Re: DllMain implementation

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 12 Feb 2008 13:14:22 -0600

Message-ID:

<Ow0NTtabIHA.4140@TK2MSFTNGP04.phx.gbl>

George wrote:

Thanks Tim,

Sorry I miss the direction in your reply.

1. What is the relationship of user provided DllMain and dllcrt0.obj?
User provided DllMain will be compiled into dllcrt0.obj (but I do not
agree, suppose we implement DllMain in foo.cpp, and then binary code
DllMain should be in foo.obj)?

No, no user code is ever placed in dllcrt0.obj.

The actual DLL entry point is always "DllMainCRTStartup" in dllcrt0.obj. It
calls "DllMain", which may be provided by the user, when it is not, a copy
in dllmain.obj inside the CRT is found and used.

2. What is the relationship between dllcrt0.obj and DllMain.obj?

regards,
George

"Tim Roberts" wrote:

"William DePalo [MVP VC++]" <willd.no.spam@mvps.org> wrote:

Here is what I'm sure of: The CRT has a source file named
Dllmain.c. The comments say it is used when no other version is
provided.

Here is what I'm not sure of: I think that it is a simple matter
specifying Dllmain.obj on the linker command line but _after_ all
of the other object modules and libraries. If the reference is
resolved early, the CRT's Dllmain won't be linked. If not, it will

It's not even that hard. DllMain.obj is included in the run-time
library (msvcrt.lib or libcmt.lib).

When we link the DLL, we specify /entry:_DllMainCRTStartup@12. The
linker goes to find that in its library list, and finds it in
dllcrt0.obj in the run-time library. That routine references
DllMain, so the linker goes to find it. If DllMain is already in
the list of known symbols (that is, if it was part of the program),
it links to that. Otherwise, it goes back to the libraries, where
it will find it in dllmain.obj. --
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.