Importing a variable from a DLL

Importing a variable from a DLL

[James Stern]

The last time I posted to this list, I was asking a
question about a mysterious symbol,
_size_of_stack_reserve__, that gdb mentioned when my
program crashed.

Mumit Khan explained that this simply meant the code
had a bug elsewhere.  I looked and sure enough, I
found an extern variable that the DLL defines and the
main executable (main.exe) uses.

Declaring the variable __declspec (dllimport) in the
appropriate source file solved that.  But then I reran
the program and encountered the same problem with
another variable.

So I hunted for documentation on dllimport.  I found
only a few lines in full-man/info/* but more in the
MSVC documentation.

As I understand it, the situtation is this:  Suppose
that source file s1.cc used in main.exe references
extern variable v1 from the DLL.  Then s1.cc has to
declare v1 as __declspec (dllimport).  If s2.cc
through s20.cc also use v1, they must do likewise.

If this is true, I'll have to change about 140
declarations in the legacy application I'm porting. 
Each will need a macro, M, that expands to either
__declspec (dllimport) or to nothing.

These declarations appear in header files so I'll also
have to properly define M in every source file that
includes one of these headers.

Is there an easier way to do this?  For example, is
there a way to specify at link time that a variable is
imported?

I realize that the application has too many extern
variables but it's legacy and not mine to rewrite.

I also realize that fixups of this nature are harder
to do for variables than for functions but I thought I
should doublecheck anyway.

Thank you.
===
-- 
Opinions expressed above are not necessarily my employer's.
James M. Stern
ITG Inc.  Culver City, CA (213) 270-7955
__________________________________________________
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Bid and sell for free at http://auctions.yahoo.com


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Re: Importing a variable from a DLL

[Mumit Khan]

James Stern <jsternitg@yahoo.com> writes:
> 
> As I understand it, the situtation is this:  Suppose
> that source file s1.cc used in main.exe references
> extern variable v1 from the DLL.  Then s1.cc has to
> declare v1 as __declspec (dllimport).  If s2.cc
> through s20.cc also use v1, they must do likewise.
> 
> If this is true, I'll have to change about 140
> declarations in the legacy application I'm porting. 
> Each will need a macro, M, that expands to either
> __declspec (dllimport) or to nothing.
> 
> These declarations appear in header files so I'll also
> have to properly define M in every source file that
> includes one of these headers.
> 
> Is there an easier way to do this?  For example, is
> there a way to specify at link time that a variable is
> imported?

Not really. I did research this issue when I first started
the dllimport/dllexport implemention, but came to the
conclusion that it just won't work given how DLLs work.

There is no silver bullet, sorry. 

Most software suffer from the unfortunate reality of bad 
modularization, and it gets in the way of making DLLs since 
the "ownership" of symbols are spread across headers shared 
by various modules. Languages like C require extra care,
which just makes matters worse.

My first lecture in a corporate software engr course I used
to teach involved long boring discussions on the importance
of design before writing the first line of code. One essential,
and often overlooked, part of the design process is the layout 
of the modules in terms of files (since files are what we typically
use in most implementations). Too bad that I myself often break 
that guideline on a regular basis.

> I realize that the application has too many extern
> variables but it's legacy and not mine to rewrite.

First thing I do is to make sure no implementation file (eg.,
C and C++ files, not headers) *ever* use extern statements,
and that each header belongs to one particular module. If your 
source files do contain extern decls, something is wrong and 
needs to be handled. My last group had the misfortune of porting 
a project of few million lines of C++, with a bit of C glue, 
and we had two engineers working 50% of the time tackling these 
issues. We did manage to turn all of the 50+ shared libraries
into DLLs, but it was a long fight.

> I also realize that fixups of this nature are harder
> to do for variables than for functions but I thought I
> should doublecheck anyway.

Functions are trivial to handle via thunks (via import
libraries created by dlltool or implib or some such tool);
it's the variables that you have to worry about, which have
no traditional way to "fix up" in the DLL model.

You'll just have to deal with it. 

