Go: SWIG doesn't work on Windows

[steeve]

Hey guys, sorry for the mistake when submitting.

I'm trying to build Go bindings for C++ lib, and while it does work on Linux, Mac, here are the steps I have to do to modify the cxx file to make it work on Windows, based in part on https://groups.google.com/forum/#!topic/golang-nuts/9L0U4Q7AtyE

First of all, change all struct swigargs to be __attribute__((__packed__)), according to https://code.google.com/p/go/source/browse/src/cmd/cgo/out.go#530 :

$ sed -i '' 's/} \*swig_a/} __attribute__((__packed__)) \*swig_a/g' file_wrap.cxx

For instance

  struct swigargs {
    long : 0;
    intgo result;
  } __attribute__((__packed__)) *swig_a = (struct swigargs *) swig_v;

However, on Win32, long long int is broken, suggested fix is, according to https://code.google.com/p/go/issues/detail?id=6541 :

$ sed -i '' 's/__attribute__((__packed__))/__attribute__((__packed__, __gcc_struct__))/g' file_wrap.cxx

  struct swigargs {
    long : 0;
    intgo result;
  } __attribute__((__packed__, __gcc_struct__)) *swig_a = (struct swigargs *) swig_v;

Also, Windows doesn't resolve symbols at compile time, so we need to properly set these symbols in a DllMain, and then have the file use them, see the dllmain file here: https://gist.github.com/steeve/7152742/raw/dllmain.cxx

Finally, we need to fix the imports, I do it like so:

# Comment out externs
    sed -i '' 's/\(extern void\)/\/\/\1/' $@
# Insert dllmain
    cat dllmain.i $@ > $@.tmp
    mv $@.tmp $@
# Fix imports by specifying then to the external dll, and commenting the x_wrap_* symbols
    sed -i '' 's/\(#pragma dynimport _ _\)/\/\/\1/' libtorrent_gc.c
    sed -i '' 's/\(#pragma dynimport.*\)\(""\)/\1"$(LIBRARY_NAME)"/g' libtorrent_gc.c
    sed -i '' 's/\(static void (\*x_wrap_\)/\/\/\1/g' libtorrent_gc.c
    sed -i '' 's/cgocall(x\(_wrap_.*\)/cgocall(\1/g' libtorrent_gc.c

And here is a snippet of my Makefile to do that:

$(SRCS) $(GOFILES): $(SWIG_FILES)
    $(SWIG) $(SWIG_FLAGS) -o $@ -outdir . $<
# It should always be like this, according to https://code.google.com/p/go/source/browse/src/cmd/cgo/out.go
    sed -i '' 's/} \*swig_a/} __attribute__((__packed__)) \*swig_a/g' $@
# Temp fix for https://code.google.com/p/go/issues/detail?id=6541
# See also https://code.google.com/p/go/issues/detail?id=5603
ifeq ($(CC), gcc)
    ifneq (,$(filter $(ARCH),x86 x64))
        sed -i '' 's/__attribute__((__packed__))/__attribute__((__packed__, __gcc_struct__))/g' $@
    endif
endif
ifeq ($(OS), Windows_NT)
# Patch SWIG generated files for succesful compilation on Windows.
# Based on https://groups.google.com/forum/#!topic/golang-nuts/9L0U4Q7AtyE
# Comment out externs
    sed -i '' 's/\(extern void\)/\/\/\1/' $@
# Insert dllmain
    cat dllmain.i $@ > $@.tmp
    mv $@.tmp $@
# Fix imports by specifying then to the external dll, and commenting the x_wrap_* symbols
    sed -i '' 's/\(#pragma dynimport _ _\)/\/\/\1/' libtorrent_gc.c
    sed -i '' 's/\(#pragma dynimport.*\)\(""\)/\1"$(LIBRARY_NAME)"/g' libtorrent_gc.c
    sed -i '' 's/\(static void (\*x_wrap_\)/\/\/\1/g' libtorrent_gc.c
    sed -i '' 's/cgocall(x\(_wrap_.*\)/cgocall(\1/g' libtorrent_gc.c
endif

[steeve]

I'll try and see if I can do it in a platform independant way through #ifdef/#define, in which case I'll submit a PR.

[fire]

Is there any progress on this?

[steeve]

Not that I know of. You might want to check this https://github.com/steeve/libtorrent-go/blob/master/Makefile#L124

[ojwb]

Looks like #262 addressed the missing __attribute__((__packed__)), but also swigargs went away in 4461c44 when support for -no-cgo was removed (because it only worked with Go versions before 1.5).

Looking at the current version of https://github.com/steeve/libtorrent-go/blob/master/Makefile I don't see any of the fixups to the SWIG generated code mentioned above.

@steeve Does that mean this issue is no longer relevant (perhaps due to the switch to always using cgo and/or changes in more recent Go versions)? Or do the linked commits above in your fork need merging?

[ojwb]

golang/go#7281 indicates that this should have worked for more than 5 years (but after all the comments above apart from mine from a few weeks ago), so I'm going to close this now.

If it doesn't seem fixed please give details and I'll be happy to reopen.