什么是cgo类型等同于指向结构的const指针？

Question

我有一个外部函数decalred在C：

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

要go定义：

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

当编译代码时，我得到类型冲突的错误

# 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*); 
      ^~~~~~~~~~ 

是什么go相当于​​3210？

Answer 1

与typedef创建类型别名的结构如下：

/* 

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类型定义，它是C.clibvlc_event_t兼容，运行cgo与-godefs选项，即

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

例如，如果结构被定义为如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; 
}; 

然后，在go兼容结构将是：

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{})) 

编辑：
添加示例结构。