What's the garbage-collection story? One gc per .so? Or is there some mechanism to share a gc instance between several .so compiled by go?

GP is not exactly right; CGO is not involved in building a shared library except for exporting C functions which can be called. It doesn't "convert Go code to C code" or anything like that.

`-buildmode=c-shared` is what produces a shared library with a C ABI exposed; https://golang.org/s/execmodes was the design document for it, which explains:

> It follows that all Go code shares a single runtime. All Go code uses the same memory allocator, the same goroutine scheduler, and in general acts as though it were linked into a single Go program. This is true even when multiple shared libraries are involved.

Thanks!