I know of one case where the developers talked about writing
a custom PE loader to deal with this (at the expense of 
compatibility of existing tools), but that project never saw
daylight outside of the South Bay cafes.

Bearer of bad news,
Mumit


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Re[2]: Importing a variable from a DLL

[Paul Sokolovsky]

Hello Mumit,

Mumit Khan <khan@xraylith.wisc.EDU> wrote:


MK> There is no silver bullet, sorry.

[]

MK> Functions are trivial to handle via thunks (via import
MK> libraries created by dlltool or implib or some such tool);
MK> it's the variables that you have to worry about, which have
MK> no traditional way to "fix up" in the DLL model.

    But at least there might be achieved proper diagnostics of such
issues, if dlltool would correctly mark data symbols in def, that
implib would only contain __imp_<symbol> and not <symbol>, and we'd
get link error when linking with object which doesn't have
__declspec(dllimport) on that symbol, and not runtime segfaults,
as we have now.

MK> You'll just have to deal with it.

MK> I know of one case where the developers talked about writing
MK> a custom PE loader to deal with this (at the expense of
MK> compatibility of existing tools), but that project never saw
MK> daylight outside of the South Bay cafes.

    It would be better just use ELF and forget about all the gore.
But rules defined by others.

MK> Bearer of bad news,
MK> Mumit



Best regards,
 Paul                            mailto:paul-ml@is.lg.ua



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Re: Re[2]: Importing a variable from a DLL

[Mumit Khan]

Paul Sokolovsky <paul-ml@is.lg.ua> writes:
> 
>     But at least there might be achieved proper diagnostics of such
> issues, if dlltool would correctly mark data symbols in def, that
> implib would only contain __imp_<symbol> and not <symbol>, and we'd
> get link error when linking with object which doesn't have
> __declspec(dllimport) on that symbol, and not runtime segfaults,
> as we have now.

Sure, but it doesn't solve James' problem. If James could create the
import libraries with all the data marked DATA, it would imply he
already *knows* which these data items are! Once you know, one tricky
part of porting a legacy application over -- after this, it's a matter 
of searching for the symbol names and attaching dllimport attribute
in user code.

However, your point is well taken that we should go and fix dlltool.
I believe I did fix it locally at one point, but definitely didn't
submit the changes (don't remember why, perhaps it didn't work).
Contributions welcome of course.

>     It would be better just use ELF and forget about all the gore.
> But rules defined by others.

Welcome to real life.

Regards,
Mumit


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Re[4]: Importing a variable from a DLL

[Paul Sokolovsky]

Hello Mumit,

Mumit Khan <khan@xraylith.wisc.EDU> wrote:

MK> Paul Sokolovsky <paul-ml@is.lg.ua> writes:
>>
>>     But at least there might be achieved proper diagnostics of such
>> issues, if dlltool would correctly mark data symbols in def, that
>> implib would only contain __imp_<symbol> and not <symbol>, and we'd
>> get link error when linking with object which doesn't have
>> __declspec(dllimport) on that symbol, and not runtime segfaults,
>> as we have now.

MK> Sure, but it doesn't solve James' problem. If James could create the
MK> import libraries with all the data marked DATA, it would imply he
MK> already *knows* which these data items are!

    I am still feeling misunderstood. Let me go with it once again,
with examples.

    So, dlltool has ability to create implib from def - it's all nice
with it, it supports DATA, sure.

    But it has ability to dump all defined symbols into def file -
about this operation I am talking. So, suppose we have library of

---a.h
extern int data;
int code();
----
---a.c

int data;

int code()
{
  return 0;
}
----

    We compile a:
gcc -c a.c

    And now creating def of it:

dlltool a.o --export-all --output-def a.def

    What we now get is:
---a.def
; G:\GCC-2.95\BIN\DLLTOOL.EXE --export-all --output-def a.def a.o
EXPORTS
        data @ 1 ;
        code @ 2 ;
----
and it's bad! Suppose we'd get instead
---a.def
; G:\GCC-2.95\BIN\DLLTOOL.EXE --export-all --output-def a.def a.o
EXPORTS
        data @ 1 DATA;
        code @ 2 ;
----

    We create implib from that:

dlltool --def a.def --ouput-lib liba.a

And then compile client application:
---client.c
#include "a.h"

int main()
{
  data=1;
}
----
When we link it

gcc client.o -la

What we get? Of course, "client.c: undefined reference to `data'"!
We now grepping, finding out that 'a.h' declares that 'data' and tag
it __declspec(dllimport). Repeat while needed.

