Linking to commercial dll's

Linking to commercial dll's

[David Westbury]

I'm attempting to link a C program that I've written to a commercial dll
(non-MS) using gcc in cygwin. I've read about everything I can find about
dll's but I can't seem to make my program access functions in the dll
correctly. My program compiles/links without errors using gcc, and even
runs correctly to some extent. Some functions work, others don't. The
functions that don't work seem to corrupt memory as evidenced by changes in
random variable values. This occurs even when the function return status
indicates success. The commercial package provides a ".dll" file, a ".lib"
file, and a ".h" file. These are obviously intended for use in a Windows
programming environment. I would appreciate if someone knowledgable would
tell me what steps would typically be required to link a program to a well
established commercial dll. For example, what might a typical gcc string
look like? Do gcc switches like "-L", and "-l" apply? Should I be linking
to the .dll or the .lib file?

Dll's seem to be an especially difficult subject in cygwin, requiring
detailed knowlege of the MS way of doing things. Are dll's not
standardized? Is a "cookbook" approach to dll linking not possible? The
issue seems to be a steep hill for an average working programmer, like me,
to climb. This limits the usefulness of cygwin as a programming environment
for me.

BTW, is cygwin intended for MS programmers wanting to explore the Unix
world or is it of more interest to Unix programmers who, like me, have an
occasional need to run Unix programs on Windows?  Programmers coming from
Unix typically won't know much about dll's so a little more introductory
documentation or pointers to such material would seem appropriate for the
cygwin site. I've been to the bookstores and haven't found much help there,
even in Windows programming texts. Apparently dll's simply work correctly
in Windows and require little explanation. One text did say something that
seemed to indicate that I should be linking against the .lib file as it
contains pointers into the .dll fine. This doesn't work for me at all
however.

Here's the gcc string I'm using that results in a partially working
executable:

gcc -g myprog.c  -o myprog  /<path to commercial dll>/filename.dll

Any help or pointers to information would be greatly appreciated.

thanks,
david




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RE: Linking to commercial dll's

[Ralf Habacker]

[-- Attachment #1: Type: text/plain, Size: 4178 bytes --]

> -----Ursprüngliche Nachricht-----
> Von: cygwin-owner@sources.redhat.com
> [mailto:cygwin-owner@sources.redhat.com]Im Auftrag von David Westbury
> Gesendet am: Donnerstag, 15. November 2001 17:11
> An: cygwin@cygwin.com
> Betreff: Linking to commercial dll's
>
> I'm attempting to link a C program that I've written to a commercial dll
> (non-MS) using gcc in cygwin. I've read about everything I can find about
> dll's but I can't seem to make my program access functions in the dll
> correctly. My program compiles/links without errors using gcc, and even
> runs correctly to some extent. Some functions work, others don't. The
> functions that don't work seem to corrupt memory as evidenced by changes in
> random variable values. This occurs even when the function return status
> indicates success. The commercial package provides a ".dll" file, a ".lib"
> file, and a ".h" file. These are obviously intended for use in a Windows
> programming environment. I would appreciate if someone knowledgable would
> tell me what steps would typically be required to link a program to a well
> established commercial dll. For example, what might a typical gcc string
> look like? Do gcc switches like "-L", and "-l" apply? Should I be linking
> to the .dll or the .lib file?
>
> Dll's seem to be an especially difficult subject in cygwin, requiring
> detailed knowlege of the MS way of doing things. Are dll's not
> standardized? Is a "cookbook" approach to dll linking not possible? The
> issue seems to be a steep hill for an average working programmer, like me,
> to climb. This limits the usefulness of cygwin as a programming environment
> for me.
>
> BTW, is cygwin intended for MS programmers wanting to explore the Unix
> world or is it of more interest to Unix programmers who, like me, have an
> occasional need to run Unix programs on Windows?  Programmers coming from
> Unix typically won't know much about dll's so a little more introductory
> documentation or pointers to such material would seem appropriate for the
> cygwin site. I've been to the bookstores and haven't found much help there,
> even in Windows programming texts. Apparently dll's simply work correctly
> in Windows and require little explanation. One text did say something that
> seemed to indicate that I should be linking against the .lib file as it
> contains pointers into the .dll fine. This doesn't work for me at all
> however.
>
> Here's the gcc string I'm using that results in a partially working
> executable:
>
> gcc -g myprog.c  -o myprog  /<path to commercial dll>/filename.dll
>
> Any help or pointers to information would be greatly appreciated.
>
1. For a deeply explanation of the windows dll format look at
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwbgen/html/ms
dn_peeringpe.asp.

2. You have to create a ld compatible import library. You can use the tools
impgen and dlltool for doing this. impgen comes from the libtool package and is
appended to this mail. dlltool is a part of the basic cygwin.

