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Subject: Question about having msvcrt*.dll and cygwin1.dll in one process
From: Jing Zhao <jing DOT zhao AT ni DOT com>
Message-ID: <OFA16137E6.A98D47F2-ON48257D93.002FE9C9-48257D93.00304419@ni.com>
Date: Mon, 17 Nov 2014 16:47:11 +0800
Content-Type: text/plain; charset="US-ASCII"

Hi all,

Cygwin FAQ 6.15 says that I cannot link with both MSVCRT*.dll and 
cygwin1.dll since they are mutually exclusive. However, I can successfully 
run a simple Windows console application which dynamically loads a dll 
(compiled in Cygwin, thus linking to cygwin1.dll). The Windows console 
application is built with Multi-threaded DLL (/MD). So what's the problem 
of linking to both MSVCRT*.dll and cygwin1.dll. Am I missing anything?

// test.cpp, DLL code compiled in Cygwin
#include <stdio.h>
int hello(void)
{
   printf ("welcome to linux world\n");
   return 7777777;
}

This is the code to produce dll
$ gcc -g -O2 -c test.cpp
$ gcc -shared -o test.dll test.o

// main application compiled in MSVC, with /MD
#include <stdio.h>
#include <Windows.h>

int main()
{
   char padding[32768];
   memset(padding, 0, sizeof(padding));

   HMODULE h = LoadLibrary(TEXT("C:\\cygwin64\\bin\\cygwin1.dll"));
   if (!h)
   {
      DWORD code = GetLastError();
      printf ("can't load cygwin1.dll, code %d\n", code);
      return 0;
   }

   typedef void (WINAPI *INITFUC)();
   INITFUC init = NULL;
   init = (INITFUC) GetProcAddress(h, "cygwin_dll_init");
   if (!init)
   {
      printf ("can't find cygwin_dll_init()\n");
      return 0;
   }
   init();

   h = LoadLibrary(TEXT("E:\\opensource\\Cygwin\\test\\test.dll"));
   if (!h)
   {
      DWORD code = GetLastError();
      printf ("can't load test.dll, code %d\n", code);
      return 0;
   }

   typedef int (*MYFUNC)(void);
   MYFUNC f = NULL;
   f = (MYFUNC) GetProcAddress(h, "_Z5hellov");
   if (!f)
   {
      printf ("can't find hello()\n");
      return 0;
   }

   int res = (*f)();
   printf ("res: %d\n", res);
   printf ("it's over\n");
   getchar();

   return 0;
}

The outputs:
welcome to linux world
res: 7777777
it's over

So everything look fine. According to the FAQ 6.15, is there anything 
that's potentially dangerous that I should be aware of, when linking both 
msvcrt.dll and cygwin1.dll? Thanks a lot!


Thanks,
Jing Zhao @ Shanghai
Ext. 3273
8+ 7023273

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