   So, it doesn't solve problem we're talking about automagically, but
it defines clean procedure, which, while being incremental, will
converge fast. Much faster than having to catch segfaults for hours
and then having users complain of occasional ones for months, as we
have now.



MK> Regards,
MK> Mumit





Best regards,
 Paul                            mailto:paul-ml@is.lg.ua



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Re: Re[4]: Importing a variable from a DLL

[Mumit Khan]

Paul Sokolovsky <paul-ml@is.lg.ua> writes:
> 
>     I am still feeling misunderstood. Let me go with it once again,
> with examples.
> 
>     So, dlltool has ability to create implib from def - it's all nice
> with it, it supports DATA, sure.

No, no, I do get your point. And, it's a very good one that I had 
ignored earlier in this thread.

I'm currently adding changes to GCC so that exported data items have
the correct "data" tag in the .drectve section. This will enable us 
to handle this in dlltool and dllwrap when the import libraries are 
created. I'll hopefully test the gcc and dlltool changes this weekend
(it's pretty trivial).

With this change, you can do the following:
  
  /* dll file. */
  __declspec(dllexport) int exported_var;
  __declspec(dllexport) void exported_fund () { }
  /* end dll file. */

  $ gcc -c dll.c
  $ dllwrap -o foo.dll --output-lib libfoo.a dll.o

Now the import library will have what Paul suggests.

Regards,
Mumit


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Re[6]: Importing a variable from a DLL

[Paul Sokolovsky]

Hello Mumit,

Mumit Khan <khan@xraylith.wisc.EDU> wrote:


MK> I'm currently adding changes to GCC so that exported data items have
MK> the correct "data" tag in the .drectve section. This will enable us
MK> to handle this in dlltool and dllwrap when the import libraries are
MK> created. I'll hopefully test the gcc and dlltool changes this weekend
MK> (it's pretty trivial).

    I also made mine, it requires changing only dlltool.c: (against
b20 distribution)

*** dlltool.c.org       Tue Oct 27 03:04:10 1998
--- dlltool.c   Thu Aug 26 22:31:32 1999
***************
*** 1144,1149 ****
--- 1144,1150 ----
      {
        asymbol *sym;
        const char *symbol_name;
+       int data;

        sym = bfd_minisymbol_to_symbol (abfd, false, from, store);
        if (sym == NULL)
***************
*** 1153,1166 ****
        if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
        ++symbol_name;

!       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, 0);

        if (add_stdcall_alias && strchr (symbol_name, '@'))
          {
          char *exported_name = xstrdup (symbol_name);
          char *atsym = strchr (exported_name, '@');
          *atsym = '\0';
!         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, 0);
        }
      }
  }
--- 1154,1171 ----
        if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
        ++symbol_name;

! /* Don't use bfd_decode_symclass() - remember, symbols with constant
!    values dumped to code section */
!       data = !(sym->flags&BSF_FUNCTION);
!
!       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, data);

        if (add_stdcall_alias && strchr (symbol_name, '@'))
          {
          char *exported_name = xstrdup (symbol_name);
          char *atsym = strchr (exported_name, '@');
          *atsym = '\0';
!         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, data);
        }
      }
  }

MK> Now the import library will have what Paul suggests.

