cmd/cgo: does not support c++ (use swig)

[alberts]

What steps will reproduce the problem?

I have some code I want to build in C++ mode but with extern "C" linkage.
However, using -x c++ with gcc breaks some of my other code.

After a make nuke, I build with:

CGO_CFLAGS=-x c++

and then I get:

cmsg.go: In function 'void _cgo_1a2aead0dab6_Cfunc_CMSG_DATA_(void*)':
cmsg.go:32:9: error: invalid conversion from 'void*' to
'_cgo_1a2aead0dab6_Cfunc_CMSG_DATA_(void*)::<anonymous struct>*' [-fpermissive]
cmsg.go: In function 'void _cgo_1a2aead0dab6_Cfunc_CMSG_LEN_(void*)':
cmsg.go:42:9: error: invalid conversion from 'void*' to
'_cgo_1a2aead0dab6_Cfunc_CMSG_LEN_(void*)::<anonymous struct>*' [-fpermissive]
cmsg.go: In function 'void _cgo_1a2aead0dab6_Cfunc_CMSG_SPACE_(void*)':
cmsg.go:52:9: error: invalid conversion from 'void*' to
'_cgo_1a2aead0dab6_Cfunc_CMSG_SPACE_(void*)::<anonymous struct>*' [-fpermissive]
make: *** [cmsg.cgo2.o] Error 1

Then with a make nuke and:

CGO_CFLAGS=-x c++ -fpermissive

I get:

cmsg.go:54:13: call of non-function C.CMSG_SPACE_
cmsg.go:54:27: call of non-function C.size_t
cmsg.go:59:13: call of non-function C.CMSG_LEN_
cmsg.go:59:25: call of non-function C.size_t
cmsg.go:64:13: call of non-function C.cmsghdr_t
cmsg.go:65:32: call of non-function C.CMSG_DATA_
make: *** No rule to make target `_cgo_run', needed by `_cgo_flags'.  Stop.

If I first build with 

CGO_CFLAGS=-x c++

until I get an error and then rebuild with

CGO_CFLAGS=-x c++ -fpermissive

without doing a make nuke in between, it builds. Seems like the -fpermissive build
should just work.

Which compiler are you using (5g, 6g, 8g, gccgo)?

6g

Which operating system are you using?

linux

Which revision are you using?  (hg identify)

tip

gcc (GCC) 4.6.0 20110120 (experimental)

Attachments:

1. cmsg.go (1725 bytes)
2. Makefile (139 bytes)
3. cmsg.c (367 bytes)

[robpike]

Comment 1:

Cgo works with C, not C++. Isn't that the issue?

Status changed to WaitingForReply.

[rsc]

Comment 2:

Cgo does not support C++.
If you'd like to add support for this kind of basic
stuff, that would be great.
But it's not super high on our priority list.
The suggested way to wrap C++ is to use SWIG.

Owner changed to r...@golang.org.

Status changed to HelpWanted.

[alberts]

Comment 3:

Thanks. The function implementations might be C++, but the function interface is C
(because of the extern "C" bit).

[rsc]

Comment 4:

Cgo does not support CGO_CFLAGS=-x c++.
It depends on a very specific way of invoking gcc, and it expects
to find a C compiler when it does that.

[alberts]

Comment 5:

I've figured it out.
The trick is to have a extern "C" header file without code and then include the C++ code
via CGO_OFILES.
Example attached.

Attachments:

1. murmur3.tar.gz (2880 bytes)

[alberts]

Comment 6:

If anyone plays with this in future, keep issue #1500 in mind.

[rsc]

Comment 7:

Labels changed: added priority-low, removed priority-medium.

[rsc]

Comment 8:

Labels changed: added priority-someday, removed priority-low.

[alberts]

Comment 10:

Some discussion on stackoverflow:
http://stackoverflow.com/questions/1713214/how-to-use-c-in-go/1721230

[ianlancetaylor]

Comment 11:

Adding SWIG support to the go tool: http://golang.org/cl/5845071/

[rsc]

Comment 12:

Possibly 4069 would help.

[rsc]

Comment 13:

SWIG is the answer for full C++.

Status changed to WorkingAsIntended.

[alberts]

Comment 14:

FWIW, for my test case (CityHash) the latest go build -ldflags '-w -hostobj' worked
perfectly for building a bunch of C++ .sysos (where one has some extern "C" functions)
into a Go binary.

[ianlancetaylor]

Comment 15:

This issue was closed by revision 8448536.

Status changed to Fixed.