cmd/cgo: Unable to use godefs with Objective-C

[anton-becker]

Please answer these questions before submitting your issue. Thanks!

go version go1.10rc1 darwin/amd64 (the same with 1.9.3)

go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/unsacrificed/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/unsacrificed/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/qr/tv0txk2s0zv4579x79sp67m00000gn/T/go-build552297888=/tmp/go-build -gno-record-gcc-switches -fno-common"

I try to go tool cgo -godefs cgodefs.go where cgodefs.go contains

// +build ignore

package tmp

//go:generate go tool cgo -godefs cgodefs.go

/*
#cgo CFLAGS: -x objective-c
#cgo LDFLAGS: -framework Cocoa
#import <Cocoa/Cocoa.h>
*/
import "C"

type CGPoint C.CGPoint

and I expect to see source with compatible CGPoint structure. Instead of it I receive a lot of compilation errors:

In file included from /Users/unsacrificed/go/src/gui/cgodefs.go:10:
In file included from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
In file included from /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:8:
/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:503:1: error: expected identifier or '('
@class NSString, Protocol;
^
/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:505:9: error: unknown type name 'NSString'
typedef NSString * NSExceptionName NS_EXTENSIBLE_STRING_ENUM;
        ^
/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:506:9: error: unknown type name 'NSString'
typedef NSString * NSRunLoopMode NS_EXTENSIBLE_STRING_ENUM;
... (100000+ lines of errors)

On the other hand I can successfully build and run GoLang app with Objective-C code.

[ianlancetaylor]

The cgo tool works with C, not Objective C. Sorry.

[anton-becker]

As far as I understand Objective-C translates to C code and this is why GoLang C package can work with Objective-C. And it looks inconsistent that godefs tools can not do this. Looks like (I'm not sure) that godefs just use wrong/ignore some compile options.

[ianlancetaylor]

It may well be that it sometimes works. Let me put it this way: we only support using the cgo tool with C, not Objective C. And I will add that the -godefs option is mainly used for internal purposes only, and is not intended to be a general purpose tool.

What are you really trying to do?

[anton-becker]

I need to pass variable of type T from Go code to Objective-C code. T is structure. So I need to create compatible (in terms of memory layout) type/variable in GoLang and pass it to Objective-C via unsafe.Pointer casting. For generic C code I use godefs in such cases.

[ianlancetaylor]

I see. I don't think there is a way to do that if the type is only defined in Objective C.

[anton-becker]

I see that godefs compile source to extract information about type (I would like to say that godefs does not parse source directly). If so then whats the diffences C code or Objective-C code if last converted to C code?
I just want to understand and may be report future request.

[ianlancetaylor]

I'm sorry, I don't know what the difference is. I see that you tried it, and that it doesn't work. I know for sure that we do not want to take on the burden of supporting Objective C going forward. I understand that it would be useful for you.

[anton-becker]

Finally I found a solution. It is possible to do what I want. The only thing I missed: godefs ignores #cgo CFLAGS: -x objective-c line. Solution - pass required CFLAGS to go binary as arguments (-- -x objective-c).
Working example:

// +build ignore

package tmp

//go:generate go tool cgo -godefs -- -x objective-c cgodefs.go

//#import <Cocoa/Cocoa.h>
import "C"

type CGPoint C.CGPoint

output:

// Created by cgo -godefs - DO NOT EDIT
// cgo -godefs -- -x objective-c cgodefs.go

package tmp

type CGPoint struct {
	X	float64
	Y	float64
}