Thanks for the very detailed info. I am certainly a lot wiser now!
Graham
On 01-11-2018 11:40 pm, R Smith wrote:
On 2018/11/01 9:51 AM, Graham Hardman wrote:
I am pleased about this, although still puzzled by the fact that the
windows LoadLibrary call returned a non null pointer and windows help
file implies that it is ok for multiple copies of same name libraries
to be loaded together.
This is an old Windows problem, one which I think they (M$) have
self-regretted so many times. What I deduced long ago after resolving
a similar problem (and I could be wrong, I don't have insider info on
it) is that Windows' idea of DLLs was to have it so the DLLs live in a
common area in the Windows system path (typically
c:\program files\common files\system) from where it can be shared to
all the lovely apps everywhere needing to use it. AND, how wonderful
would it be if, when a DLL is updated, everyone benefits from the new
version. Right?
So what they did is the DLL loader seems to have a rule of loading the
DLL FIRST from the windows common DLL area, and only if not available
there (because that's where the updatedest DLL would be, right?) then
see if local DLL exists next to the app and loads it.
Most applications play nice and do not install their greasy little
DLLs to the Widows DLL common area, but just hug them locally (in the
app folder). But some, thinking probably they were doing the right
thing, do install DLLs to the common area, and sometimes this includes
a DLL like sqlite3.dll which starts causing problems for everyone else
using it, because they don't actually distribute/install newest
versions, so everything else loading the same library gets the last
installed version of the last app to successfully install to the
common area.
Even this wouldn't be such a catastrophe if the common area DLLs
weren't usually "in use" by the services it supports, so it can't be
updated by anyone else, unless you KNOW which service(s) is/are using
it and stop all of them.
This leaves a new application install with very few options, if you
can't get Windows to use your local DLL, nor can you update the
"in-use" one in the common area... How then do you cause the system to
use your newest DLL? There are a few hacky ways:
1 - The easiest is to just use a different name (and to do your part
by not installing it for all).
2 - Use a side-by-side isolation assembly.
See:
https://docs.microsoft.com/en-us/windows/desktop/SbsCs/isolated-applications-and-side-by-side-assemblies-portal
This option is fine if you develop new software to be isolated
from any DLL conflicts from the start, but it's a LOT of effort to
just fix one conflict.
3 - This is my favourite - Use DLL redirection, which involves simply
adding a .local file to your app path (so myapp.exe must be
accompanied by myapp.local).
This does require and extra file to maintain, but it's relatively
easy and means you don't need to be concerned with sorting out DLL
naming or isolation.
I believe the file's presence matters, but its content is
irrelevant (I usually add a line like "** Force local DLLs **" as the
content), which makes Widows look locally first when loading DLLs.
See:
https://docs.microsoft.com/en-us/windows/desktop/dlls/dynamic-link-library-redirection
NB! - The .local file will force local redirection for ALL DLLs you
use, some of which may not be intended, like if you supply xxx.dll as
a back-up, but would prefer Windows to use it's internal (and possibly
newer) xxx.dll if present.
NB2! - As is the standard for M$, every fix must have a fallback... so
known common DLLs cannot be redirected like this (there is a list
maintained in the Windows registry in
HKLM/SYSTEM/CurrentControlSet/Control/Session Manager/KnownDLLs/ of
what Windows regards as "common" DLLs. Make sure your DLL is not in
there, for if it is, options 1 & 2 above are your only remaining
venues).
HTH - Cheers!
Ryan
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