Date: Sat, 13 May 2000 08:18:33 -0700 From: nathan@khmere.com To: Alfred Perlstein <bright@wintelcom.net> Cc: "hackers@FreeBSD.ORG" <hackers@FreeBSD.ORG> Subject: Re: mremap help ? or no support for FreeBSD ? so do what ? Message-ID: <391D7249.14E1E77B@khmere.com> References: <391D39E0.934224E4@khmere.com> <20000512235721.A28383@fw.wintelcom.net> <391D61DA.C9793AAC@khmere.com> <20000513011918.C28383@fw.wintelcom.net>
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Alfred Perlstein wrote:
> * nathan@khmere.com <nathan@khmere.com> [000513 00:41] wrote:
> > Alfred Perlstein wrote:
> >
> > > * nathan@khmere.com <nathan@khmere.com> [000512 21:54] wrote:
> > > > I know that this was discussed in the past but I can't find out what to
> > > > do ?
> > > >
> > > > In Linux if I have to resize a mmap 'ed object I can just use mremap....
> > > > but in FreeBSD if, I want to resize it what do I do ?
> > > >
> > > > I have tried writing past where I know the end is and it kinda works ?
> > > > but why ?
> > > >
> > > > Is their a better solution besides just writing to the file and then
> > > > calling msync ?
> > > >
> > > > Is their new plans to make a mremap call for FreeBSD 4.x ?
> > >
> > > no.
> > >
> > > >
> > > > Or am I just sh%t out of luck ?
> > >
> > > Possibly, but if you describe what you are trying to accomplish
> > > there may be some advice available. Your misuse of msync makes me
> > > think that a rethinking of what you are trying to accomplish may
> > > be a good idea. Please explain what makes you need mremap which
> > > is not portable to any version of unix. I'm assuming you want your
> > > app to work on Solaris and other commercial systems.
> > >
> > > --
> > > -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
> > > "I have the heart of a child; I keep it in a jar on my desk."
> > >
> > > To Unsubscribe: send mail to majordomo@FreeBSD.org
> > > with "unsubscribe freebsd-hackers" in the body of the message
> >
> > Sorry !
> >
> > Ok here is a snipit of what I am trying to do....
> >
> > fstat(fd, &st);
> > if( (base = (caddr_t *) mmap(0, st.st_size, ( PROT_READ | PROT_WRITE ) ,
> > MAP_SHARED, fd, 0)) == MAP_FAILED)
> > {
> > MSG_ERR("cannot mmap file, exiting ! ");
> > _exit(1);
> > }
> >
> > /* say st.st_size = 200, and I want to add to the end of the file more data
> > */
> > base += 200; /* we are now at the end */
> >
> > rec = (stuct rec_t *) base;
> > rec->len = 200;
> > /* we are writing to the buff, but we are past.... */
> > memmove(rec->data, data, 100);
> > msync(base, 0, (st.st_len + rec->len), MS_ASYNC);
> >
> > the struct rec_t is an example of a stuct that I want to append to the buf
> > and make another reccord.
> > If base was a regular pointer we would segfault.... cause we are past the
> > pointers buf....
> > but it kinda works.. meaning it will write and add and sometimes and not
> > segfault... sometimes...
> >
> > or I can do this:
> >
> > rec = (stuct rec_t *) base;
> > rec->len = 200;
> > memmove(rec->data, data, 100);
> > write(fd, (char *) rec, rec->len);
> > msync(base, 0, (st.st_len + rec->len), MS_ASYNC);
> >
> > After I append to the file and then msync. I should be able to read the
> > reccord that I just added from base.
> > base should reflect the file.... right ?
> >
> > I want to make things simple and only append to the buffer "base" and not
> > have to re-open the file and write to it then msync it back to mem. I know
> > that some of this under FreeBSD "automatic" meaning that the changes made to
> > the file are seen by "base" even though I do not call msync..... but in
> > order to keep things clear, I will msync.
> >
> > I hope that this wasn't too confusing... I am not good at explanations.... or
> > spelling....
>
> The first code snippet is completely incorrect, this is _not_ how
> you use the mmap() interface and I'm suprised it works at all.
> Writing after the mmap'd area should cause a SEGV of BUS signal to
> be sent to your process.
>
> You really haven't provided the code you're using on Linux to use mremap()
> which I'm assuming looks something like:
>
> > rec = (stuct rec_t *) base;
> > rec->len = 200;
> > memmove(rec->data, data, 100);
> > write(fd, (char *) rec, rec->len);
> ->>> mremap(arguments to extend the map);
> > msync(base, 0, (st.st_len + rec->len), MS_ASYNC);
>
> (derived from the second snippet)
>
> Now i'm confused, why are you mmap()'ing like this? I know mremap()
> offers a slightly easier interface, but you could simulate with some
> code like so:
>
> (you've already mmap()'d a page aligned and multiple of page sized
> region)
> if (!mmap(base + offset, additional length,
> PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, offset)) {
>
> unmap file and remap it.
> };
>
> which would tell the OS to map in the end of the file at the end
> of the map you already have.
>
> problems which can then happen is that if you mmap() multiple files
> without specifying a fixed address then they may be mmap'd into
> your address space right next to each other so that the fixed
> mapping will fail.
>
> A more flexible way would be to maintain a structure for each file
> that keeps track of all the base and length addresses that you've
> mmap'd in the file. Then before writing to the memory address you
> can do a quick lookup to figure out where the OS has mapped in that
> particular offset into your address space. You could use a hash
> on the fd and a binary tree to store that infomation pretty easily.
>
> Basically implement a simple mmap() manager and you should be set.
>
> best of luck,
> --
> -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
> "I have the heart of a child; I keep it in a jar on my desk."
>
> To Unsubscribe: send mail to majordomo@FreeBSD.org
> with "unsubscribe freebsd-hackers" in the body of the message
Thanks....
but not to sound too pesky...
When you wrote
if (!mmap(base + offset, additional length,
PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, offset)) {
This may sound silly but after this is done then the file will reflect the changes
?
and now the file will extend beyond the original size ?
why do you need to unmap it ? if the file was mmap 'ed with MAP_SHARED and not
MAP_FIXED wouldn't the changes be made to all objects ? then you only need to
msync back the diff if any ? (or at all)
See I want to get around using the fd at all. I just want to open the file then
close it and just reference it from mem only. With Linux I think that you can do
this by calling mremap Linux man : "mremap expands (or shrinks) an existing
memory mapping"
So it would be kinda like realloc but the changes would be seen by all
objects...... ? and then I can close the fd and only keep track of 1 object. If I
need to add to it ....... mremap on it.
Or am I just way off in my understanding ? I know that I did misuse the mmap in
the top snipit but I was just playing.....
Thank you
nathan
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