Here is an example for a dll named 'libxslt.dll':

1. compile the tool impgen with the appended source file

	gcc -o impgen impgen.c
	cp ./impgen /usr/local/bin


2. create a file with all exported symbols from the dll (it's called 'def'
file).

	impgen libxslt.dll >libxslt.def


3. create an import library with this def file

	dlltool -d libxslt.def -l libxslt.a


4. Now you can link your app with the dll (in truth with the import library)
	Note:
	There may be some compiling problems with
	the include file from your dll in case of
	redefined symbols or compiler syntay.
	It may be nessesary to patch this.

	gcc -o yourapp yourapp.c -L. -lxslt

	Note2:
	You can verify using the import library with the -Wl,--verbose flag in you
command line.

	gcc -o yourapp yourapp.c -L. -lxslt -Wl,--verbose

	Then you see some lines with

	...
	attempt to open /usr/lib/w32api/libkernel32.dll.a failed
	attempt to open /usr/lib/w32api/libkernel32.a succeeded
	...

	which shows, that you are linking the .a file, not the dll.


Perhaps that helps

PS: To the cygwin cracks: If I have written something wrong, please correct
this.

Regards

Ralf Habacker


[-- Attachment #2: impgen.c --]
[-- Type: application/octet-stream, Size: 3754 bytes --]

 /* impgen.c starts here */
 /*   Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.

  This file is part of GNU libtool.

  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  */

 #include <stdio.h>		/* for printf() */
 #include <unistd.h>		/* for open(), lseek(), read() */
 #include <fcntl.h>		/* for O_RDONLY, O_BINARY */
 #include <string.h>		/* for strdup() */
 
 /* O_BINARY isn't required (or even defined sometimes) under Unix */
 #ifndef O_BINARY
 #define O_BINARY 0
 #endif
 
 static unsigned int
 pe_get16 (fd, offset)
      int fd;
      int offset;
 {
   unsigned char b[2];
   lseek (fd, offset, SEEK_SET);
   read (fd, b, 2);
   return b[0] + (b[1]<<8);
 }

 static unsigned int
 pe_get32 (fd, offset)
     int fd;
     int offset;
 {
   unsigned char b[4];
   lseek (fd, offset, SEEK_SET);
   read (fd, b, 4);
   return b[0] + (b[1]<<8) + (b[2]<<16) + (b[3]<<24);
 }

 static unsigned int
 pe_as32 (ptr)
      void *ptr;
 {
   unsigned char *b = ptr;
   return b[0] + (b[1]<<8) + (b[2]<<16) + (b[3]<<24);
 }

 int
 main (argc, argv)
     int argc;
     char *argv[];
 {
     int dll;
     unsigned long pe_header_offset, opthdr_ofs, num_entries, i;
     unsigned long export_rva, export_size, nsections, secptr, expptr;
     unsigned long name_rvas, nexp;
     unsigned char *expdata, *erva;
     char *filename, *dll_name;

     filename = argv[1];

     dll = open(filename, O_RDONLY|O_BINARY);
     if (dll < 1)
 	return 1;

     dll_name = filename;

     for (i=0; filename[i]; i++)
 	if (filename[i] == '/' || filename[i] == '\\'  || filename[i] == ':')
 	    dll_name = filename + i +1;

     pe_header_offset = pe_get32 (dll, 0x3c);
     opthdr_ofs = pe_header_offset + 4 + 20;
     num_entries = pe_get32 (dll, opthdr_ofs + 92);

     if (num_entries < 1) /* no exports */
 	return 1;

     export_rva = pe_get32 (dll, opthdr_ofs + 96);
     export_size = pe_get32 (dll, opthdr_ofs + 100);
     nsections = pe_get16 (dll, pe_header_offset + 4 +2);
     secptr = (pe_header_offset + 4 + 20 +
 	      pe_get16 (dll, pe_header_offset + 4 + 16));

     expptr = 0;
     for (i = 0; i < nsections; i++)
     {
 	char sname[8];
 	unsigned long secptr1 = secptr + 40 * i;
 	unsigned long vaddr = pe_get32 (dll, secptr1 + 12);
 	unsigned long vsize = pe_get32 (dll, secptr1 + 16);
 	unsigned long fptr = pe_get32 (dll, secptr1 + 20);
 	lseek(dll, secptr1, SEEK_SET);
 	read(dll, sname, 8);
 	if (vaddr <= export_rva && vaddr+vsize > export_rva)
 	{
 	    expptr = fptr + (export_rva - vaddr);
 	    if (export_rva + export_size > vaddr + vsize)
 		export_size = vsize - (export_rva - vaddr);
 	    break;
 	}
     }

     expdata = (unsigned char*)malloc(export_size);
     lseek (dll, expptr, SEEK_SET);
     read (dll, expdata, export_size);
     erva = expdata - export_rva;

     nexp = pe_as32 (expdata+24);
     name_rvas = pe_as32 (expdata+32);

     printf ("EXPORTS\n");
     for (i = 0; i<nexp; i++)
     {
 	unsigned long name_rva = pe_as32 (erva+name_rvas+i*4);
 	printf ("\t%s @ %ld ;\n", erva+name_rva, 1+ i);
     }

     return 0;
 }
 /* impgen.c ends here */


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RE: Linking to commercial dll's