MK> Regards,
MK> Mumit


Best regards,
 Paul                            mailto:paul-ml@is.lg.ua



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Re: Re[6]: Importing a variable from a DLL

[Mumit Khan]

Paul Sokolovsky <paul-ml@is.lg.ua> writes:
> 
>     I also made mine, it requires changing only dlltool.c: (against
> b20 distribution)
> 
> *** dlltool.c.org       Tue Oct 27 03:04:10 1998
> --- dlltool.c   Thu Aug 26 22:31:32 1999
> ***************
> *** 1144,1149 ****
> --- 1144,1150 ----
>       {
>         asymbol *sym;
>         const char *symbol_name;
> +       int data;
> 
>         sym = bfd_minisymbol_to_symbol (abfd, false, from, store);
>         if (sym == NULL)
> ***************
> *** 1153,1166 ****
>         if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
>         ++symbol_name;
> 
> !       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, 0);
> 
>         if (add_stdcall_alias && strchr (symbol_name, '@'))
>           {
>           char *exported_name = xstrdup (symbol_name);
>           char *atsym = strchr (exported_name, '@');
>           *atsym = '\0';
> !         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, 0);
>         }
>       }
>   }
> --- 1154,1171 ----
>         if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
>         ++symbol_name;
> 
> ! /* Don't use bfd_decode_symclass() - remember, symbols with constant
> !    values dumped to code section */
> !       data = !(sym->flags&BSF_FUNCTION);
> !
> !       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, data);
> 
>         if (add_stdcall_alias && strchr (symbol_name, '@'))
>           {
>           char *exported_name = xstrdup (symbol_name);
>           char *atsym = strchr (exported_name, '@');
>           *atsym = '\0';
> !         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, data);
>         }
>       }
>   }
> 

Thanks.

This is only a partial solution for --export-all case. The other one is
handled by looking at .drectve section for symbols exported explicitly
via dllexport attribute. See scan_drectve_symbols in dlltool.c. A small
GCC change is necessary to support that one. 

btw, please use the -p option when creating diffs; it helps to see the
function name in the patch.

Regards,
Mumit


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Re: Re[6]: Importing a variable from a DLL

[Mumit Khan]

Paul Sokolovsky <paul-ml@is.lg.ua> writes:
> 
>     I also made mine, it requires changing only dlltool.c: (against
> b20 distribution)
> 
> *** dlltool.c.org       Tue Oct 27 03:04:10 1998
> --- dlltool.c   Thu Aug 26 22:31:32 1999
> ***************
> *** 1144,1149 ****
> --- 1144,1150 ----
>       {
>         asymbol *sym;
>         const char *symbol_name;
> +       int data;
> 
>         sym = bfd_minisymbol_to_symbol (abfd, false, from, store);
>         if (sym == NULL)
> ***************
> *** 1153,1166 ****
>         if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
>         ++symbol_name;
> 
> !       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, 0);
> 
>         if (add_stdcall_alias && strchr (symbol_name, '@'))
>           {
>           char *exported_name = xstrdup (symbol_name);
>           char *atsym = strchr (exported_name, '@');
>           *atsym = '\0';
> !         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, 0);
>         }
>       }
>   }
> --- 1154,1171 ----
>         if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
>         ++symbol_name;
> 
> ! /* Don't use bfd_decode_symclass() - remember, symbols with constant
> !    values dumped to code section */
> !       data = !(sym->flags&BSF_FUNCTION);
> !
> !       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, data);
> 
>         if (add_stdcall_alias && strchr (symbol_name, '@'))
>           {
>           char *exported_name = xstrdup (symbol_name);
>           char *atsym = strchr (exported_name, '@');
>           *atsym = '\0';
> !         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, data);
>         }
>       }
>   }
> 

Thanks.

This is only a partial solution for --export-all case. The other one is
handled by looking at .drectve section for symbols exported explicitly
via dllexport attribute. See scan_drectve_symbols in dlltool.c. A small
GCC change is necessary to support that one. 

btw, please use the -p option when creating diffs; it helps to see the
function name in the patch.

