x/mobile: Getting undefined reference while generating binding using gomobile

[jay11ca39]

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

$ go version: go1.12.4 linux/amd64

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

go env Output

$ go env
GOARCH="amd64"

GOBIN="/usr/local/go"

GOCACHE="/home/--/.cache/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOOS="linux"

GOPATH="My Go path"

GOPROXY=""

GORACE=""

GOROOT="/usr/local/go"

GOTMPDIR=""

GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD=""

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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build242944229=/tmp/go-build -gno-record-gcc-switches"

What did you do?

1. I have go package, I am trying to generate go binding for that using gomobile tool.
   (a) gomobile bind -o my.aar -target=android mygopackage

2. Details about my go package [mygopackage]:
   (a) $ tree
   ├── file.go
   └── res.c

   (b) Details about file.go:
   (i) I am including my res.c
   (ii) I am just redirecting my function call to c file.

   (c) Details about my res.c file:
   (i) I am including one header for getting interfaces: #include <ifaddrs.h> :
   http://man7.org/linux/man-pages/man3/getifaddrs.3.html
   and I am calling one API: getifaddrs()

Issue:
While generating the go mobile binding it is giving undefined reference :
$ gomobile bind -o my.aar -target=android mygopackage

gomobile: go build -buildmode=c-shared -o=/tmp/gomobile-work-063445150/android/src/main/jniLibs/armeabi-v7a/libgojni.so gobind failed: exit status 2
'#mygopackage'
src/mygopackage/res.c:15: error: undefined reference to 'getifaddrs'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I have browsed all issues but no issue is related to this.

[jay11ca39]

Hello @eliasnaur
Can you please help me out in this

[jay11ca39]

As per my analysis this symbol is present in "libc" library. But I am not sure how gomobile internally links "libc" while we run gomobile tool to generate .aar file.

[eliasnaur]

According to a quick Google search, getifaddrs is not available before Android API level 24. Gomobile builds with a lower API level (21 if I remember correctly). You should be able to work around the issue while maintaining Android compatibility by using dlopen and dlsym to access getifaddrs.

[AlexRouSg]

@eliasnaur is there anything preventing android from having a command line arg to set the API level? I saw IOS has one and find it weird android doesn't.

[eliasnaur]

Nothing other than no-one has implemented it yet.

[jay11ca39]

@eliasnaur ,
Is there is any way to specify API level in gomobile bind command ?

[AlexRouSg]

@jay11ca39 if you want to implement it, look at https://github.com/golang/mobile/blob/master/cmd/gomobile/env.go#L311 and https://github.com/golang/mobile/blob/master/cmd/gomobile/build.go#L220

[jay11ca39]

@eliasnaur ,
I have checked the minimum API level in the mainfest file of one of the generated .aar file using gomobile:
<uses-sdk android:minSdkVersion="15"/></manifest>

Then, it means it is taking minimum API level as 15.

But, Then I tried to include some symbols that is introduced in Android API level: 8
Header file: ethtool.h
Symbol : ethtool_cmd_speed
Link: https://android.googlesource.com/platform/external/kernel-headers/+/froyo/original/linux/ethtool.h

But, it is also giving the undefined reference error:
src/mygopackage/res.c:20: error: undefined reference to 'ethtool_cmd_speed'

As per this testing I got confused that reported issue is related to minimum API level or something else.

Can you please provide your opinion on this.
Thanks in advance.

[eliasnaur]

The uses-sdk declaration is for Java code. A -D__ANDROID_API__=$API is needed somewhere. Perhaps it is enough to use

#cgo CFLAGS -D__ANDROID_API__=24

in your .c file.

[jay11ca39]

Tried to place in .c file but it did not help.
Tried to explicitly set it using export command in go env from command line, but result is same.

@eliasnaur ,
As I specified in my last comment, even I used the symbol that is there from API level 8 but it given the same error for those symbols as well: ethtool_cmd_speed

I assume minimum API level in gomobile is at least more than 8.

[AlexRouSg]

@jay11ca39 Can you link to docs showing ethtool_cmd_speed is available since API 8? What you linked is the android kernel headers and those are not available to apps.

[jay11ca39]

@AlexRouSg ,
I thought those will be available to apps. I did not find any doc saying that it is introduced in API 8.

@eliasnaur ,
As currently, gomobile bind command is not taking command line argument for API-Level.
Is there is any way to set manually in gomobile code to set API level and build it..

[AlexRouSg]

@jay11ca39

#31905 (comment)

if you want to implement it, look at https://github.com/golang/mobile/blob/master/cmd/gomobile/env.go#L311 and https://github.com/golang/mobile/blob/master/cmd/gomobile/build.go#L220

"it" referring to the api version command line arg.

[jay11ca39]

#31905 (comment)

[AlexRouSg]

@jay11ca39 what is that last comment suppose to mean? You asked how to get gomobile to use a different API level and I told you where to start looking.

[gopherbot]

Change https://golang.org/cl/176077 mentions this issue: cmd/gomobile: add arg to set android api level

[jay11ca39]

Hello @eliasnaur
I have one basic query regarding build gomobile source code:

1. go get golang.org/x/mobile/cmd/gomobile

2. Then, i have taken the changes from https://golang.org/cl/176077

3. Now, I want to rebuild the gomobile.

I did not find steps to build gomobile code from source code.

Can you, let me know how to to build from source ?

[jay11ca39]

#31905 (comment)
@eliasnaur @AlexRouSg @andybons
Can anybody please provide views on how to build from source? so that I can verify whether given patch solved this issue or not.

[AlexRouSg]

Anything you can obtain with go get is just a normal Go program/package, so you just build it the same way you build any normal Go program/package.

Please see https://golang.org/doc/code.html#remote

But the changes have been merged, this means you can go get -u golang.org/x/mobile/cmd/gomobile to update.

[eliasnaur]

You should be able to just run go install golang.org/x/mobile/cmd/... after applying the change.

[jay11ca39]

Thanks @eliasnaur , @AlexRouSg for all your help and suggestions.

I am able to build my code using -androidapi command line flag introduced in: https://go-review.googlesource.com/c/mobile/+/176077