Re: MinGW build and FTGL library for Octave 3.1.55

[Benjamin Lindner]

Alexander Mamonov wrote:
> Hello Benjamin,
>
> Many thanks for the excellent work!
> Can you please share with me the forbidden arcane knowledge of what
> combination of compiler flag variables and configure options will
> result in .oct files being linked dynamically to libgcc and libstdc++?
> I tried different combinations of -shared, -shared-libgcc, and
> --enable-shared, but the .oct files are still being linked statically.

Well it's certainly not forbidden, but thinking of it, it is not really 
well documented ...

So here we go:

You must distinguish between the shared libgcc and the shared libstd++.
Linking against shared libgcc is simple, provide -shared-libgcc at 
linking stage.

Linking against shared libstd++ is a bit more tricky. Needless to say, 
you will need a version of gcc which includes a shared libstd++. TDM's 
mingw port 4.3.0-2 does, for example. (mind the -2 !).
The shared version of libstd++ is called libstd++_s, hence you must add 
-lstdc++_s. Mind that the default libstd++ will be added implicitly, so 
be careful of the order of the linker libraries.
Then, you should link with --enable-auto-import. This is not mandatory, 
*but*, if you do not compile with -D_DLL, the externals referenced into 
standard c++ library will not be maked as __dllimport__, and therefore 
are not found in the shared library's import libaray, *unless* you link 
with --enable-auto-import, *and* since the static version of libstd++ is 
implicitly added at linker stage this will silenty use the static 
version of libstd++, although you might have explicitly specified -lstd++_s.
So to save me the debugging pain, I always specify -D_DLL, thus shared 
and only shared libstd++ is used, and if there is a problem, I get a 
linker error.

So here is what I do:
  CXXFLAGS += -D_DLL
  LDFLAGS += -shared-libgcc
and then add
  -lstdc++_s
to the linker libraries at the proper position.

try the hello world example

#include <iostream>
int main()
{
   std::cout << "Hello World!\n";
   return 0;
}

once as
  g++ -D_DLL -shared-libgcc -s -o hello_shared.exe hello.cc -lstdc++_s
(mind the order of object files and linker libraries!)
and once as
  g++ -s -o hello_static.exe hello.cc

and (this is also possible)
  g++ -shared-libgcc -s -o hello_sharedgcc.exe hello.cc
which will link against shared libgcc but static libstd++

I get
$ ls -la hello*.*
-rw-r--r-- 1 admin Admins     82 May 22 22:37 hello.cc
-rwxr-xr-x 1 admin Admins   5120 May 22 22:38 hello_shared.exe
-rwxr-xr-x 1 admin Admins 441344 May 22 22:44 hello_sharedgcc.exe
-rwxr-xr-x 1 admin Admins 460288 May 22 22:38 hello_static.exe


Now the really tricky part comes, when you cannot specify 
"-shared-libgcc" because the tool you use, for example mkoctfile, does 
not recognize this flag (libtool doesn't either, by the way).
The workaround I use here, is to specify -shared-libgcc as part of CXX's 
 name, as in CXX="g++ -shared-libgcc"

hope this helps
benjamin