How do you statically link a c library in go using cgo?

Question

So there's a bunch of stuff on the group that suggests you can do this in go (although not on the cgo documentation):

package bridge

import "fmt"

// #cgo CFLAGS: -I/Users/doug/projects/c/go-bridge/include
// #cgo LDFLAGS: /Users/doug/projects/c/go-bridge/build/libgb.a
// #include <junk.h>
import "C"

func Run() {
  fmt.Printf("Invoking c library...\n")
  C.x(10)
  fmt.Printf("Done\n")
}

However, it doesn't seem to work:

/var/folders/.../bridge.a(bridge.cgo2.o)(__TEXT/__text): x: not defined

This seems to work fine using a dynamic library, and inspecting the generated files, it actually has the symbol 'x' in there:

/var/folders/rg/hj4s3qlj3sz1d1b5p50ws0vc0000gn/T/go-build442792776/bridge/_obj/_cgo_.o:
0000000100001048 S _NXArgc 
0000000100001050 S _NXArgv 
0000000100001060 S ___progname 
0000000100000dc0 T __cgo_2d7eefe3d6d4_Cfunc_x
0000000100000da0 T __cgo_allocate 
0000000100000db0 T __cgo_panic
0000000100000000 T __mh_execute_header 
0000000100000d90 T _crosscall2
0000000100001058 S _environ
                 U _exit 
0000000100000d80 T _main
                 U _puts 
0000000100001000 s _pvars 
0000000100000de0 T _x                <------- Exists
                 U dyld_stub_binder 
0000000100000d40 T start

but obviously is just a marker in bridge.cgo2.o:

/var/folders/rg/hj4s3qlj3sz1d1b5p50ws0vc0000gn/T/go-build442792776/bridge.a(bridge.cgo2.o):
0000000000000368 s EH_frame0
0000000000000000 T __cgo_2d7eefe3d6d4_Cfunc_x
0000000000000380 S __cgo_2d7eefe3d6d4_Cfunc_x.eh
                 U _x

What am I doing wrong?

For ref, the c header:

int x(int y);

And code:

#include <junk.h>
#include <stdio.h>

int x(int y) {
  printf("Hello World\n");
  return y;
}

--

Edit:

No, -L and -l don't work either; there's actually some specific discussion on the google group that this (-l/blah/blah.a) does not work for cgo, and the correct syntax is in fact to omit the -l and just list the .a file... but hey, if it'd worked, I'd totally just use it. But it doesn't:

dougs-mini:go doug$ go run test.go
# bridge
ld: library not found for -l/Users/doug/projects/c/go-bridge/build/libgb.a
collect2: ld returned 1 exit status
dougs-mini:go doug$ ls -l /Users/doug/projects/c/go-bridge/build/libgb.a
-rw-r--r--  1 doug  staff  872 25 May 14:02 /Users/doug/projects/c/go-bridge/build/libgb.a

verbose version:

dougs-mini:go doug$ go build -work -x test.go
WORK=/var/folders/rg/hj4s3qlj3sz1d1b5p50ws0vc0000gn/T/go-build354497708
mkdir -p $WORK/bridge/_obj/
cd /Users/doug/projects/c/go-bridge/go/src/bridge
/Users/doug/projects/go/go/pkg/tool/darwin_amd64/cgo -objdir $WORK/bridge/_obj/ -- -I/Users/doug/projects/c/go-bridge/include -I $WORK/bridge/_obj/ bridge.go
/Users/doug/projects/go/go/pkg/tool/darwin_amd64/6c -FVw -I $WORK/bridge/_obj/ -I /Users/doug/projects/go/go/pkg/darwin_amd64 -o $WORK/bridge/_obj/_cgo_defun.6 -DGOOS_darwin -DGOARCH_amd64 $WORK/bridge/_obj/_cgo_defun.c
gcc -I . -g -O2 -fPIC -m64 -pthread -fno-common -I/Users/doug/projects/c/go-bridge/include -I $WORK/bridge/_obj/ -o $WORK/bridge/_obj/_cgo_main.o -c $WORK/bridge/_obj/_cgo_main.c
gcc -I . -g -O2 -fPIC -m64 -pthread -fno-common -I/Users/doug/projects/c/go-bridge/include -I $WORK/bridge/_obj/ -o $WORK/bridge/_obj/_cgo_export.o -c $WORK/bridge/_obj/_cgo_export.c
gcc -I . -g -O2 -fPIC -m64 -pthread -fno-common -I/Users/doug/projects/c/go-bridge/include -I $WORK/bridge/_obj/ -o $WORK/bridge/_obj/bridge.cgo2.o -c $WORK/bridge/_obj/bridge.cgo2.c
gcc -I . -g -O2 -fPIC -m64 -pthread -fno-common -o $WORK/bridge/_obj/_cgo_.o $WORK/bridge/_obj/_cgo_main.o $WORK/bridge/_obj/_cgo_export.o $WORK/bridge/_obj/bridge.cgo2.o -l/Users/doug/projects/c/go-bridge/build/libgb.a
# bridge
ld: library not found for -l/Users/doug/projects/c/go-bridge/build/libgb.a
collect2: ld returned 1 exit status

It's worth noting that the failure when you try to link like this (using -l) is typical of gcc failing to link because you're attempting to combine a set of object files.

ie. This:

gcc -I . -g -O2 -fPIC -m64 -pthread -fno-common -o ... -l/path/libgb.a

Will never compile under gcc; you must link a static library like this:

gcc -I . -g -O2 -fPIC -m64 -pthread -fno-common -o ... /path/libgb.a

ie. It is absolutely not that I'm missing a -l or -L.

Answer 1

Turns out my code is 100% fine; it was a copy of Go 1.0; under go 1.1 this works. Under go 1.0, it doesn't.

(it's a bit lame answering my own question, I know; but the 'use -L -l answers below aren't right either; it had nothing to do with that).

A working solution example is up on github here for anyone who find's this question later:

https://github.com/shadowmint/go-static-linking

in short that looks like:

CGO_ENABLED=0 go build -a -installsuffix cgo -ldflags '-s' src/myapp/myapp.go

see also: https://github.com/golang/go/issues/9344

Answer 2

You just have to link with -Ldirectory -lgb.

$ cat >toto.c
int x( int y ) { return y+1; }
$ cat >toto.h
int x(int);
$ gcc -O2 -c toto.c
$ ar q libgb.a toto.o
$ cat >test.go
package main

import "fmt"

// #cgo CFLAGS: -I.
// #cgo LDFLAGS: -L. -lgb 
// #include <toto.h>
import "C"

func main() {
  fmt.Printf("Invoking c library...\n")
  fmt.Println("Done ", C.x(10) )
}
$ go build test.go
$ ./test
Invoking c library...
Done  11

Answer 3

A straightforward Makefile to link Go code with a dynamic or static library:

static:
    gcc -c gb.c
    ar -rcs libgb.a gb.o
    go build -ldflags "-linkmode external -extldflags -static" bridge.go

dynamic:
    gcc -shared -o libgb.so gb.c
    go build bridge.go

Directives in bridge.go:

/*
#cgo CFLAGS: -I.
#cgo LDFLAGS: -L. -lgb
#include "gb.h"
*/
import "C"
...

Answer 4

Try:

// #cgo LDFLAGS: -l/Users/doug/projects/c/go-bridge/build/libgb.a

You missed the -l in the LDFLAGS directive.