cmd/cgo: can't find type of function in shared library #32374

aacebedo · 2019-05-31T21:02:40Z

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

go version go1.12.5 linux/amd64

Does this issue reproduce with the latest release?

Yes

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


GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/tmp/test"
GOPROXY=""
GORACE=""
GOROOT="/opt/go"
GOTMPDIR=""
GOTOOLDIR="/opt/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-build134582758=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I am trying to create a golang program which will use an external C library. I want to be able to use the "go get" without having to manually run swig.

I have created the following structure:

|-- libfoo
|   |-- foo.c
|   |-- foo.h
|   `-- libfoo.so
`-- src
    |-- example_test.go
    |-- lib.go
    `-- libfoo.swig

In the libfoo folder I have a simple shared library. The files contain: foo.c:

#include "foo.h"

int foo(int c){
    return c+1;
};

foo.h

int foo(int c);

That I've compiled it with:

gcc -o ./libfoo.so -fPIC -shared ./foo.c

Then the golang files: libfoo.swig

%module libfoo

%{
  extern int foo(int a);
%}

extern int foo(int a);

lib.go

package libfoo

/*
#cgo LDFLAGS: -L../libfoo -lfoo
*/

import "C"

And finally example_test.go

package libfoo

import (
    "testing"
    "fmt"
)

func TestFoo(*testing.T) {
    i := Foo(1)
    fmt.Printf("%v\n",i)
}

When I try to build with "go build -x" It seems that the cgo directives located in the "lib.go" file are not taken into account as I do not see the flags appearing in the output. Here is the output

06aa7e308d6:/mnt/data/src# go build -x
WORK=/tmp/go-build551116655
mkdir -p $WORK/b001/
swig -version
cd $WORK
/opt/go/pkg/tool/linux_amd64/compile -o ./b001/_go_.o -trimpath ./b001 -p main -complete -goversion go1.12.5 -D _$WORK -c=4 ./swig_intsize.go
cd /mnt/data/src
swig -go -cgo -intgosize 64 -module libfoo -o $WORK/b001/libfoo_wrap.c -outdir $WORK/b001/ libfoo.swig
CGO_LDFLAGS='"-g" "-O2"' /opt/go/pkg/tool/linux_amd64/cgo -objdir $WORK/b001/ -importpath _/mnt/data/src -- -I $WORK/b001/ -g -O2 ./lib.go $WORK/b001/_libfoo_swig.go
cd $WORK
gcc -fno-caret-diagnostics -c -x c - || true
gcc -Qunused-arguments -c -x c - || true
gcc -fdebug-prefix-map=a=b -c -x c - || true
gcc -gno-record-gcc-switches -c -x c - || true
cd $WORK/b001
TERM='dumb' gcc -I /mnt/data/src -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I ./ -g -O2 -o ./_x001.o -c _cgo_export.c
TERM='dumb' gcc -I /mnt/data/src -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I ./ -g -O2 -o ./_x002.o -c lib.cgo2.c
TERM='dumb' gcc -I /mnt/data/src -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I ./ -g -O2 -o ./_x003.o -c _libfoo_swig.cgo2.c
TERM='dumb' gcc -I /mnt/data/src -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I ./ -g -O2 -o ./_x004.o -c libfoo_wrap.c
TERM='dumb' gcc -I /mnt/data/src -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I ./ -g -O2 -o ./_cgo_main.o -c _cgo_main.c
cd /mnt/data/src
TERM='dumb' gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -o $WORK/b001/_cgo_.o $WORK/b001/_cgo_main.o $WORK/b001/_x001.o $WORK/b001/_x002.o $WORK/b001/_x003.o $WORK/b001/_x004.o -g -O2
# _/mnt/data/src
/tmp/go-build551116655/b001/_x004.o: In function `_wrap_foo_libfoo_0f106433f15c8754':
/tmp/go-build/libfoo_wrap.c:255: undefined reference to `foo'
collect2: error: ld returned 1 exit status

What did you see instead?

f I set the CGO_LDFLAGS using an environment variable, it works. However I want to be able to just use "go get" without having those variables before.

I don't understand why go does not take my #cgo directive into account. Did I miss something ?

The text was updated successfully, but these errors were encountered:

ianlancetaylor · 2019-05-31T21:29:28Z

The cgo tool needs to figure out the type of foo in order to permit calling it from Go code. It tries to do this by linking a program and examing the debug info. It doesn't know how to get the type from the C header file, and it doesn't know how to get the type from a shared library. So I suspect that this general approach won't work at present. There's no reason in principle that it couldn't work, but someone would have to do the work.

aacebedo · 2019-06-01T08:07:29Z

What I don't understand is that if I set env vars for CGO_LDFLAGS with the content of the #cgo directive I have in the lib.go file it works.

In the documentation it is said that "go build" shall use that directive in any of the go files of the project and use it during build. It seems not be the case.

AlexRouSg · 2019-06-01T17:07:38Z

You have a newline between import "C" and the comment. Remove it so it looks like

/*
#cgo LDFLAGS: -L../libfoo -lfoo
*/
import "C"

aacebedo · 2019-06-01T19:30:33Z

That's working !
I didn't see anything in the documentation about not letting a newline here.
Is this a normal behavior ?

AlexRouSg · 2019-06-01T19:34:05Z

Yes it is

See https://golang.org/cmd/cgo/

If the import of "C" is immediately preceded by a comment, that comment, called the preamble, is used as a header when compiling the C parts of the package.

aacebedo · 2019-06-01T20:24:17Z

Thanks ! I didn't see this word.
I am closing the issue.

ianlancetaylor · 2019-06-03T06:09:04Z

@AlexRouSg Thanks for spotting that.

ianlancetaylor changed the title ~~Build issue with SWIG file and external C library~~ cmd/cgo: can't find type of function in shared library May 31, 2019

ianlancetaylor added the NeedsInvestigation label May 31, 2019

ianlancetaylor added this to the Unplanned milestone May 31, 2019

aacebedo closed this as completed Jun 1, 2019

golang locked and limited conversation to collaborators Jun 2, 2020

gopherbot added the FrozenDueToAge label Jun 2, 2020

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cmd/cgo: can't find type of function in shared library #32374

cmd/cgo: can't find type of function in shared library #32374

aacebedo commented May 31, 2019

ianlancetaylor commented May 31, 2019

aacebedo commented Jun 1, 2019

AlexRouSg commented Jun 1, 2019

aacebedo commented Jun 1, 2019

AlexRouSg commented Jun 1, 2019

aacebedo commented Jun 1, 2019

ianlancetaylor commented Jun 3, 2019

cmd/cgo: can't find type of function in shared library #32374

cmd/cgo: can't find type of function in shared library #32374

Comments

aacebedo commented May 31, 2019

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

Does this issue reproduce with the latest release?

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

What did you do?

What did you see instead?

ianlancetaylor commented May 31, 2019

aacebedo commented Jun 1, 2019

AlexRouSg commented Jun 1, 2019

aacebedo commented Jun 1, 2019

AlexRouSg commented Jun 1, 2019

aacebedo commented Jun 1, 2019

ianlancetaylor commented Jun 3, 2019

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

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