[Ralf Habacker]

Here is another link with some documentation about this stuff

http://www.pms.informatik.uni-muenchen.de/mitarbeiter/panne/cygwin/using_dlls.ht
m

Regards
Ralf


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Re: Linking to commercial dll's

[Tim Prince]

[-- Attachment #1: Type: text/plain, Size: 559 bytes --]

Sven Panne
  Contact Curriculum Vitae Research Teaching Haskell libraries Cygwin stuff
Links Amazing pic...   Sven Panne   NOTE: I've left university and work for
BetaResearch now
----- Original Message -----
From: "Ralf Habacker" <Ralf.Habacker@freenet.de>
To: "Cygwin" <cygwin@sources.redhat.com>
Sent: Saturday, November 17, 2001 11:32 AM
Subject: RE: Linking to commercial dll's


> Here is another link with some documentation about this stuff
>
>
http://www.pms.informatik.uni-muenchen.de/mitarbeiter/panne/cygwin/using_dll
s.ht
> m
>
> Regards
> Ralf


[-- Attachment #2: star.gif --]
[-- Type: application/octet-stream, Size: 89 bytes --]

RE: Linking to commercial dll's

[Ralf Habacker]

> -----Ursprüngliche Nachricht-----
> Von: cygwin-owner@sources.redhat.com
> [mailto:cygwin-owner@sources.redhat.com]Im Auftrag von Ralf Habacker
> Gesendet am: Freitag, 16. November 2001 22:03
> An: Cygwin
> Betreff: RE: Linking to commercial dll's
<snip>

> 1. For a deeply explanation of the windows dll format look at
> http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwbg
> en/html/ms
> dn_peeringpe.asp.
>
> 2. You have to create a ld compatible import library. You can use the tools
> impgen and dlltool for doing this. impgen comes from the libtool
> package and is
> appended to this mail. dlltool is a part of the basic cygwin.
>
> Here is an example for a dll named 'libxslt.dll':
>
> 1. compile the tool impgen with the appended source file
>
> 	gcc -o impgen impgen.c
> 	cp ./impgen /usr/local/bin
>
>
> 2. create a file with all exported symbols from the dll (it's called 'def'
> file).
>
> 	impgen libxslt.dll >libxslt.def

I should note that there is another tool pexports, which does the same as impgen
with the enhancement of exporting data symbols
You can found pexports under http://www.is.lg.ua/~paul/devel/binutils.html

Regards
Ralf



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Re: Linking to commercial dll's

[Charles Wilson]

David Westbury wrote:

> 
> Dll's seem to be an especially difficult subject in cygwin, requiring
> detailed knowlege of the MS way of doing things.

Unfortunately, yes.

> Are dll's not
> standardized? 

Unfortunately, no.

> Is a "cookbook" approach to dll linking not possible? 

Not yet.  Soon this will be possible, but not quite yet (we're still
waiting for the final piece, libtool-with-autoimport-support).  Work is
progressing.

> The
> issue seems to be a steep hill for an average working programmer, like
me,
> to climb. 

Yep.  I'm afraid that right now, you must actually learn some new
stuff.  Eventually it will be turnkey, but not yet.

See http://www.neuro.gatech.edu/users/cwilson/cygutils/V1.1/dll-stuff 

--Chuck

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Re: Linking to commercial dll's

[David Westbury]

Jade,

I tried your recommended gcc string but no luck. Compiler reports that it
can't resolve references to functions in the dll. Same as if I specify no
filename.lib at all.

Thanks for reponse. Any other ideas greatly appreciated.

david





                                                                                                                   
                    Jade                                                                                           
                    <jade@titan.um       To:     "David Westbury" <David.Westbury@ipaper.com>                      
                    cc.cu>               cc:                                                                       
                                         Subject:     Re: Linking to commercial dll's                              
                    11/15/2001                                                                                     
                    08:44 AM                                                                                       
                                                                                                                   
                                                                                                                   




El jue, 15 nov 2001, escribiste:
>
> gcc -g myprog.c  -o myprog  /<path to commercial dll>/filename.dll

Sorry for sending tthis twice but i made a mistake
you should type :

>gcc -g myprog.c  -o myprog  /<path to commercial lib files of
commarcial dll>/filename.lib --
Jade




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