cmd/go: how to pass -mmacosx-version-min=10.8 from cmd/go down to MacOS clang?

[akamensky]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.7.4 darwin/amd64

What operating system and processor architecture are you using (go env)?

MacOS 10.12.2

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH=""
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/rk/g9b46vzx70g86vhvqtmwtjxw0000gn/T/go-build661102909=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

Build a static library to be used in another project:
GOPATH=$(pwd) go install -buildmode=c-archive main
Then try to use that library in a Qt5.7 project.

What did you expect to see?

Main QTt5.7 projects builds successfully

What did you see instead?

The project build fails, because apparently the MacOS clang by default will compile with compatibility to latest OS version.

libmain.a(000000.o)) was built for newer OSX version (10.12) than being linked (10.8)

COMMENT

It is possible to specify a compatibility version when using MacOS clang++. For example here is how Qt does it with -mmacosx-version-min flag:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -headerpad_max_install_names -stdlib=libc++ -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -mmacosx-version-min=10.8 -Wl,-rpath,/Users/username/Qt5.7.1/5.7/clang_64/lib -o <redacted>

However I seem to be unable to pass ANY CFLAGS as then the build script dies:

$ CFLAGS="-mmacosx-version-min=10.8" GOPATH=/Users/username/IdeaProjects/eswift-client go install -x -buildmode=c-archive main
WORK=/var/folders/rk/g9b46vzx70g86vhvqtmwtjxw0000gn/T/go-build609762181
$ 

[bradfitz]

What makes you think it died?

It looks like it just decided there was no work to do.

(Probably because the CFLAGS environment variable doesn't affect Go and/or is not part of Go's buildid signature.)

Have you tried Go's -extldflags flags? See https://golang.org/pkg/cmd/link/

I think this will work if you use the right arguments.

[akamensky]

@bradfitz -extldflags are passed during the linking time, it is not passed during compile time. The compatibility mark is set compile-time AFAIK.

[akamensky]

@bradfitz
Wow, that is really fast closing issue without even reading into details. The report is not that process died, the report is that there is no way to set compatibility mark (yes I tried -extldflags, but again those are passed to linked; it is clang again just by conincidence, with gcc it would be gcc/g++ and ld)

[akamensky]

@bradfitz Moreover go build same as go install does not have -extldflags. It has -ldflags, but then it tries to use go link as linked which fails (because the buildmode is set ti c-archive)

[bradfitz]

Does go {build,install} -ldflags=-extldflags=-mmacosx-version-min=10.8 your/go/cmd not work?

[akamensky]

@bradfitz No. The library still compiled as minimum version 10.12 compatible

With -x it shows /usr/local/go/pkg/tool/darwin_amd64/link -o $WORK/cinterface/_obj/exe/a.out.a -L $WORK -L /Users/alexey.kamenskiy/IdeaProjects/eswift-client/pkg/darwin_amd64 -extld=clang -buildmode=c-archive -buildid=50ff8b062a12a6887dcebe054e651e2b348418e6 -extldflags=-mmacosx-version-min=10.8 $WORK/cinterface.a as command, but does not show underlying clang command...

[bradfitz]

Delete your target binary and re-run the go build command with all those flags, but also include -x to print the output. Is it actually running what you expect?

[akamensky]

Updated the comment, I always use -x to see full output. And of course I deleted pkg/ before doing that.

Below is full output. It passes -extldflags to go link, but the final .a is still 10.12 only (also even with -x it does not show clang linking call)

