add.go

package main

import "C"

//export GoAdd
func GoAdd(a, b int) int {
	return a + b
}

func main() {}

add_test.c

// +build ignore

#include <stdio.h>
#include "add.h"

int main() {
  int x = GoAdd(12, 34);
  printf("%d\n", x);
  return 0;
}

build-add-cgo.sh

#!/bin/bash

set -euxo pipefail

cmddir=$(pwd)
rm -rf "$cmddir/_obj"
mkdir -p "$cmddir/_obj"
tmpdir="$cmddir/_obj"

CFLAGS="-pthread -g -O2 -fPIC"
export CGO_CFLAGS=$CFLAGS
export CGO_LDFLAGS=-fPIC

# build stdlib
objdir=$tmpdir/stdlib
mkdir -p $objdir/pkg
cp -r $GOROOT/src $objdir/src
cp -r $GOROOT/pkg/tool $objdir/pkg/tool
cp -r $GOROOT/pkg/include $objdir/pkg/include
export GOROOT=$objdir
go install -installsuffix=shared -gcflags=-shared -ldflags=-shared std
go install -installsuffix=shared -gcflags='-shared -+' -ldflags=-shared runtime/cgo

# compile example.com/repo
srcdir=$cmddir
objdir=$tmpdir/repo
mkdir -p $objdir

go tool cgo -objdir $objdir -importpath example.com/repo -exportheader $objdir/_cgo_install.h $srcdir/add.go
gcc -c -o $objdir/_cgo_export.o $CFLAGS $objdir/_cgo_export.c
gcc -c -o $objdir/add.cgo2.o $CFLAGS $objdir/add.cgo2.c
gcc -c -o $objdir/_cgo_main.o $CFLAGS $objdir/_cgo_main.c
gcc -o $objdir/_cgo_.o -pthread $objdir/{_cgo_export,add.cgo2,_cgo_main}.o
go tool cgo -dynpackage main -dynimport $objdir/_cgo_.o -dynout $objdir/_cgo_import.go
go tool compile -o $objdir/_pkg_.a -shared -p main -installsuffix shared -pack $objdir/*.go
go tool pack r $objdir/_pkg_.a $objdir/{_cgo_export,add.cgo2}.o

# link
mkdir $objdir/tmp
go tool link -o $objdir/repo.so -tmpdir=$objdir/tmp -installsuffix=shared -buildmode=c-shared -linkmode=external -linkshared -extld=gcc $objdir/_pkg_.a

# compile add_test
srcdir=$cmddir
objdir=$tmpdir/add_test
mkdir -p $objdir

gcc -c -o $objdir/add_test.o $CFLAGS -iquote $tmpdir/repo $srcdir/add_test.c

# link add_test
gcc -o $objdir/add_test $objdir/add_test.o $tmpdir/repo/repo.so