From: "Shawn O. Pearce" <spearce@spearce•org>
To: "Stefan-W. Hahn" <stefan.hahn@s-hahn•de>
Cc: git@vger•kernel.org
Subject: Re: [PATCH] Replacing the system call pread() with real mmap().
Date: Sun, 7 Jan 2007 06:24:45 -0500 [thread overview]
Message-ID: <20070107112445.GE10351@spearce.org> (raw)
In-Reply-To: <20070107111712.GB9909@scotty.home>
"Stefan-W. Hahn" <stefan.hahn@s-hahn•de> wrote:
> +# Define NO_PREAD if you have a problem with pread() system call (i.e.
> +# cygwin.dll before v1.5.22). This needs NO_MMAP not to be set, because pread()
> +# is emulated using mmap.
Better to emulate pread using lseek/xread/lseek:
off_t p = lseek(fd, offset, SEEK_SET);
xread(fd, buf, count);
lseek(fd, p, SEEK_SET);
If you look at the history for contrib/mmap.c you will see an
implementation like that before it was called git_mmap().
This works OK in Git as we only ever have one thread, therefore
there is no race condition during the seek/read/seek calls. Its also
usually faster on platforms like Mac OS X where mmap() is horrible.
If the platform is lacking a good pread() it may also be the case
that it lacks a good mmap(); in this case it would be desirable to
set both NO_MMAP and NO_PREAD.
--
Shawn.
next prev parent reply other threads:[~2007-01-07 11:24 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-01-06 17:03 problem with git clone on cygwin Stefan-W. Hahn
2007-01-06 21:40 ` Juergen Ruehle
[not found] ` <20070106215919.GB8041@scotty.home>
2007-01-07 9:30 ` Stefan-W. Hahn
2007-01-07 10:46 ` Juergen Ruehle
2007-01-07 11:21 ` Stefan-W. Hahn
2007-01-07 6:00 ` Shawn O. Pearce
2007-01-07 11:17 ` [PATCH] Replacing the system call pread() with real mmap() Stefan-W. Hahn
2007-01-07 11:24 ` Shawn O. Pearce [this message]
2007-01-07 16:36 ` Stefan-W. Hahn
2007-01-07 17:10 ` Johannes Schindelin
2007-01-07 20:01 ` Stefan-W. Hahn
2007-01-09 18:51 ` [PATCH] Test for failing pread() on cygwin Stefan-W. Hahn
2007-01-09 18:51 ` [PATCH] Replacing the system call pread() with lseek()/xread()/lseek() sequence Stefan-W. Hahn
2007-01-09 20:21 ` Junio C Hamano
2007-01-07 11:18 ` [PATCH] Test for failing pread() on cygwin Stefan-W. Hahn
2007-01-08 0:19 ` Horst H. von Brand
2007-01-08 17:09 ` Stefan-W. Hahn
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