Using empty struct properly with CGO

Question

Working with gssapi.h

struct gss_name_struct;
typedef struct gss_name_struct * gss_name_t;

I am trying to figure out how to properly initialize a variable containing this by

var output_name C.gss_name_t = &C.struct_gss_name_struct{}

But the functions like gss_import_name act like if I was passing null pointer to them. What is the correct way to properly initialize and use these empty structs with CGO?

Answer 1

Go's strict typing makes typedefs a pain to work with. The best way to make your Go look clear is to write a small wrapper function in C to build the struct exactly how you want it. In this case though, go is using a zero-length byte array for an empty C struct, which you can verify below. You can declare it directly in go, and convert it when necessary.

Since C isn't strict with types, using type inference is often the easiest way to assign the type that Go expects. There's also a trick using the cgo tool to show the type declarations you need. Using go tool cgo -godefs filename.go will output the cgo definitions for your types. As you see though, the go equivalent types could get a little messy.

// statement in the original .go file
//var output_name C.gss_name_t = &C.struct_gss_name_struct{}

// output from cgo -godefs
// var output_name *[0]byte = &[0]byte{}

// or more succinctly
output_name := &[0]byte{}

// output_name can be converted directly to a C.gss_name_t
fmt.Printf("%+v\n", output_name)
fmt.Printf("%+v\n", C.gss_name_t(output_name))