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Date: Thu, 10 May 2007 11:06:42 -0700
From: "kalasad mailu" <kalasadmailu AT gmail DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Please Help!! Calling Socket function in a dll file (that is created using cygwin library), by a Microsoft Visual C++ program resulting in infinite loop
In-Reply-To: <46425FB7.8C763D51@dessent.net>
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This question is also regarding my efforts to create a dll file using
g++ in cygwin and use it in VC++.

The below program works fine and the output is also shown below. But
IF I UNCOMMENT THE TWO LINES in my file dtest.cpp i.e.

//#include<iostream>
//std::cout<<"hello naumskara\n";

and do the same steps. Dll is created successfully, and the MSVC++
compiles fine. But when I try to run it, I get the following msg when
it tries to execute the init(); (to initialize the cygwin environment)
function(MSVC++ file):
------------------------------------------------------------------------------------------------------------
First-chance exception at 0x610b48b6 in new3.exe: 0xC0000005: Access
violation reading location 0x00000004.
-------------------------------------------------------------------------------------------------------------

I am running the MSVC++ program from the IDE.

Could you please suggest me how to make the program(MSVC++) work after
un-commenting the line.

Thanks.


------------------------------------------------------
dtest.cpp FILE THAT I AM USING TO CREATE THE DLL FILE
------------------------------------------------------
#include<stdio.h>
//#include<iostream>
#define SOCKETTEST_BUILD_DLL
#include "dtest.h"
int called()
{
	printf("Ctor 400 called\n");
	//std::cout<<"hello naumskara\n";
	return 400;
}

int c = called();

DLL_IMPORT_EXPORT int test_dll()
{
	printf("%d printed from dll",c);
	return c;
}

------------------------------------------------------
dtest.h FILE THAT I AM USING TO CREATE THE DLL FILE
------------------------------------------------------
#ifdef SOCKETTEST_BUILD_DLL
#define DLL_IMPORT_EXPORT __declspec(dllexport)
#else
#define DLL_IMPORT_EXPORT __declspec(dllimport)
#endif

extern "C" {
DLL_IMPORT_EXPORT int test_dll();
}

-------------------------------------------------------
COMMANDS THAT I USED TO CREATE THE DLL AND LIB FILES
-------------------------------------------------------

$ g++ -shared -o dtestdll.dll dtest.cpp
-Wl,--output-def,dtestdll.def,--out-implib,libdtestdll.a

C:\Program Files\Microsoft Visual Studio .NET 2003\Vc7\bin\lib"
/machine:i386 /DEF:dtestdll.def


------------------------------------------------------------------
MSVC++ PROGRAM USING THE DLL FILE, THAT IS CREATED IN CYGWIN USING g++
------------------------------------------------------------------

#include<stdio.h>
#include "dtest.h"
#include<Windows.h>

void main(){
	HMODULE h = LoadLibrary("cygwin1.dll");
	void (*init)() = (void(__cdecl *)(void))GetProcAddress(h, "cygwin_dll_init");
	init();
    int k= test_dll();
	printf("\nthe OSA value returned is %d\n", k);
	Sleep(5000);
	
}

---------------------------------
OUTPUT FROM MY MSVC++ PROGRAM:
--------------------------------
Ctor 400 called
400 printed from dll
the OSA value returned is 400



On 5/9/07, Brian Dessent <brian AT dessent DOT net> wrote:
> kalasad mailu wrote:
>
> > I used the socket.h file provided by cygwin(/usr/include/cygwin/.) and
> > wrote a program to create socket, made this a dll file and used this
> > dll in VC++.
> >
> > I assume using of the socket.h( form cygwin) did all the conversion
> > form the linux system calls to the windows system calls and made my
> > socket program work. Please correct me if my understanding is wrong.
>
> A header file does not implement anything.  It contains no code at all
> (except in the case of C++ or inlined functions.)  It only describes an
> interface that is actually implemented in a library.  In the case of
> Cygwin, the standard C library is implemented by cygwin1.dll, along with
> many POSIX functions.
>
> > Now I want to try the same process for simple c++ functions like
> > "cout". I don't find any cygwin header files like iostream.h or
> > stream.h in the cygwin directory.
>
> cout is part of the C++ standard library (STL) and is implemented by
> libstdc++ which is part of gcc.
>
> > When I created this stand alone program.
> >
> > #include <iostream>
> >
> > int main () {
> >     std::cout << "Hello World\n";
> >     return 0;
> > }
> >
> > I guess this picked the header file from
> > "cygwin\lib\gcc\i686-pc-cygwin\3.4.4\include\c++\" (a gcc header file)
> >
> > Is there no iostream header file by cygwin (/usr/include/cygwin)?
>
> First of all, stop looking in /usr/include/cygwin for things, and stop
> worrying about which directory header files are in.  That has no bearing
> on what library implements a specific function, as all headers for all
> installed libraries are in /usr/include.  Some of these headers are
> provided by Cygwin.  The ones in the "cygwin" subdirectory are only for
> Cygwin-specific things, but this is only a small fraction of the
> functionality provided by Cygwin.  For example, all of the stadard C
> library functions (such as stdio.h, stdlib.h, io.h, stdint.h, and on and
> on) are all implemented by Cygwin, i.e. cygwin1.dll, and are in
> /usr/include.  And many other libraries as well, such as zlib, gettext,
> libintl, and so on.  So, stop confusing a header file with the library
> that implements the code and realize that the location a header file
> resides on disk has no relationship at all to the implementation.
>
> > I am running into header file conflict when I also include both cygwin
> > header files and the gcc header files. How to solve the conflict?
>
> No, there's no conflict.  Gcc implements some libraries of its own, and
> Cygwin provides most of the standard C library.
>
> > Is it possible to make a dll that uses the functions defined by the
> > gcc header file(***not the cygwin header file i.e. present in
> > (/usr/include/cygwin)***) and use it in the MSVC++?
>
> Stop thinking of things by directory!  Look at the headers in the cygwin
> package (cygcheck -l cygwin).  This represents everything implemented by
> cygwin1.dll.  See that there are many things outside of
> /usr/include/cygwin.
>
> Now look at the libraries provided by gcc for the C++ runtime (cygcheck
> -l gcc-g++).  See that gcc implements most of the C++ runtime
> functionality.  There are all designed to perfectly coexist, there is no
> problem with using both in a given program or DLL.
>
> As to using something in MSVC, first of all libstdc++ is gcc specific,
> so there is no way to use it at the source level with anything else.
> YOu can link to it at the binary level, i.e. call code written by g++
> from MSVC, but only if the interface is 'extern "C"', because the two
> compilers have different C++ ABIs which cannot coexist.
>
> > If including the header file from gcc works then why is cygwin
> > providing separate header files(like stdio.h, stdlib.h and etc in
> > /usr/include/cygwin)?
>
> Different headers provide different things.  /usr/include/stdio.h
> provides the standard C library interface.  /usr/include/sys/stdio.h
> provides low level implementation details.  You do not normally write
> #include <sys/stdio.h>, as it is included by other headers
> automatically.  Stop focusing on the location of things, and think of it
> as an interface.  When you use #include <stdio.h> you are using a
> certain interface; the details of how it is implemented are not your
> concern.  stdio.h could include /usr/include/foo/bar/baz.h if it wanted
> to.
>
> Brian
>
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