Regards,
Mumit


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Re[6]: Importing a variable from a DLL

[Paul Sokolovsky]

Hello Mumit,

Mumit Khan <khan@xraylith.wisc.EDU> wrote:


MK> I'm currently adding changes to GCC so that exported data items have
MK> the correct "data" tag in the .drectve section. This will enable us
MK> to handle this in dlltool and dllwrap when the import libraries are
MK> created. I'll hopefully test the gcc and dlltool changes this weekend
MK> (it's pretty trivial).

    I also made mine, it requires changing only dlltool.c: (against
b20 distribution)

*** dlltool.c.org       Tue Oct 27 03:04:10 1998
--- dlltool.c   Thu Aug 26 22:31:32 1999
***************
*** 1144,1149 ****
--- 1144,1150 ----
      {
        asymbol *sym;
        const char *symbol_name;
+       int data;

        sym = bfd_minisymbol_to_symbol (abfd, false, from, store);
        if (sym == NULL)
***************
*** 1153,1166 ****
        if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
        ++symbol_name;

!       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, 0);

        if (add_stdcall_alias && strchr (symbol_name, '@'))
          {
          char *exported_name = xstrdup (symbol_name);
          char *atsym = strchr (exported_name, '@');
          *atsym = '\0';
!         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, 0);
        }
      }
  }
--- 1154,1171 ----
        if (bfd_get_symbol_leading_char (abfd) == symbol_name[0])
        ++symbol_name;

! /* Don't use bfd_decode_symclass() - remember, symbols with constant
!    values dumped to code section */
!       data = !(sym->flags&BSF_FUNCTION);
!
!       def_exports (xstrdup (symbol_name) , 0, -1, 0, 0, data);

        if (add_stdcall_alias && strchr (symbol_name, '@'))
          {
          char *exported_name = xstrdup (symbol_name);
          char *atsym = strchr (exported_name, '@');
          *atsym = '\0';
!         def_exports (exported_name, xstrdup (symbol_name), -1, 0, 0, data);
        }
      }
  }

MK> Now the import library will have what Paul suggests.

MK> Regards,
MK> Mumit


Best regards,
 Paul                            mailto:paul-ml@is.lg.ua



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Re: Re[4]: Importing a variable from a DLL

[Mumit Khan]

Paul Sokolovsky <paul-ml@is.lg.ua> writes:
> 
>     I am still feeling misunderstood. Let me go with it once again,
> with examples.
> 
>     So, dlltool has ability to create implib from def - it's all nice
> with it, it supports DATA, sure.

No, no, I do get your point. And, it's a very good one that I had 
ignored earlier in this thread.

I'm currently adding changes to GCC so that exported data items have
the correct "data" tag in the .drectve section. This will enable us 
to handle this in dlltool and dllwrap when the import libraries are 
created. I'll hopefully test the gcc and dlltool changes this weekend
(it's pretty trivial).

With this change, you can do the following:
  
  /* dll file. */
  __declspec(dllexport) int exported_var;
  __declspec(dllexport) void exported_fund () { }
  /* end dll file. */

  $ gcc -c dll.c
  $ dllwrap -o foo.dll --output-lib libfoo.a dll.o

Now the import library will have what Paul suggests.

Regards,
Mumit


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Re[4]: Importing a variable from a DLL

[Paul Sokolovsky]

Hello Mumit,

Mumit Khan <khan@xraylith.wisc.EDU> wrote:

MK> Paul Sokolovsky <paul-ml@is.lg.ua> writes:
>>
>>     But at least there might be achieved proper diagnostics of such
>> issues, if dlltool would correctly mark data symbols in def, that
>> implib would only contain __imp_<symbol> and not <symbol>, and we'd
>> get link error when linking with object which doesn't have
>> __declspec(dllimport) on that symbol, and not runtime segfaults,
>> as we have now.

MK> Sure, but it doesn't solve James' problem. If James could create the
MK> import libraries with all the data marked DATA, it would imply he
MK> already *knows* which these data items are!

    I am still feeling misunderstood. Let me go with it once again,
with examples.

    So, dlltool has ability to create implib from def - it's all nice
with it, it supports DATA, sure.

