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	Fri, 26 Jun 2020 02:31:42 -0700 (PDT)
To:	<cygwin AT cygwin DOT com>
Subject:	Using cygwin-dll with msvc-exe
Date:	Fri, 26 Jun 2020 11:31:40 +0200
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From:	Kristian Ivarsson via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	sten DOT kristian DOT ivarsson AT gmail DOT com
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Hi all

Our task is to use an own cygwin-gcc-dll imported and used in a msvc-exe
(64-bit-system)

According to https://cygwin.com/faq.html#faq.programming.msvc-gcc-objects it
seems like it would be possible by doing it like
https://cygwin.com/cygwin-ug-net/dll.html

The msvc-exe compiles and links with no problems, but crashes during start
up in runtime (as soon as there are any reference to the (extern "C")
functions in the DLL (they aren't even called))

We're linking agains the import .lib and using implicit loading (i.e. no
LoadLibrary)

The console output is:

...
   $ /cygdrive/c/Repos/Trunk/Debug64/my_msvc_program.exe
         0 [main] my_msvc_program (17392) child_copy: cygheap read copy
failed, 0x180343408..0x18036E1D8, done 0, windows pid 17392, Win32 error 6
       582 [main] my_msvc_program (17392)
C:\Repos\Trunk\Debug64\my_msvc_program.exe: *** fatal error - ccalloc would
have returned NULL
...


A snippet from strace output is:

...
     193    5437 [main] my_msvc_program (12608) fhandler_pipe::create:
CreateFile: name \\.\pipe\cygwin-e022582115c10879-12608-sigwait
     198    5635 [main] my_msvc_program (12608) fhandler_pipe::create: pipe
write handle 0x130
     171    5806 [main] my_msvc_program (12608) dll_crt0_0: finished
dll_crt0_0 initialization
   --- Process 12608 thread 2084 created
    1050    6856 [sig] my_msvc_program (12608) wait_sig: entering ReadFile
loop, my_readsig 0x12C, my_sendsig 0x130
   --- Process 12608, exception c0000005 at 00000004a9e3b012
...



I guess I'm experiencing the same issue as
https://stackoverflow.com/questions/25787344/using-dll-compiled-with-cygwin-
inside-visual-studio-2010 (it is using explicit loading of the DLL though)


Just linking the dll works fine, but as soon as a symbol is referenced, it
seems like the linker/loader make things end up in a weird way. The only odd
thing I can see is that the loader (ldd) finds our dll before cygwin1.dll
and thus our own dll is loaded before cygwin.dll


So, should it be possible to use cygwin-gcc-built-dll:s in msvc-built-exe:s
(or msvc-built-dll:s) ? If so, does anyone have a qualified idea of what
might be missing/wrong in what we're doing ?


Best regards,
Kristian

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