什么是 cgo 类型，相当于指向结构的 const 指针？

Question

我在 C 中贴标了一个外部函数：

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

待定义 go:

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

编译代码时，出现类型冲突错误

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

go 等同于 const struct libvlc_event_t* 是什么？

Answer 1

使用 typedef 为结构创建类型别名，如下所示：

/*

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 兼容 C.clibvlc_event_t、运行 cgo 和 -godefs 选项的类型定义，即

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

例如，如果结构定义为 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;
};

那么，go 中的兼容结构将是：

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。

什么是 cgo 类型，相当于指向结构的 const 指针？

What is the cgo type equivalent to a const pointer to a struct?

go

cgo