    But it has ability to dump all defined symbols into def file -
about this operation I am talking. So, suppose we have library of

---a.h
extern int data;
int code();
----
---a.c

int data;

int code()
{
  return 0;
}
----

    We compile a:
gcc -c a.c

    And now creating def of it:

dlltool a.o --export-all --output-def a.def

    What we now get is:
---a.def
; G:\GCC-2.95\BIN\DLLTOOL.EXE --export-all --output-def a.def a.o
EXPORTS
        data @ 1 ;
        code @ 2 ;
----
and it's bad! Suppose we'd get instead
---a.def
; G:\GCC-2.95\BIN\DLLTOOL.EXE --export-all --output-def a.def a.o
EXPORTS
        data @ 1 DATA;
        code @ 2 ;
----

    We create implib from that:

dlltool --def a.def --ouput-lib liba.a

And then compile client application:
---client.c
#include "a.h"

int main()
{
  data=1;
}
----
When we link it

gcc client.o -la

What we get? Of course, "client.c: undefined reference to `data'"!
We now grepping, finding out that 'a.h' declares that 'data' and tag
it __declspec(dllimport). Repeat while needed.

   So, it doesn't solve problem we're talking about automagically, but
it defines clean procedure, which, while being incremental, will
converge fast. Much faster than having to catch segfaults for hours
and then having users complain of occasional ones for months, as we
have now.



MK> Regards,
MK> Mumit





Best regards,
 Paul                            mailto:paul-ml@is.lg.ua



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Re: Re[2]: Importing a variable from a DLL

[Mumit Khan]

Paul Sokolovsky <paul-ml@is.lg.ua> writes:
> 
>     But at least there might be achieved proper diagnostics of such
> issues, if dlltool would correctly mark data symbols in def, that
> implib would only contain __imp_<symbol> and not <symbol>, and we'd
> get link error when linking with object which doesn't have
> __declspec(dllimport) on that symbol, and not runtime segfaults,
> as we have now.

Sure, but it doesn't solve James' problem. If James could create the
import libraries with all the data marked DATA, it would imply he
already *knows* which these data items are! Once you know, one tricky
part of porting a legacy application over -- after this, it's a matter 
of searching for the symbol names and attaching dllimport attribute
in user code.

However, your point is well taken that we should go and fix dlltool.
I believe I did fix it locally at one point, but definitely didn't
submit the changes (don't remember why, perhaps it didn't work).
Contributions welcome of course.

>     It would be better just use ELF and forget about all the gore.
> But rules defined by others.

Welcome to real life.

Regards,
Mumit


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Re[2]: Importing a variable from a DLL

[Paul Sokolovsky]

Hello Mumit,

Mumit Khan <khan@xraylith.wisc.EDU> wrote:


MK> There is no silver bullet, sorry.

[]

MK> Functions are trivial to handle via thunks (via import
MK> libraries created by dlltool or implib or some such tool);
MK> it's the variables that you have to worry about, which have
MK> no traditional way to "fix up" in the DLL model.

    But at least there might be achieved proper diagnostics of such
issues, if dlltool would correctly mark data symbols in def, that
implib would only contain __imp_<symbol> and not <symbol>, and we'd
get link error when linking with object which doesn't have
__declspec(dllimport) on that symbol, and not runtime segfaults,
as we have now.

MK> You'll just have to deal with it.

MK> I know of one case where the developers talked about writing
MK> a custom PE loader to deal with this (at the expense of
MK> compatibility of existing tools), but that project never saw
MK> daylight outside of the South Bay cafes.

    It would be better just use ELF and forget about all the gore.
But rules defined by others.

