I'm giving cross-compilation from linux to windows another go. I'm on an ubuntu focal based system (Linux Mint 20 to be exact). AFAICT I have to

• I start by installing mingw: apt get mingw-w64

• Then I have to compile and install dependencies one by one. For the time being I'm just doing zlib, libpng and freetype.

• Then compile allegro with Toolchain-mingw.cmake

• (If you know an easier way, do tell!)

I found some useful instructions on cross-compiling various dependencies for OpenTTD which I'm adapting from

Actually compiling the dependencies goes well, here is the script I used:

Freetype configure ends with the following

configure:

Library configuration:
  external zlib: yes (pkg-config)
  bzip2:         no
  libpng:        yes (pkg-config)
  harfbuzz:      no

Which looks like everything is OK.

Then I continue compiling from my working copy of allegro:

mkdir Build/Mingw-w64
cd Build/Mingw-w64
cmake -DCMAKE_TOOLCHAIN_FILE=../../cmake/Toolchain-mingw.cmake ../..

Unfortunately, freetype is found but not picked up. cmake reports (full output attached):

-- Found Freetype: /usr/i686-w64-mingw32/lib/libfreetype.a (found version "2.10.1") 
-- Performing Test TTF_COMPILES
-- Performing Test TTF_COMPILES - Failed
-- Performing Test TTF_COMPILES_WITH_EXTRA_DEPS
-- Performing Test TTF_COMPILES_WITH_EXTRA_DEPS - Failed
CMake Warning at addons/CMakeLists.txt:137 (message):
  FreeType doesn't compile.  Disabling support.

CMakeError log is attached, I think this is the interesting bit:

/usr/bin/i686-w64-mingw32-gcc  -msse -W -Wall -Wpointer-arith -Wmissing-declarations -Wstrict-prototypes -Wdeclaration-after-statement -DTTF_COMPILES_WITH_EXTRA_DEPS    -Wl,--whole-archive CMakeFiles/cmTC_a42c1.dir/objects.a -Wl,--no-whole-archive  -o cmTC_a42c1.exe -Wl,--out-implib,libcmTC_a42c1.dll.a -Wl,--major-image-version,0,--minor-image-version,0 @CMakeFiles/cmTC_a42c1.dir/linklibs.rsp
/usr/bin/i686-w64-mingw32-ld: /usr/i686-w64-mingw32/lib/libfreetype.a(sfnt.o): in function `Load_SBit_Png':
/home/martijn/prg/alleg/allegro-buildenv/alleg5-crosscompile/freetype-2.10.1/src/sfnt/pngshim.c:290: undefined reference to `png_create_read_struct'
/usr/bin/i686-w64-mingw32-ld: /home/martijn/prg/alleg/allegro-buildenv/alleg5-crosscompile/freetype-2.10.1/src/sfnt/pngshim.c:300: undefined reference to `png_create_info_struct'
/usr/bin/i686-w64-mingw32-ld: /home/martijn/prg/alleg/allegro-buildenv/alleg5-crosscompile/freetype-2.10.1/src/sfnt/pngshim.c:308: undefined reference to `png_set_longjmp_fn'
/usr/bin/i686-w64-mingw32-ld: /home/martijn/prg/alleg/allegro-buildenv/alleg5-crosscompile/freetype-2.10.1/src/sfnt/pngshim.c:441: undefined reference to `png_destroy_read_struct'
/usr/bin/i686-w64-mingw32-ld: /home/martijn/prg/alleg/allegro-buildenv/alleg5-crosscompile/freetype-2.10.1/src/sfnt/pngshim.c:314: undefined reference to `png_set_read_fn'
/usr/bin/i686-w64-mingw32-ld: /home/martijn/prg/alleg/allegro-buildenv/alleg5-crosscompile/freetype-2.10.1/src/sfnt/pngshim.c:316: undefined reference to `png_read_info'

Lot's of undefined references to png functions. Apparently, freetype is not happy finding libpng after all. Why not? Any suggestions on how to troubleshoot this further?

Thanks Edgar, that does the trick! My cmake command is now:

cmake  -DCMAKE_TOOLCHAIN_FILE=../../cmake/Toolchain-mingw.cmake \
  -DFREETYPE_PNG=on \
  -DFREETYPE_ZLIB=on \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo \
  -DWANT_DEMO=off \
  -DWANT_EXAMPLES=off \
  -DWANT_DOCS=off \
  ../..

png, zlib, freetype, ogg and vorbis are included now. I finally managed to cross-compile one of my games with it. Next, I'll give DUMB a go.

Some further questions:

• Is there a way to tell cmake to look for libraries in /usr/local/i686-w64-mingw32 instead of /usr/i686-w64-mingw32? It would be nice to keep my homebrew stuff separate from the packaging system. I tried -DCMAKE_INCLUDE_PATH:PATH=/usr/local/i686-w64-mingw32/include -DCMAKE_LIBRARY_PATH:PATH=/usr/local/i686-w64-mingw32/lib </code> but that doesn't seem to have any effect.

