doc: for -buildmode=c-shared, document that //export requires import "C"

[asmaloney]

This may just be a misunderstanding of how this is supposed to work - if so I'd appreciate some guidance.

What version of Go are you using (go version)?
go version go1.5beta3 darwin/amd64

What operating system and processor architecture are you using?
Mac OS X 10.10.4 Intel Core i7

What did you do?
I'm trying to build a C archive to link to a C++ app. I read the release notes and the Go Execution Modes doc. The second document states:

The only callable symbols will be those functions marked as exported (by any package), as described in the cgo documentation.

I wrote a small test:

package main

import "log"

//export Foo
func Foo() {
    log.Println("Foo")
}

func main() {
    // unused
}

go build -buildmode=c-archive

What did you expect to see?

I expected it to create foo.a and a header file with Foo exported.

What did you see instead?

It generated foo.a, but no header file.

nm also doesn't show my function being exported:

$ nm -g foo.a

foo.a(000000.o):
                 U ___stack_chk_fail
                 U ___stack_chk_guard
                 U ___stderrp
00000000000001f0 T __cgo_sys_thread_start
0000000000000360 T __cgo_wait_runtime_init_done
                 U _abort
00000000000004e1 T _crosscall_amd64
                 U _fprintf
                 U _fputc
                 U _free
                 U _fwrite
                 U _malloc
                 U _pthread_attr_destroy
                 U _pthread_attr_getstacksize
                 U _pthread_attr_init
                 U _pthread_cond_broadcast
                 U _pthread_cond_wait
                 U _pthread_create
                 U _pthread_key_create
                 U _pthread_key_delete
                 U _pthread_mutex_lock
                 U _pthread_mutex_unlock
                 U _pthread_setspecific
                 U _pthread_sigmask
                 U _setenv
                 U _strerror
                 U _unsetenv
0000000000000470 T _x_cgo_free
0000000000000000 T _x_cgo_init
0000000000000430 T _x_cgo_malloc
00000000000003c0 T _x_cgo_notify_runtime_init_done
0000000000000400 T _x_cgo_setenv
0000000000000300 T _x_cgo_sys_thread_create
0000000000000480 T _x_cgo_thread_start
0000000000000420 T _x_cgo_unsetenv

foo.a(go.o):
00000000000b49f0 T __cgo_panic
0000000000054fe0 T __cgo_topofstack
00000000000b4a20 T _crosscall2
                 U _x_cgo_free
                 U _x_cgo_init
                 U _x_cgo_malloc
                 U _x_cgo_notify_runtime_init_done
                 U _x_cgo_setenv
                 U _x_cgo_sys_thread_create
                 U _x_cgo_thread_start
                 U _x_cgo_unsetenv

[ianlancetaylor]

You need to actually import "C" in the file with the //export comment. Otherwise cgo is not invoked and the //export comment is ignored.

[asmaloney]

Thanks Ian! Misunderstanding on my part - I didn't realize cgo was involved at all in this - I thought your new stuff was just borrowing the export syntax - so I didn't dig into the cgo docs.

I see you added the documentation label - that's great.

Would it be feasible to recognize that no user code is being exported and produce a warning to point them in the right direction? If the developer is using -buildmode=c-archive they are likely expecting something of theirs to be exported.

[ianlancetaylor]

I can't think of a simple way to check for no functions being exported, and I'm not sure it's worth coming up with a complex way. The fact that it doesn't work will be obvious.

[gopherbot]

CL https://golang.org/cl/13080 mentions this issue.

[gopherbot]

CL https://golang.org/cl/13096 mentions this issue.