MK> Bearer of bad news,
MK> Mumit



Best regards,
 Paul                            mailto:paul-ml@is.lg.ua



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Re: Importing a variable from a DLL

[Mumit Khan]

James Stern <jsternitg@yahoo.com> writes:
> 
> As I understand it, the situtation is this:  Suppose
> that source file s1.cc used in main.exe references
> extern variable v1 from the DLL.  Then s1.cc has to
> declare v1 as __declspec (dllimport).  If s2.cc
> through s20.cc also use v1, they must do likewise.
> 
> If this is true, I'll have to change about 140
> declarations in the legacy application I'm porting. 
> Each will need a macro, M, that expands to either
> __declspec (dllimport) or to nothing.
> 
> These declarations appear in header files so I'll also
> have to properly define M in every source file that
> includes one of these headers.
> 
> Is there an easier way to do this?  For example, is
> there a way to specify at link time that a variable is
> imported?

Not really. I did research this issue when I first started
the dllimport/dllexport implemention, but came to the
conclusion that it just won't work given how DLLs work.

There is no silver bullet, sorry. 

Most software suffer from the unfortunate reality of bad 
modularization, and it gets in the way of making DLLs since 
the "ownership" of symbols are spread across headers shared 
by various modules. Languages like C require extra care,
which just makes matters worse.

My first lecture in a corporate software engr course I used
to teach involved long boring discussions on the importance
of design before writing the first line of code. One essential,
and often overlooked, part of the design process is the layout 
of the modules in terms of files (since files are what we typically
use in most implementations). Too bad that I myself often break 
that guideline on a regular basis.

> I realize that the application has too many extern
> variables but it's legacy and not mine to rewrite.

First thing I do is to make sure no implementation file (eg.,
C and C++ files, not headers) *ever* use extern statements,
and that each header belongs to one particular module. If your 
source files do contain extern decls, something is wrong and 
needs to be handled. My last group had the misfortune of porting 
a project of few million lines of C++, with a bit of C glue, 
and we had two engineers working 50% of the time tackling these 
issues. We did manage to turn all of the 50+ shared libraries
into DLLs, but it was a long fight.

> I also realize that fixups of this nature are harder
> to do for variables than for functions but I thought I
> should doublecheck anyway.

Functions are trivial to handle via thunks (via import
libraries created by dlltool or implib or some such tool);
it's the variables that you have to worry about, which have
no traditional way to "fix up" in the DLL model.

You'll just have to deal with it. 

I know of one case where the developers talked about writing
a custom PE loader to deal with this (at the expense of 
compatibility of existing tools), but that project never saw
daylight outside of the South Bay cafes.

Bearer of bad news,
Mumit


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Importing a variable from a DLL

[James Stern]

The last time I posted to this list, I was asking a
question about a mysterious symbol,
_size_of_stack_reserve__, that gdb mentioned when my
program crashed.

Mumit Khan explained that this simply meant the code
had a bug elsewhere.  I looked and sure enough, I
found an extern variable that the DLL defines and the
main executable (main.exe) uses.

Declaring the variable __declspec (dllimport) in the
appropriate source file solved that.  But then I reran
the program and encountered the same problem with
another variable.

So I hunted for documentation on dllimport.  I found
only a few lines in full-man/info/* but more in the
MSVC documentation.

As I understand it, the situtation is this:  Suppose
that source file s1.cc used in main.exe references
extern variable v1 from the DLL.  Then s1.cc has to
declare v1 as __declspec (dllimport).  If s2.cc
through s20.cc also use v1, they must do likewise.

If this is true, I'll have to change about 140
declarations in the legacy application I'm porting. 
Each will need a macro, M, that expands to either
__declspec (dllimport) or to nothing.

These declarations appear in header files so I'll also
have to properly define M in every source file that
includes one of these headers.

Is there an easier way to do this?  For example, is
there a way to specify at link time that a variable is
imported?

I realize that the application has too many extern
variables but it's legacy and not mine to rewrite.

I also realize that fixups of this nature are harder
to do for variables than for functions but I thought I
should doublecheck anyway.

Thank you.
===
-- 
Opinions expressed above are not necessarily my employer's.
James M. Stern
ITG Inc.  Culver City, CA (213) 270-7955
__________________________________________________
Do You Yahoo!?
Bid and sell for free at http://auctions.yahoo.com


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