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Date:	Sun, 29 Aug 2010 19:08:59 -0400
From:	=?ISO-8859-1?Q?Tom=E1s_Staig?= <tomas DOT staig AT gmail DOT com>
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To:	cygwin AT cygwin DOT com
Subject:	Linking shared libraries problem
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  Hi,
I have been trying to port some software from Linux (Scientific 
Linux/RedHat) to windows using Cygwin. I have been able to port most of 
it with little changes but I encountered a problem when linking shared 
libraries. It seems that the chain of dependencies is not included when 
linking. Furthermore, ldd does not show the dependency libraries as in 
Linux. I have tried both using the import libraries (%.dll.a) and 
linking the dll files (%.dll) directly.

I have arranged a small example program that reproduces this effect.
Used Ubuntu 8.04 to and "CYGWIN_NT-5.1" version "1.7.6(0.230/5/3) 
2010-08-16 16:06" on top of a 32-bits Windows XP Machine to test the 
above examples.

x.h:
int add(int a, int b);

x.cpp:
#include "x.h"
int add(int a, int b) {return a + b;}

y.h:
int sub(int a, int b);

y.cpp:
#include "y.h"
int sub(int a, int b){return a - b;}

main.cpp:
#include <stdio.h>
#include "x.h"
#include "y.h"

int main()
{
    printf("%d\n",add(1,1));
    printf("%d\n",sub(2,1));
    return 0;
}

Makfile in Linux:
#Compiling
all:
    g++ -c x.cpp
    g++ -c y.cpp
    g++ -shared -Wl,-whole-archive y.o -Wl,-no-whole-archive -o liby.so
    g++ -shared -Wl,-whole-archive x.o -Wl,-no-whole-archive -L./ -ly -o 
libx.so
    g++ -o main main.cpp -L./ -lx

Makefile in Cygwin:
all:
    g++ -c x.cpp
    g++ -c y.cpp
    g++ -shared y.o -Wl,-out-implib=liby.dll.a -Wl,-export-all-symbols 
-Wl,-enable-auto-import -Wl,-whole-archive -Wl,-no-whole-archive -o liby.dll
    g++ -shared x.o -Wl,-out-implib=libx.dll.a -Wl,-export-all-symbols 
-Wl,-enable-auto-import -Wl,-whole-archive -Wl,-no-whole-archive -L./ 
-ly -o libx.dll
    g++ -o main main.cpp -L./ -lx

Linux does not produce any output and the program 'main' works.
Cygwin Output:
/tmp/cc0LNesq.o:main.cpp:(.text+0x4a): undefined reference to `sub(int, 
int)'
collect2: ld returned 1 exit status
make: *** [all] Error 1

If I use ldd in Linux for libx.so:
$ ldd libx.so
     linux-gate.so.1 =>  (0xb772f000)
     liby.so (0xb7729000)
     libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0xb7635000)
     libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xb75f9000)
     libgcc_s.so.1 => /lib/libgcc_s.so.1 (0xb75ee000)
     libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb749f000)
     /lib/ld-linux.so.2 (0xb7730000)

and in Cygwin for libx.dll:
$ ldd libx.dll
     ntdll.dll => /cygdrive/c/WINDOWS/system32/ntdll.dll (0x7c910000)
     kernel32.dll => /cygdrive/c/WINDOWS/system32/kernel32.dll (0x7c800000)

As you can see, there is no reference to liby.dll. I could add the 
library (-ly) directly to the compiling line of main and it works, but 
the truth is that it would not be a good approach, since in the software 
I'm trying to port, there are several dependent modules, so the last 
ones would have an incredibly large list of dependencies.

So, am I doing something wrong? Is there any way to add the dependency 
to be shown with ldd or any workaround(maybe a linker flag or something) 
to make the above example work?
Thanks in advance.

Best Regards,
Tomas.

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