cgo类型中与指向结构体的常量指针等效的类型是什么?

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英文:

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

问题

我在C中有一个外部函数声明:

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

go中定义如下:

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

在编译代码时,我遇到了类型冲突错误:

# 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中与const struct libvlc_event_t*等效的是什么?

英文:

I have an extern function decalred in C:

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

To be defined in go:

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

When compiling the code, I get type conflicting errors

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

What is the go equivalent to const struct libvlc_event_t*?

答案1

得分: 2

创建一个使用typedef的结构体类型别名,如下所示:

typedef const struct libvlc_event_t clibvlc_event_t;
extern void goCallback(clibvlc_event_t*, void*);

导入"C"包

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

如果你想知道与C.clibvlc_event_t兼容的go类型定义,可以使用-godefs选项运行cgo,例如:

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

例如,如果结构体定义如下所示:https://docs.libreoffice.org/avmedia/html/Types_8hxx_source.html

struct libvlc_event_t
{
    int   type;  // 事件类型
    void *p_obj; // 发出该事件的对象

    union // 目前我们不需要这个。
    {
     struct {
        const char *dummy1;
        const char *dummy2;
      } padding;
    } u;
};

那么,在go中兼容的结构体将是:

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

//将C结构体转换为Go结构体
ev := (*VLCEvent)(unsafe.Pointer(event))

//将Go结构体转换为C结构体
p := (*C.clibvlc_event_t)(unsafe.Pointer(&VLCEvent{}))

编辑:
添加示例结构体。

英文:

Create type alias to the struct with typedef as follows:

/*

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

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

If you want to know go type definition which is compatible to C.clibvlc_event_t, run cgo with -godefs options, i.e.

go tool cgo -godefs &lt;YOUR-GO-FILE&gt;

For example, if the struct is defined as in 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&#39;t need this.
    {
     struct {
        const char *dummy1;
        const char *dummy2;
      } padding;
    } u;
};

then, the compatible struct in go will be:

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(&amp;VLCEvent{}))

EDIT:
Add example struct.

huangapple
  • 本文由 发表于 2017年6月23日 01:37:36
  • 转载请务必保留本文链接:https://go.coder-hub.com/44706042.html
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