  
  

• I also notice that freetype has a lot of optional dependencies. What would be the use of e.g. harfbuzz?

edit; and:

• Where do get the source code for libDUMB? The code here: https://github.com/kode54/dumb seems to require SDL as dependency. Or should I stick to http://dumb.sourceforge.net/ ?

Ok, a little further. I figured out minimp3 and I got DUMB almost working.

For reference, I fetch & cross-compile DUMB with:

Custom Toolchain-mingw.cmake for DUMB:

With all of that, DUMB compiles, and allegro's cmake picks it up.

edit(2):

I got a new problem. I suddenly started showing up so I thought it was related to DUMB, but it's actually caused by static compilation (with -DWANT_STATIC_RUNTIME=on)

It's strangely inconsistent though, sometimes it works and sometimes it doesn't For now I'm skipping the static build.

It's also similar to this: https://stackoverflow.com/questions/11267877/mingw-multiple-definition-of-unwind-resume

[ 71%] Building C object addons/image/CMakeFiles/allegro_image.dir/tga.c.obj
[ 72%] Building C object addons/image/CMakeFiles/allegro_image.dir/dds.c.obj
[ 72%] Building C object addons/image/CMakeFiles/allegro_image.dir/identify.c.obj
[ 73%] Building CXX object addons/image/CMakeFiles/allegro_image.dir/gdiplus.cpp.obj
[ 74%] Linking CXX shared library ../../lib/allegro_image-debug-5.2.dll
/usr/bin/i686-w64-mingw32-ld: /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a(unwind-sjlj.o):(.text+0x400): multiple definition of `_Unwind_SjLj_Register'; ../../lib/liballegro-debug.dll.a(d000067.o):(.text+0x0): first defined here
/usr/bin/i686-w64-mingw32-ld: /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a(unwind-sjlj.o):(.text+0x490): multiple definition of `_Unwind_SjLj_Unregister'; ../../lib/liballegro-debug.dll.a(d000070.o):(.text+0x0): first defined here
/usr/bin/i686-w64-mingw32-ld: /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a(unwind-sjlj.o):(.text+0x780): multiple definition of `_Unwind_SjLj_Resume'; ../../lib/liballegro-debug.dll.a(d000068.o):(.text+0x0): first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [addons/image/CMakeFiles/allegro_image.dir/build.make:187: lib/allegro_image-debug-5.2.dll] Error 1
make[1]: *** [CMakeFiles/Makefile2:407: addons/image/CMakeFiles/allegro_image.dir/all] Error 2
make: *** [Makefile:130: all] Error 2
The command '/bin/sh -c ./build-alleg5-mingw-debug.sh &&        ./build-alleg5-mingw-monolith.sh &&     ./build-alleg5-mingw-release.sh &&      ./build-alleg5-mingw-static.sh &&         rm -rf /home/builder/allegro5/Build' returned a non-zero code: 2
make: *** [makefile:7: build] Error 2

How do I end up with multiple definitions of libgcc_eh.a(unwind-sjlj.o) (whatever that is?)

Any advice to get static compilation working?

That makes sense, but the error occurs during the build of allegro, not my exe (allegro_image addon to be precise). Could the same conflict occur between allegro and one of the dependent libs? How do I find out which lib contains the offending duplicate definition?

Still no luck with the static library. I can give more details though (edit, I found a workaround, but I still have questions...)

Here are the preparation steps:

mkdir -p /home/builder/allegro5/Build/MingwStatic
cd /home/builder/allegro5/Build/MingwStatic
cmake \
  -DCMAKE_TOOLCHAIN_FILE=../../cmake/Toolchain-mingw.cmake \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo \
  -DWANT_STATIC_RUNTIME=on \
  -DFREETYPE_PNG=on \
  -DFREETYPE_ZLIB=on \
  -DWANT_DEMO=off -DWANT_EXAMPLES=off -DWANT_DOCS=off \
  ../..

Here is the exact command that is failing:

make VERBOSE=1
/usr/bin/i686-w64-mingw32-g++   -msse -W -Wall -Wpointer-arith -O2 -g -DNDEBUG -static-libgcc -static-libstdc++ -static -lpthread -shared -o ../../lib/allegro_image-5.2.dll -Wl,--out-implib,../../lib/liballegro_image.dll.a -Wl,--major-image-version,5,--minor-image-version,2 -Wl,--whole-archive CMakeFiles/allegro_image.dir/objects.a -Wl,--no-whole-archive @CMakeFiles/allegro_image.dir/linklibs.rsp

Do you see anything wrong with this?

