From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
To: caml-list@inria.fr
Subject: Re: [Caml-list] mmap() and strings
Date: Thu, 09 Dec 2004 10:42:04 +0900 (JST) [thread overview]
Message-ID: <20041209.104204.95951378.garrigue@math.nagoya-u.ac.jp> (raw)
In-Reply-To: <20041209.100901.75479815.garrigue@math.nagoya-u.ac.jp>
From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
> The main problem is way string length is represented.
> What you have to do is create a pseudo block header inside a bigarray.
> The simplest way is to first create a string of the right size, and
> then copy it byte by byte to the bigarray, starting with index (-4)
> (for a 32-bit machine) and ending at ((len/4+1)*4) (the last by of the
> last word of the string encodes part of the length), using
> String.unsafe_get or String.unsafe_blit (more subtle).
> Then you want to get a pointer at offset 4 in the string.
> Not too hard either:
> (Obj.magic
> (!(snd (Obj.magic biga : Obj.t * int ref)) + 2)
> : string)
Sorry, the above code is wrong.
The right one is
let str_of_bigarray biga =
(Obj.magic (snd (Obj.magic biga : Obj.t * int) + 2) : string)
The "+2" is intended to add 4 to the pointer stored in the second word
of the bigarray, which happens to be the pointer to the raw data.
And for initialization
let copy_string s biga =
String.unsafe_blit s (-4) (str_of_bigarray biga) (-4)
((String.length s / 4 + 2)*4)
After this, you can use it as a normal string.
(Again, this code comes with no warranty, use at your own risk)
Jacques Garrigue
next prev parent reply other threads:[~2004-12-09 1:42 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-12-08 20:04 Julien Cristau
2004-12-08 20:24 ` [Caml-list] " Basile STARYNKEVITCH
2004-12-08 20:50 ` Julien Cristau
2004-12-09 1:09 ` Jacques Garrigue
2004-12-09 1:42 ` Jacques Garrigue [this message]
2004-12-09 10:32 ` David Baelde
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