如何将 **C.char 数据传回调用者的缓冲区？

Question

我将构建一个 Go 共享对象二进制文件（.DLL 和 .so），将字符串传回 Java。为了计算从 Go 传递的 C 字符串，我写了这个：

package main

/*
#include <stdlib.h>
*/
import "C"
import (
    "log"
    "unsafe"
)

//export passBackHello
func passBackHello(buf **C.char) C.int {
    str := "Hello World!"
    length := len(str)
    cString := C.CString(str) // returns *C.char
    defer C.free(unsafe.Pointer(cString))
    log.Println("In passBackHello: cString:", C.GoStringN(cString, C.int(length)))
    *buf = C.CString(str) // works
    *buf = cString        // doesn't work
    log.Println("In passBackHello: buf:", C.GoStringN(*buf, C.int(length)))

    return C.int(length)
}

func main() {
    buf := make([]byte, 8192) //create my buffer
    cStrPointer := (**C.char)(unsafe.Pointer(&buf[0]))
    defer C.free(unsafe.Pointer(cStrPointer))
    lengthCint := passBackHello(cStrPointer)
    log.Println("In main: length:", int(lengthCint))
    log.Println("In main: buf:", C.GoStringN(*cStrPointer, lengthCint))
    log.Println("In main: buf:", C.GoString(*cStrPointer))
}

当我在函数 passBackHello 中使用 *buf = C.CString(str) 时，它起作用了：

2018/03/31 19:33:54 In passBackHello: cString: Hello World!
2018/03/31 19:33:54 In passBackHello: buf: Hello World!
2018/03/31 19:33:54 In main: length: 12
2018/03/31 19:33:54 In main: buf: Hello World!
2018/03/31 19:33:54 In main: buf: Hello World!
exit status 3221226356

当我在函数 passBackHello 中使用 *buf = cStringPointer 时，它在 buf 中显示，而在 passBackHello 中显示，但在 main 中不显示：

2018/03/31 19:33:05 In passBackHello: cString: Hello World!
2018/03/31 19:33:05 In passBackHello: buf: Hello World!
2018/03/31 19:33:05 In main: length: 12
2018/03/31 19:33:05 In main: buf:  ⌂3     X☺�
2018/03/31 19:33:05 In main: buf:  ⌂3
exit status 3221226356

我需要让 *buf = cStringPointer 版本工作，因为那个版本有 C 字符串变量的 C.free。我是运行go版本go1.10.1windows/amd64.

更新

在应用 Azeem 的答案和一些其他清理后，工作的 Go 代码如下所示：

//export passBackHello
func passBackHello(cStrPointer **C.char) C.int {
    str := "Hello World!"
    length := len(str)
    *cStrPointer = C.CString(str) // copies *C.char into caller's buffer
    log.Println("In passBackHello *cStrPointer:", C.GoStringN(*cStrPointer, C.int(length)))
    return C.int(length)
}

func main() {
    buf := make([]byte, 8192) //create my buffer
    cStrPointer := (**C.char)(unsafe.Pointer(&buf[0]))
    defer C.free(unsafe.Pointer(cStrPointer))
    lengthCint := passBackHello(cStrPointer)
    log.Println("In main: length:", int(lengthCint))
    log.Println("In main: *cStrPointer:", C.GoStringN(*cStrPointer, lengthCint))
    log.Println("In main: *cStrPointer:", C.GoString(*cStrPointer))
}

为了完整起见，Java 调用者看起来像这样：

// allocate a void**
final PointerByReference decCStringPointer = new PointerByReference();
// call the C function
Integer decLength = gpg.passBackHello(decCStringPointer);
// extract the void* that was allocated in C
final Pointer p = decCStringPointer.getValue();
// extract the null-terminated string from the Pointer
final String decValue = p.getString(0);
System.out.printf("decrypted length: %d\n", decLength);
System.out.printf("decrypted value: %s\n", decValue);

看起来缓冲区在C内存中。如何从 Java 中释放它？

Answer 1

根据C.CString函数的文档：

// Go string to C string
// The C string is allocated in the C heap using malloc.
// It is the caller's responsibility to arrange for it to be
// freed, such as by calling C.free (be sure to include stdlib.h
// if C.free is needed).
func C.CString(string) *C.char

您不需要在您的函数中释放它。应该释放它的是调用者（即 main 函数）。

从您的 passBackHello 中删除 defer C.free(unsafe.Pointer(cString)) 行，它应该可以工作！

注意:
您不需要那个缓冲区（即 buf），因为它是正在返回的指针并且已经分配了内存。如果需要，您可以将其复制到缓冲区中并释放指针。

如何将 **C.char 数据传回调用者的缓冲区？

How do I pass **C.char data back to caller's buffer?

jna

go

cgo