From: sos AT prospect DOT com DOT ru (Sergey Okhapkin)
Subject: RE: Another problem with DLL's
22 Mar 1997 07:04:40 -0800
Sender: daemon AT cygnus DOT com
Approved: cygnus DOT gnu-win32 AT cygnus DOT com
Distribution: cygnus
Message-ID: <01BC36B5.D95F4050.cygnus.gnu-win32@sos>
Original-To: Valery Fine <Valeri DOT Faine AT cern DOT ch>, "'John Cerney'" <j-cerney1 AT ti DOT com>
Original-Cc: "gnu-win32 AT cygnus DOT com" <gnu-win32 AT cygnus DOT com>,
        "'Geoffrey Noer'"
	 <noer AT cygnus DOT com>,
        "'Jeremy Allison'" <jra AT cygnus DOT com>
Encoding: 51 TEXT, 52 UUENCODE
X-MS-Attachment: dll.diff 0 00-00-1980 00:00
Original-Sender: owner-gnu-win32 AT cygnus DOT com

John Cerney wrote:
>
> When running, the thing crashes when the DLL function is called. An 
access
> violation occurs in cygwin.dll. The foo.dll that is built uses cygwin.dll 
for
> its printf function. If I take the printf out of the foo.dll code the 
VC++
> main2.exe will run just fine.
>

This problem is due to struct reent_data in a dll and pointer to it 
(_impure_ptr) remains uninitialized after dll loading. The patch below 
solves this problem. This patch moves reent_data from application's private 
address space to cygwin.dll and adds pointer to this structure to 
cygwin.dll export table. Auto-initialisation code of your dll now can 
initialize _impure_ptr properly. Also, this patch modifies fork() code to 
copy contents of parent's reent_data to the child. Allpy this patch (it 
modifies files dcrt0.cc, fork.cc and winsup.h), add "reent_data" to export 
list in cygwin.din, and rebuild cygwin.dll. Add the following file to your 
dll:

#include <windows.h>
#include <stdio.h>
extern struct _reent *_impure_ptr, 
*__imp_reent_data;
int WINAPI dll_main(HANDLE a, DWORD reason, void *q)
{ 
switch (reason){
   case DLL_PROCESS_ATTACH: break;
   case 
DLL_PROCESS_DETACH: break;
   case DLL_THREAD_ATTACH:  break;
   case 
DLL_THREAD_DETACH:  break;
  };
  _impure_ptr=__imp_reent_data;
  return 
1;
}
main(){}

and build your dll (I've used three-pass method as for cygwin.dll). Ld's 
command line should contain "-e _dll_main AT 12".

 

--
Sergey Okhapkin
Moscow, Russia
Looking for a job

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`
end

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