On Saturday, 18 August 2012 at 01:06:10 UTC, Michel Fortin wrote:
On 2012-08-17 20:49:21 +0000, "Paulo Pinto"
<[email protected]> said:
On Friday, 17 August 2012 at 17:42:24 UTC, Michel Fortin wrote:
Quote from <http://xenophilia.org/winvunix.html>:
In Unix, a shared object (.so) file contains code to be used
by the program, and also the names of functions and data
that it expects to find in the program. When the file is
joined to the program, all references to those functions and
data in the file's code are changed to point to the actual
locations in the program where the functions and data are
placed in memory. This is basically a link operation.
In Windows, a dynamic-link library (.dll) file has no
dangling references. Instead, an access to functions or data
goes through a lookup table. So the DLL code does not have
to be fixed up at runtime to refer to the program's memory;
instead, the code already uses the DLL's lookup table, and
the lookup table is modified at runtime to point to the
functions and data.
For this just looks at two different ways to implement dynamic
loading.
Having Windows experience since the Windows 3.1 days,
alongside with
many other OS besides the typical Linux fan, I always find sad
this vision
of Linux == UNIX, right, Windows wrong.
I never intended to mean Windows == wrong. What I am saying is
that Windows does not have a dynamic linker capable of changing
pointers to symbols in the code it loads (which is what a
linker does). Code compiled for Windows need instead to refer
to a separate lookup table when accessing symbols from others
DLLs. Neither approach is wrong; each has different tradeoffs.
Implementing a non-fragile ABI would be possible using a lookup
table of runtime calculated values for field and vtable
offsets, and those could be implemented both through a dynamic
linker with DLLs, but accessing the lookup table adds some
runtime overhead each time you need to access this symbol in
the code because of the indirection.
In the original linked article (which I wrote) what was
proposed was to have the dynamic linker calculate offsets for
fields and vtable entries and insert those offsets directly in
the code (just like a linker does when it resolves symbols).
But for that you'd need a custom linker (both static and
dynamic), and probably a custom shared library format. So it's
a huge task, especially when you consider that it should run on
multiple platforms. But this same approach could make the C++
ABI non-fragile too.
Wouldn't DLL redirection with Toolhelp32 help here?
But you're right the main problem is the portability across OS,
specially since most tend to have a C like linker, instead of a
more
intelligent one.
This is can only be improved with higher level linkers, which is a
bit what happens with the solutions where the binaries have
bytecode
which is only compiled at load time, like .NET and JVM in their
default offerings. Or even older Lillith system, or Native Oberon
with bytecode modules.
So, a solution would be either something as you describe, or
having everyone use a D based OS. :)
--
Paulo
