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| author | Simon Marlow <simonmar@microsoft.com> | 2006-04-07 02:05:11 +0000 |
|---|---|---|
| committer | Simon Marlow <simonmar@microsoft.com> | 2006-04-07 02:05:11 +0000 |
| commit | 0065d5ab628975892cea1ec7303f968c3338cbe1 (patch) | |
| tree | 8e2afe0ab48ee33cf95009809d67c9649573ef92 /docs/comm/rts-libs/foreignptr.html | |
| parent | 28a464a75e14cece5db40f2765a29348273ff2d2 (diff) | |
| download | haskell-0065d5ab628975892cea1ec7303f968c3338cbe1.tar.gz | |
Reorganisation of the source tree
Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.
The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level. The build system now makes no
pretense at being multi-project, it is just the GHC build system.
No doubt this will break many things, and there will be a period of
instability while we fix the dependencies. A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.
Diffstat (limited to 'docs/comm/rts-libs/foreignptr.html')
| -rw-r--r-- | docs/comm/rts-libs/foreignptr.html | 68 |
1 files changed, 68 insertions, 0 deletions
diff --git a/docs/comm/rts-libs/foreignptr.html b/docs/comm/rts-libs/foreignptr.html new file mode 100644 index 0000000000..febe9fe422 --- /dev/null +++ b/docs/comm/rts-libs/foreignptr.html @@ -0,0 +1,68 @@ +<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> +<html> + <head> + <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"> + <title>The GHC Commentary - why we have <tt>ForeignPtr</tt></title> + </head> + + <body BGCOLOR="FFFFFF"> + + <h1>On why we have <tt>ForeignPtr</tt></h1> + + <p>Unfortunately it isn't possible to add a finalizer to a normal + <tt>Ptr a</tt>. We already have a generic finalization mechanism: + see the Weak module in package lang. But the only reliable way to + use finalizers is to attach one to an atomic heap object - that + way the compiler's optimiser can't interfere with the lifetime of + the object. + + <p>The <tt>Ptr</tt> type is really just a boxed address - it's + defined like + + <pre> +data Ptr a = Ptr Addr# +</pre> + + <p>where <tt>Addr#</tt> is an unboxed native address (just a 32- + or 64- bit word). Putting a finalizer on a <tt>Ptr</tt> is + dangerous, because the compiler's optimiser might remove the box + altogether. + + <p><tt>ForeignPtr</tt> is defined like this + + <pre> +data ForeignPtr a = ForeignPtr ForeignObj# +</pre> + + <p>where <tt>ForeignObj#</tt> is a *boxed* address, it corresponds + to a real heap object. The heap object is primitive from the + point of view of the compiler - it can't be optimised away. So it + works to attach a finalizer to the <tt>ForeignObj#</tt> (but not + to the <tt>ForeignPtr</tt>!). + + <p>There are several primitive objects to which we can attach + finalizers: <tt>MVar#</tt>, <tt>MutVar#</tt>, <tt>ByteArray#</tt>, + etc. We have special functions for some of these: eg. + <tt>MVar.addMVarFinalizer</tt>. + + <p>So a nicer interface might be something like + +<pre> +class Finalizable a where + addFinalizer :: a -> IO () -> IO () + +instance Finalizable (ForeignPtr a) where ... +instance Finalizable (MVar a) where ... +</pre> + + <p>So you might ask why we don't just get rid of <tt>Ptr</tt> and + rename <tt>ForeignPtr</tt> to <tt>Ptr</tt>. The reason for that + is just efficiency, I think. + + <p><small> +<!-- hhmts start --> +Last modified: Wed Sep 26 09:49:37 BST 2001 +<!-- hhmts end --> + </small> + </body> +</html> |