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Re: --enable-auto-import extension
- From: "David A. Cobb" <superbiskit at cox dot net>
- To: Charles Wilson <cwilson at ece dot gatech dot edu>
- Cc: egor duda <binutils at sources dot redhat dot com>, cygwin-apps at sources dot redhat dot com
- Date: Sun, 07 Jul 2002 16:39:28 -0400
- Subject: Re: --enable-auto-import extension
- Organization: CoxNet User
- References: <email@example.com> <3D1D08A1.firstname.lastname@example.org>
An (obviously untimely) thought. Would not this technique offer a
solution to the deficiency that a Windoze "shared object" (.dll) cannot
reference symbols in the .EXE that loads it? It's a thought -- the
linker(?) [or lib-tool] could generate some sort of standard thunk in
the DLL identifying "Undefined" symbols which the calling program could
plug in at runtime [invisibly to the user, of course, as part of the
dll-load, lib-init processing]. It should not even be horribly slower
initiating, because it is simply shifting into the application itself
the same work done by a X-Nix loader that the Win loader can't do.
I'll also second Chuck's WooHoo!
Charles Wilson wrote:
First: Woo Hoo! Thanks for looking in to this problem Egor!!
egor duda wrote:
Clean way to handle such situations (other than convincing
Microsoft to change a loader) is to add some runtime support for
The idea is to add a vector of "cygwin internal pseudo-relocation"
entries to executable data section and to perform relocations of
appropriate data manually at program startup.
But why is this cygwin-specific? It seems that it's equally
applicable to mingw (e.g. native) DLLs, just as mingw's gcc can use
the current auto-import feature, even though MSVC can't understand or
Attached is a proof-of-concept patch to ld and simple testcase.
Tested, and works:
data=111 222 333
If this idea is worthwhile, i think i should add more things to the
Absolutely. Of course, it would still need to be *rigorously* tested
to insure that
a) DLLs built this way could still be linked-to by "regular" code
that doesn't violate the current limitations. (e.g. suppose I as the
cygintl-2.dll maintainer built the next cygintl-2.dll using this
spiffy new ld.exe; so now, cygintl-2.dll has the extra reloc table.
Q1: will existing code that relied on the OLD cygintl-2.dll (without
the additional reloc table) and does NOT try to access
data-with-addend, STILL work if I drop in the new DLL? [e.g. runtime
Q2: Could I relink old code (that again, does NOT try to access
data-with-addend) to the new DLL using an OLD ld.exe? (e.g. an
enduser of cygintl-2.dll who hasn't updated their binutils) [e.g
linktime backwards compat]
Q3: Is the new DLL usable by windows tools, provided a suitable import
library is generated? (I'm thinking here of mingw folks who build DLLs
and implibs for use by others with MSVC -- granted, MSVC can't use
auto-import at all, much less your extention. But the same linker
will be used even to build "regular" DLLs...we can't break that.)
1. Make cygreloc generation conditional via --enable-cygwin-reloc or
something like that.
At first, yes, it does need to be conditional -- and default to OFF,
And, it should probably not be "cygwin" specific. --enable-data-reloc?
2. If linker creates at least one cygreloc entry, it should emit
reference to some external symbol, say 'cygwin_process_cygreloc' so
that if object contains non-empty cygreloc vector it'd be
guaranteed that it can't be linked with runtime without cygreloc
Okay, that takes care of "new style" exe accidentally linking at
runtime to "old style" DLL. Still, that leaves compatibility
existing "old style" EXE ---> "new style" DLL
linking a "new" old style EXE using the old linker ---> against a
"new style" DLL
This chunk of code (in pe-dll.c)
+ if (pe_dll_extra_pe_debug)
+ printf ("creating cygreloc entry for %s (addend=%d)\n",
+ fixup_name, addend);
+ b = make_cygreloc_fixup_entry (name, fixup_name, addend,
+ add_bfd_to_link (b, b->filename, &link_info);
doesn't seem to get called in your example -- but it should, if I
understand correctly...What's the deal?
Anyway, because I can't see any "creating cygreloc entry..." debug
messages, I'm not quite sure exactly where the cygreloc vector GOES --
into the client .o, or into the DLL. I had assumed the DLL, but your
point #2 above confuses that issue for me...
(cygreloc --> addend_reloc?)
3. Make relocations a bit more flexible by adding type and size
(possible 64-bit support?)
I dunno -- that's a tall order. This addend-offset problem affects
structs and arrays -- which come in all SORTS of specific types with
different field orders and sizes. Also, what about recursive offsets?
bob = a.foo.bar.baz ?
Granted, fixing 64 bit types (long long), simple arrays, and simple
structs will go a LONG way to solving the problem in practical terms
-- but until EVERY case is covered, we still need to detect the
failure cases and warn at link time (not runtime).
Nice work so far, but it'll need LOTS of testing and verification, as
you can well imagine. Unfortunately, my time will be VERY VERY
limited over the next six weeks to help with this sort of thing -- or
for any cygwin-related stuff. Thesis Defense approaches...
David A. Cobb, Software Engineer, Public Access Advocate
"By God's Grace I am a Christian man, by my actions a great sinner." -- The Way of a Pilgrim; R. M. French, tr.
Life is too short to tolerate crappy software.