the
     normal functions.

     This macro was introduced as part of the Large File Support
     extension (LFS).
</quote>


> Does this work as before on non-LFS systems?

The text above doesn't seem to guarantee this.

At least the binary kernel I use (compiled on a 2.4 host with this
ubd lfs patch included) runs fine on a machine with glibc 2.1.3 (does
understand LFS) and a 2.2.16 kernel (not LFS-enabled). I see the
open system call being made with the O_LARGEFILE LFS flag, but this
flag is apparently ignored by the kernel. So all would seem well in
this case.

This was about the oldest system I could find. I would expect really
ancient glibc's to ignore the _FILE_OFFSET_BITS define, so I don't
expect problems there.

Anyone with a libc5 system willing to do some tests for me?


cheers,
Lennert

----- End forwarded message -----