cmd/link: MSVC support (MSVC object + linking with MSVC)

I understand that the go linker cannot currently link msvc object files and also recognize that this issue is likely to be a low priority. However, it would be nice to support this because it would somewhat simplify windows workflow. This issue is mainly to understand how much effort this would be and/or what would be required.

[bradfitz]

/cc @alexbrainman

[alexbrainman]

@xoviat what is the problem that you are having? I need to be able to reproduce it here. So, please, provide all steps I will need to follow to reproduce this.

Thank you

Alex

PS: I won't have computer until end of July. I will have a look at this then.

what is the problem that you are having?

I haven't tried it yet, but I would like to specifically call c functions in msvc object files by linking them as .syso with the go linker. Everything that I have read indicates that this is not possible but I will create a procedure to reproduce the specific error that occurs.

[alexbrainman]

specifically call c functions in msvc object files by linking them as .syso with the go linker

Have you tried building these into a DLL, and use them from inside of DLL?

I will create a procedure to reproduce the specific error that occurs.

Please, do. Thank you.

Alex

Have you tried building these into a DLL, and use them from inside of DLL?

That was actually my original plan. I am using swig so it is not so convenient to compile the generated c code separately and then rewrite the functions as DLL exports. It's not difficult, but it is annoying when the workflow with gcc is just go generate; go build.

Alright, I have a procedure. Start with these files:

hello.go:

package main

/*
	extern void hello();
*/
import "C"

func main() {
	C.hello()
}

hello.c:

#include <stdio.h>

extern void hello()
{
    printf("Hello World from C");
}

then run these commands (with msvc on path):

cl /c hello.c
mv hello.obj hello.syso
mv hello.c hello.c.bak
go build

Result:
Warning: corrupt .drectve at end of def file

When running the produced file:

Exception 0xc0000005 0x8 0x13 0x13
PC=0x13
signal arrived during external code execution

main._Cfunc_hello()
        _//_obj/_cgo_gotypes.go:41 +
main.main()
        C://hello.go:9 +0x27

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
        C:/Program Files/Go/src/runtime/asm_amd64.s:2197 +0x1
rax     0x4a5960
rbx     0xc042045f78
rcx     0x4a9a20
rdi     0xc042045f78
rsi     0x4adc60
rbp     0xc042045f38
rsp     0x6dfd68
r8      0xc042016340
r9      0x0
r10     0xc04204faa0
r11     0x4783c2
r12     0x0
r13     0x6
r14     0x0
r15     0xf1
rip     0x13
rflags  0x10216
cs      0x33
fs      0x53
gs      0x2b

[alexbrainman]

I do not have cl command installed on my pc. How do I install msvc?

Thank you.

Alex

You need the "build tools 2017" here. Microsoft actually now allows anyone to use visual studio for free as long as it's not for commercial development. If it's too much trouble, I can just give you the object file if you want.

If only c++ was not a thing, I wouldn't have to worry about this. But it is, so the pain continues...

[alexbrainman]

You need the "build tools 2017" here.

Got it. Thank you.

If it's too much trouble, I can just give you the object file if you want.

Yes, please, post hello.obj somewhere.

Thinking about this some more. You are actually using gcc to link object file compiled with msvc compiler. Can you try and do your exercise, but replacing "go build" step with gcc linking hello.obj to a C program? Perhaps it has been done before. I suspect, if we know how to do that, we might be able to do similar with Go.

Alex

AFAIK lld https://github.com/llvm-mirror/lld supports msvc object files.

Object file is here: https://github.com/xoviat/msvcgo/blob/master/hello.syso