Sender: rich AT phekda DOT freeserve DOT co DOT uk
Message-ID: <3D0C85B3.146E1AC1@phekda.freeserve.co.uk>
Date: Sun, 16 Jun 2002 13:33:55 +0100
From: Richard Dawe <rich AT phekda DOT freeserve DOT co DOT uk>
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To: djgpp-workers AT delorie DOT com
Subject: Re: DJGPP and the Large File Summit (LFS)
References: <10206142111 DOT AA17751 AT clio DOT rice DOT edu>
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Reply-To: djgpp-workers AT delorie DOT com

Hello.
Charles Sandmann wrote:
[snip]
> One thing I've always thought is that lseek and llseek should be the
> same code, either with a compile define or a common stub routine.  We
> also have many places in the libc which do seeks via int calls instead
> of calling a common low level seek.  There is at least one other routine
> I've seen that was essentially a clone of another routine (and should
> be fixed, but I can't remember right now).
[snip]

Combining lseek and llseek into a common stub routine isn't quite as
straightforward as it sounds. What do you do, when you have to call an FSEXT
handler? Which do you pass in: __FSEXT_lseek or __FSEXT_llseek?

As part of implementing LFS, it seems like a common stub routine is the only
way forward, because of the possibilities of small file vs. large file and
small file-only function vs. large file-supporting function. What I currently
have in my tree is this:

    lseek   -\
              +-> __lseek64_internal
    lseek64 -/

__lseek64_internal does all the work - checking that the maximum offset will
not be exceeded, calling the appropriate FSEXT handler (lseek vs. lseek64 (aka
llseek)). __lseek64_internal takes a flag which indicates whether large-file
operations are allowed (allow_large_file). There are three cases:

1. lseek calls __lseek64_internal; allow_large_file == 0: maximum offset is
LONG_MAX;
2. lseek64 calls __lseek64_internal; allow_large_file == 1; the file was
opened in small-file mode: maximum offset is LONG_MAX;
3. lseek64 calls __lseek64_internal; allow_large_file == 1; the file was
opened in large-file mode: maximum offset is ULONG_MAX.

Cases 1 & 2 call the __FSEXT_lseek handler. Case 3 calls the __FSEXT_lseek64
(aka __FSEXT_llseek) handler.

There is a similar hierarchy for fseek, fseeko64. These use __lseek64_internal
as well:

    fseeko -> fseek -\
                     +-> __fseeko64_internal -> __lseek64_internal
    fseeko64 --------/

__fseeko64_internal is just fseek modified to use *64 functions and pass the
allow_large_file flag to __lseek64_internal.

Bye, Rich =]

-- 
Richard Dawe [ http://www.phekda.freeserve.co.uk/richdawe/ ]