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.
--
Michel Fortin
[email protected]
http://michelf.ca/
