什麼是cgo類型等同於指向結構的const指針？

Question

我有一個外部函數decalred在C：

//extern void goCallback(const struct libvlc_event_t*, void*); 

要go定義：

//export goCallback 
func goCallback(event unsafe.Pointer, userData unsafe.Pointer) { 
    log.Fatal("TODO goCallback") 
} 

當編譯代碼時，我得到類型衝突的錯誤

# github.com/tarrsalah/libvlc-go 
In file included from $WORK/github.com/tarrsalah/libvlc-go/_obj/_cgo_export.c:3:0: 
cgo-gcc-export-header-prolog:42:13: error: conflicting types for ‘goCallback’ 
In file included from $WORK/github.com/tarrsalah/libvlc-go/_obj/_cgo_export.c:3:0: 
../../../tarrsalah/libvlc-go/event_manager.go:6:13: note: previous declaration of ‘goCallback’ was here 
//extern void goCallback(const struct libvlc_event_t*, void*); 
      ^~~~~~~~~~ 
/tmp/go-build855229382/github.com/tarrsalah/libvlc-go/_obj/_cgo_export.c:17:6: error: conflicting types for ‘goCallback’ 
void goCallback(void* p0, void* p1) 
     ^~~~~~~~~~ 
In file included from $WORK/github.com/tarrsalah/libvlc-go/_obj/_cgo_export.c:3:0: 
../../../tarrsalah/libvlc-go/event_manager.go:6:13: note: previous declaration of ‘goCallback’ was here 
//extern void goCallback(const struct libvlc_event_t*, void*); 
      ^~~~~~~~~~ 

是什麼go相當於​​3210？

Answer 1

與typedef創建類型別名的結構如下：

/* 

typedef const struct libvlc_event_t clibvlc_event_t; 
extern void goCallback(clibvlc_event_t*, void*); 
*/ 
import "C" 

//export goCallback 
func goCallback(event *C.clibvlc_event_t, userData unsafe.Pointer) { 
    log.Fatal("TODO goCallback") 
} 

如果你想知道go類型定義，它是C.clibvlc_event_t兼容，運行cgo與-godefs選項，即

go tool cgo -godefs <YOUR-GO-FILE> 

例如，如果結構被定義爲如https://docs.libreoffice.org/avmedia/html/Types_8hxx_source.html

struct libvlc_event_t 
{ 
    int type; // event type 
    void *p_obj; // object emitting that event 

    union // so far we don't need this. 
    { 
    struct { 
     const char *dummy1; 
     const char *dummy2; 
     } padding; 
    } u; 
}; 

然後，在go兼容結構將是：

type VLCEvent struct { 
    Type  int32 
    Pad_cgo_0 [4]byte 
    Obj  *byte 
    U   [16]byte 
} 

//Cast C struct to Go struct 
ev := (*VLCEvent)(unsafe.Pointer(event)) 

//Cast Go struct to C struct 
p := (*C.clibvlc_event_t)(unsafe.Pointer(&VLCEvent{})) 

編輯：
添加示例結構。