handle

Documentation

Overview

Copyright 2021 The Go Authors. All rights reserved. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.

Index

• type Handle
  • func New(v interface{}) Handle
  • func (h Handle) Delete()
  • func (h Handle) Value() (interface{}, error)

Types

type Handle

type Handle uintptr

Handle provides a way to pass values that contain Go pointers (pointers to memory allocated by Go) between Go and C without breaking the cgo pointer passing rules. A Handle is an integer value that can represent any Go value. A Handle can be passed through C and back to Go, and Go code can use the Handle to retrieve the original Go value.

The underlying type of Handle is guaranteed to fit in an integer type that is large enough to hold the bit pattern of any pointer. The zero value of a Handle is not valid, and thus is safe to use as a sentinel in C APIs.

For instance, on the Go side:

package main

/*
#include <stdint.h> // for uintptr_t

extern void MyGoPrint(uintptr_t handle);
void myprint(uintptr_t handle);
*/
import "C"
import "runtime/cgo"

//export MyGoPrint
func MyGoPrint(handle C.uintptr_t) {
	h := cgo.Handle(handle)
	val := h.Value().(string)
	println(val)
	h.Delete()
}

func main() {
	val := "hello Go"
	C.myprint(C.uintptr_t(cgo.NewHandle(val)))
	// Output: hello Go
}

and on the C side:

#include <stdint.h> // for uintptr_t

// A Go function
extern void MyGoPrint(uintptr_t handle);

// A C function
void myprint(uintptr_t handle) {
    MyGoPrint(handle);
}

func New

func New(v interface{}) Handle

New returns a handle for a given value.

The handle is valid until the program calls Delete on it. The handle uses resources, and this package assumes that C code may hold on to the handle, so a program must explicitly call Delete when the handle is no longer needed.

The intended use is to pass the returned handle to C code, which passes it back to Go, which calls Value.

func (Handle) Delete

func (h Handle) Delete()

func (Handle) Value

func (h Handle) Value() (interface{}, error)

Value returns the associated Go value for a valid handle.

The method panics if the handle is invalid.