如何在go包之间传递C对象？

Question

When I try to pass a C.int from package main to a function in a helper package called common, I get the following error:

main.go:24: cannot use argc (type C.int) as type common.C.int in argument to common.GoStrings

From common.go:

/* 
    ...
*/
import "C"

...

func GoStrings(argc C.int, argv **C.char) (args []string) {
    // do stuff
}

From main.go:

/*
#cgo LDFLAGS: -lpam -fPIC
#define PAM_SM_AUTH

#include <security/pam_appl.h>
*/
import "C"

...

func pam_sm_authenticate(pamh *C.pam_handle_t, flags, argc C.int, argv **C.char) C.int {
    args := common.GoStrings(argc, argv)
    ...
}

Is there any way to pass these objects back and forth? I've tried type casting to e.g. common.C.int, but that doesn't seem to be valid syntax. I'd like to be able to call GoStrings from multiple different main programs, and it seems like that should be allowable.

Answer 1

Unfortunately you can't pass C types between packages. You'll need to perform any necessary type conversions within the package that is importing the C types. As per the documentation:

Cgo translates C types into equivalent unexported Go types. Because the translations are unexported, a Go package should not expose C types in its exported API: a C type used in one Go package is different from the same C type used in another.

If you have common C translation methods that you use, consider using go generate with a helper script to create these in each package where it is required from a master source file. Not as nice as solution as having a common library but much better than manually updating files in multiple packages.