go - 如何在 Go 中填写 void* C 指针？

Question

我正在尝试与 Go 中的一些 C 代码进行交互。使用 cgo，这一直是相对简单的，直到我遇到这个(相当常见的)案例:需要传递一个指向本身包含指向某些数据的指针的结构的指针。如果不求助于将结构的创建放入 C 代码本身，我似乎无法弄清楚如何从 Go 中做到这一点，我不想这样做。这是一个说明问题的片段:

package main

// typedef struct {
//     int   size;
//     void *data;
// } info;
//
// void test(info *infoPtr) {
//     // Do something here...
// }
import "C"

import "unsafe"

func main() {
    var data uint8 = 5
    info := &C.info{size: C.int(unsafe.Sizeof(data)), data: unsafe.Pointer(&data)}
    C.test(info)
}

虽然编译正常，但尝试运行它会导致:

panic: runtime error: cgo argument has Go pointer to Go pointer

在我的例子中，传递给 C 调用的数据不会在调用后保留(即，有问题的 C 代码深入结构，复制它需要的内容，然后返回)。

Answer 1

参见 "Passing pointers" cgo 文档中的部分:

Go code may pass a Go pointer to C provided the Go memory to which it points does not contain any Go pointers.

还有:

These rules are checked dynamically at runtime. The checking is controlled by the cgocheck setting of the GODEBUG environment variable. The default setting is GODEBUG=cgocheck=1, which implements reasonably cheap dynamic checks. These checks may be disabled entirely using GODEBUG=cgocheck=0. Complete checking of pointer handling, at some cost in run time, is available via GODEBUG=cgocheck=2.

如果您运行您提供的代码段:

GODEBUG=cgocheck=0 go run snippet.go

然后就没有 panic 了。但是，正确的方法是使用 C.malloc(或从其他地方获取“C 指针”):

package main

// #include <stdlib.h>
// typedef struct {
//     int   size;
//     void *data;
// } info;
//
// void test(info *infoPtr) {
//     // Do something here...
// }
import "C"

import "unsafe"

func main() {
    var data uint8 = 5

    cdata := C.malloc(C.size_t(unsafe.Sizeof(data)))
    *(*C.char)(cdata) = C.char(data)
    defer C.free(cdata)

    info := &C.info{size: C.int(unsafe.Sizeof(data)), data: cdata}
    C.test(info)
}

之所以有效，是因为虽然常规的 Go 指针是不允许的，但 C.malloc 返回一个“C 指针”:

Go pointer means a pointer to memory allocated by Go (such as by using the & operator or calling the predefined new function) and the term C pointer means a pointer to memory allocated by C (such as by a call to C.malloc). Whether a pointer is a Go pointer or a C pointer is a dynamic property determined by how the memory was allocated.

请注意，您需要包含 stdlib.h 才能使用 C.free。