[alexey.kamenskiy@nc-macbook:eswift-client]$ CFLAGS="-mmacosx-version-min=10.8" GOPATH=/Users/alexey.kamenskiy/IdeaProjects/eswift-client go install -x -buildmode=c-archive -ldflags="-extldflags=-mmacosx-version-min=10.8" cinterface
WORK=/var/folders/rk/g9b46vzx70g86vhvqtmwtjxw0000gn/T/go-build680834420
mkdir -p $WORK/cinterface/_obj/
mkdir -p $WORK/cinterface/_obj/exe/
cd /Users/alexey.kamenskiy/IdeaProjects/eswift-client/src/cinterface
CGO_LDFLAGS="-g" "-O2" /usr/local/go/pkg/tool/darwin_amd64/cgo -objdir $WORK/cinterface/_obj/ -importpath cinterface -exportheader=$WORK/cinterface/_obj/_cgo_install.h -- -I $WORK/cinterface/_obj/ main.go
cd $WORK
clang -fdebug-prefix-map=a=b -c trivial.c
clang -gno-record-gcc-switches -c trivial.c
cd /Users/alexey.kamenskiy/IdeaProjects/eswift-client/src/cinterface
clang -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -fno-common -I $WORK/cinterface/_obj/ -g -O2 -o $WORK/cinterface/_obj/_cgo_main.o -c $WORK/cinterface/_obj/_cgo_main.c
clang -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -fno-common -I $WORK/cinterface/_obj/ -g -O2 -o $WORK/cinterface/_obj/_cgo_export.o -c $WORK/cinterface/_obj/_cgo_export.c
clang -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -fno-common -I $WORK/cinterface/_obj/ -g -O2 -o $WORK/cinterface/_obj/main.cgo2.o -c $WORK/cinterface/_obj/main.cgo2.c
clang -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -fno-common -o $WORK/cinterface/_obj/_cgo_.o $WORK/cinterface/_obj/_cgo_main.o $WORK/cinterface/_obj/_cgo_export.o $WORK/cinterface/_obj/main.cgo2.o -g -O2
/usr/local/go/pkg/tool/darwin_amd64/cgo -objdir $WORK/cinterface/_obj/ -dynpackage main -dynimport $WORK/cinterface/_obj/_cgo_.o -dynout $WORK/cinterface/_obj/_cgo_import.go
cd $WORK
clang -no-pie -c trivial.c
cd /Users/alexey.kamenskiy/IdeaProjects/eswift-client/src/cinterface
clang -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -fno-common -o $WORK/cinterface/_obj/_all.o $WORK/cinterface/_obj/_cgo_export.o $WORK/cinterface/_obj/main.cgo2.o -g -O2 -Wl,-r -nostdlib
/usr/local/go/pkg/tool/darwin_amd64/compile -o $WORK/cinterface.a -trimpath $WORK -p main -buildid 50ff8b062a12a6887dcebe054e651e2b348418e6 -D _/Users/alexey.kamenskiy/IdeaProjects/eswift-client/src/cinterface -I $WORK -I /Users/alexey.kamenskiy/IdeaProjects/eswift-client/pkg/darwin_amd64 -pack $WORK/cinterface/_obj/_cgo_gotypes.go $WORK/cinterface/_obj/main.cgo1.go $WORK/cinterface/_obj/_cgo_import.go
pack r $WORK/cinterface.a $WORK/cinterface/_obj/_all.o # internal
cd .
/usr/local/go/pkg/tool/darwin_amd64/link -o $WORK/cinterface/_obj/exe/a.out.a -L $WORK -L /Users/alexey.kamenskiy/IdeaProjects/eswift-client/pkg/darwin_amd64 -extld=clang -buildmode=c-archive -buildid=50ff8b062a12a6887dcebe054e651e2b348418e6 -extldflags=-mmacosx-version-min=10.8 $WORK/cinterface.a
mkdir -p /Users/alexey.kamenskiy/IdeaProjects/eswift-client/pkg/darwin_amd64/
mv $WORK/cinterface/_obj/_cgo_install.h /Users/alexey.kamenskiy/IdeaProjects/eswift-client/pkg/darwin_amd64/cinterface.h
mv $WORK/cinterface/_obj/exe/a.out.a /Users/alexey.kamenskiy/IdeaProjects/eswift-client/pkg/darwin_amd64/cinterface.a
[alexey.kamenskiy@nc-macbook:eswift-client]$

[bradfitz]

And of course I deleted pkg/ before doing that.

The output from the linker doesn't go in pkg/, it goes to your $GOPATH/bin or $GOBIN.

Anyway, I'll reopen this and let @ianlancetaylor decide whether there's a bug. If anything, the bug is that our pkg directory causes confusion and needs to be replaced with a proper cache. That is #4719.

[minux]

Have you tried setting $CGO_CFLAGS for go build?

[ianlancetaylor]

You can use -ldflags="-v -extldflags=-mmacosx-version-min=10.8" (I added -v) to see exactly how the Go linker invokes the clang linker. Certainly the -extldflags option should be being passed to clang.

The Go linker also invokes dsymutil on Darwin. Perhaps that is the problem here. Try invoking the go tool with -ldflags=-s to see if that makes a difference.

[rsc]

CGO_CFLAGS=-mmacosx-version-min=10.8 CGO_LDFLAGS=-mmacosx-version-min=10.8 should suffice. In fact only the CGO_CFLAGS is really needed, but setting the CGO_LDFLAGS lets you see a link error if you add -ldflags -v to the go command:

$ cat x.go
package main

import "C"

//export GoHello
func GoHello() {
	println("hello")
}

func main() {}

$ CGO_CFLAGS=-mmacosx-version-min=10.8 \
  CGO_LDFLAGS=-mmacosx-version-min=10.8 \
  go build -ldflags -v x.go

...
ld: warning: object file (/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-link-038149023/000001.o) was built for newer OSX version (10.12) than being linked (10.8)
...
$

The reason this is happening is that there are .o files in the standard library that were not compiled with this CGO_CFLAGS setting, and they are being linked into the binary too. To force rebuilding of everything, the best approach (the one that works with go install as well) is to use a custom installsuffix, which keeps those build artifacts separate from the normal ones:

$ CGO_CFLAGS=-mmacosx-version-min=10.8 \
  CGO_LDFLAGS=-mmacosx-version-min=10.8 \
  go build -installsuffix qt -ldflags -v x.go

...
...no warning here...
...
$

Sorry for the confusion. I expect that a future version of cmd/go that is more careful about reuse of compiled archives will end up recompiling even the standard library automatically when you set CGO_CFLAGS this way, making -installsuffix unneccessary. But for now, that's the best workaround.