Contents of referred file in addons/image/CMakeFiles/allegro_image.dir/linklibs.rsp:

 ../../lib/liballegro.dll.a /usr/i686-w64-mingw32/lib/libgdiplus.a -luuid -luser32 -lgdi32 -lcomdlg32 -lole32 -lwinmm -lkernel32 -lpsapi -lshlwapi /usr/i686-w64-mingw32/lib/libd3d9.a /usr/i686-w64-mingw32/lib/libdinput8.a -lopengl32 -lglu32 -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32

Here is again the (same) error I'm getting:

/usr/bin/i686-w64-mingw32-ld: /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a(unwind-sjlj.o):(.text+0x400): multiple definition of `_Unwind_SjLj_Register'; ../../lib/liballegro.dll.a(d000067.o):(.text+0x0): first defined here
/usr/bin/i686-w64-mingw32-ld: /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a(unwind-sjlj.o):(.text+0x490): multiple definition of `_Unwind_SjLj_Unregister'; ../../lib/liballegro.dll.a(d000070.o):(.text+0x0): first defined here
/usr/bin/i686-w64-mingw32-ld: /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a(unwind-sjlj.o):(.text+0x780): multiple definition of `_Unwind_SjLj_Resume'; ../../lib/liballegro.dll.a(d000068.o):(.text+0x0): first defined here

I've examined a few object files with nm like you say.

nm /usr/lib/gcc/i686-w64-mingw32/9.3-win32/libgcc_eh.a | grep SjLj_Register
nm: unwind-dw2.o: no symbols
00000400 T __Unwind_SjLj_Register

nm CMakeFiles/allegro.dir/objects.a | grep SjLj_Register
    U __Unwind_SjLj_Register

As far as I can tell this is correct, libgcc_eh.a contains the type definition, CMakeFiles/allegro.dir/objects.a lists the symbol as undefined. This is how it should be, right?

edit:

So I found a workaround. If I manually run that one compilation step with the extra flags "-Wl,-allow-multiple-definition" (as adviced here https://stackoverflow.com/questions/11267877/mingw-multiple-definition-of-unwind-resume) that one step completes, and I can finish continue the entire library with make.

Would there be a downside to just always adding those flags?

edit 2:

Also, is there a way to override those flags when you invoke cmake?
I tried calling cmake with -DCMAKE_STATIC_LINKER_FLAGS=-Wl,-allow-multiple-definition. I can confirm that the flag appears in CMakeCache.txt, but compilation fails anyway with the same error as above.

edit 3:

So, weirdly enough, it works when I pass -DCMAKE_SHARED_LINKER_FLAGS=-Wl,-allow-multiple-definition to cmake when doing the static build of allegro.

All that remains now is figuring out what can be improved to the allegro build files so this hack isn't necessary.

What am I looking for exactly? I notice that the -Wl,-allow-multiple-definition flag ends up in link.txt, not linklibs.rsp.

I never understood cmake-gui. The tool just baffles me. The only useful thing it does is that, by scanning a long list of (undocumented) options, you might have a chance of guessing the right one that you need to change.

edit: ok here is what I see.

# contents of addons/image/CMakeFiles/allegro_image.dir/linklibs.rsp
 ../../lib/liballegro.dll.a /usr/i686-w64-mingw32/lib/libgdiplus.a -luuid -luser32 -lgdi32 -lcomdlg32 -lole32 -lwinmm -lkernel32 -lpsapi -lshlwapi /usr/i686-w64-mingw32/lib/libd3d9.a /usr/i686-w64-mingw32/lib/libdinput8.a -lopengl32 -lglu32 -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 

# contents of addons/font/CMakeFiles/allegro_font.dir/linklibs.rsp
 ../../lib/liballegro.dll.a -luser32 -lgdi32 -lcomdlg32 -lole32 -lwinmm -lkernel32 -lpsapi -lshlwapi /usr/i686-w64-mingw32/lib/libd3d9.a /usr/i686-w64-mingw32/lib/libdinput8.a -lopengl32 -lglu32 -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 

# contents of addons/color/CMakeFiles/allegro_color.dir/linklibs.rsp
 ../../lib/liballegro.dll.a -luser32 -lgdi32 -lcomdlg32 -lole32 -lwinmm -lkernel32 -lpsapi -lshlwapi /usr/i686-w64-mingw32/lib/libd3d9.a /usr/i686-w64-mingw32/lib/libdinput8.a -lopengl32 -lglu32 -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 

# contents of addons/primitives/CMakeFiles/allegro_primitives.dir/linklibs.rsp
 ../../lib/liballegro.dll.a -luser32 -lgdi32 -lcomdlg32 -lole32 -lwinmm -lkernel32 -lpsapi -lshlwapi /usr/i686-w64-mingw32/lib/libd3d9.a /usr/i686-w64-mingw32/lib/libdinput8.a -lopengl32 -lglu32 -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 

# contents of addons/ttf/CMakeFiles/allegro_ttf.dir/linklibs.rsp 
 ../../lib/liballegro_font.dll.a /usr/i686-w64-mingw32/lib/libfreetype.dll.a ../../lib/liballegro.dll.a -luser32 -lgdi32 -lcomdlg32 -lole32 -lwinmm -lkernel32 -lpsapi -lshlwapi /usr/i686-w64-mingw32/lib/libd3d9.a /usr/i686-w64-mingw32/lib/libdinput8.a -lopengl32 -lglu32 -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32