objective-c - 将 UnsafeMutablePointer 传递给函数时出错

Question

我是一名使用 swift 的 ios 开发者。我正在使用 sdk 与 ip 摄像头通信，从中检索数据，并将其显示给用户。我有一个 sdk 附带的项目，它是用 obj c 编写的，并且运行没有问题。问题是我无法将参数从函数传递到预期的参数类型。SDK 很旧，我使用静态库和桥接头将其集成，并且能够成功连接到相机。我认为sdk是用c或c++写的，我基本上是参数转换有问题。

Fos.h

typedef struct   
{ 
int                       channel;
long long                 time;
unsigned int              index;
FOSMEDIATYPE              type;     //Is video or audio
FOSDECFMT                 fmt;      //The format of video or audio.
short                     isKey;
short                     multiAudioPage;
int                       frameTag; 
union{ 
    FOSVIDEO_INFO      video;       //Media type is video.
    FOSAUDIO_INFO      audio;       //Media type is audio.
} media;
unsigned long long      pts;        //Pts.
unsigned int              len;      //Size of data.
char                      data[0];  //Just data.
}ATTRIBUTE_PACKED FOSDEC_DATA;    //Save the video or audio data,include video or audio information.

.

Objective C 函数，当前有效

- (IBAction)login:(id)sender {
//[self._imageView add]
NSString* str = [self._textField text];
const char* url = [str UTF8String];
char ip[128];
int port;
char usr[64];
char pwd[64];
int count = sscanf(url, "%[^:]:%d/%[^:]:%s", ip, &port, usr, pwd);
if (count != 4) {
    return;
}

mHandle = FosSdk_Create(ip, "", usr, pwd, port, port, FOSIPC_H264, FOSCNTYPE_IP);
//mHandle = FosSdk_Create(ip, "FASDFDSAFDASFASD", usr, pwd, port, port, FOSIPC_H264, FOSCNTYPE_P2P);
if (mHandle == FOSHANDLE_INVALID) {
    return;
}
[self._Login setEnabled:NO];
[self._Logout setEnabled:YES];

[NSThread detachNewThreadSelector:@selector(RunInThread:) toTarget:self withObject:^(void *param){

    int usrPrivilege = 0;
    FOSCMD_RESULT ret = FosSdk_Login(mHandle, &usrPrivilege, 500);
    if (FOSCMDRET_OK != ret) {
        return;
    }

    ret = FosSdk_OpenVideo(mHandle, FOSSTREAM_SUB, 500);
    if (FOSCMDRET_OK != ret) {
        return;
    }
    //char* framebuf = (char*)malloc(512*1024);
    FOSDEC_DATA* data = NULL;

    int outlen = 0;
    while (mHandle) {
// FosSdk_RetainHandle(mHandle, &usrPrivilege);
        if ( FOSCMDRET_OK == FosSdk_GetVideoData(mHandle, (char**)&data, &outlen, FOSDECTYPE_RGB24) && outlen>0)
        {

            if (data->type == FOSMEDIATYPE_VIDEO) {
                //int a=0;
                //a++;
                [self imageFromAVPicture:data->data width:data->media.video.picWidth height:data->media.video.picHeight];
            }
            else{
            }
        }
        usleep(20*1000);
    }
 }];
}

.

fos.h中的方法签名

    FOSSDK FOSCMD_RESULT FOSAPI FosSdk_GetVideoData(FOSHANDLE handle, char **data, int *outLen, FOSDECFMT videoFmt);

.

从 objective-c 调用时

FosSdk_GetVideoData(<#unsigned int handle#>, <#char **data#>, <#int *outLen#>, <#FOSDECFMT videoFmt#>)
}];

.

从swift调用时

FosSdk_GetVideoData(handle: UInt32,
                   data: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!,
                   outLen: UnsafeMutablePointer<Int32>!,
                   videoFmt: FOSDECFMT)

.

错误是数据参数，正是:

Cannot convert value of type 'UnsafeMutablePointer<FOSDEC_DATA>' to expected argument type 'UnsafeMutablePointer<Int8>?'

.

我试过将数据声明更改为此，但它给了我这个错误

let data = UnsafeMutablePointer<Int8>(&fosdecData)
Ambiguous use of 'init'

.

这个指针有点复杂，但我能够使用同样使用 unsafeMutablePointers 的方法将我的设备与相机配对。但是，我不知道如何将这个空数据对象传递给函数。

SDK 已过时且文档错误，这无济于事。无论如何，我需要让它工作。希望能得到一些帮助，谢谢各位码农!.

编辑，感谢 OOPer.这是 swift 中的更新版本，它实际上会产生意外错误。从函数返回的结果现在是 0，所以它是正确的。但是，在尝试访问原始 FosdedData 结构时，我有一个 Command failed due to signal: Segmentation fault: 11。我怎么能解决这个问题？抱歉问题很长，我想准确一点。

var mhandle : UInt32 = FOS_HANDLE_INIT.rawValue

override func viewDidLoad() {
    super.viewDidLoad()

    //These functions basically do camera pairing, and write result on mhandle, which is equal to 0 , if functions work, otherwise 1
    initializeFOS()
    startEzLinkFOS()
    sdkCreateFOS()
    loginFOS()
    openVideoFOS()        

    var data: UnsafeMutablePointer<FOSDEC_DATA>? = nil
    var outlen : Int32 = 0
    while self.mhandle > 0 {
        let resultOOPer = withUnsafeMutablePointer(to: &data) { pointerToFosdecData in
            pointerToFosdecData.withMemoryRebound(to: UnsafeMutablePointer<Int8>?.self, capacity: 1, { (pointerToInt8Pointer) in

                FosSdk_GetVideoData(self.mhandle, pointerToInt8Pointer, &outlen, FOSDECTYPE_RGB24)
            })
        }

        if resultOOPer == FOSCMDRET_OK && outlen > 0{
            if let unwrData = data {

                //if unwrData.pointee.type == FOSMEDIATYPE_VIDEO{
                //Produced Output when i try access pointee, which is actually the fosdecData struct i need:
                //Command failed due to signal: Segmentation fault: 11
                //}
            }
        }
        usleep(20*1000)
    }


}

Answer 1

首先，使用UnsafeMutablePointer的初始化器以这种方式可能会产生意想不到的结果，你永远不应该这样做:

let data = UnsafeMutablePointer<FOSDEC_DATA>(&fosdecData)

Swift 编译器可能会为 inout 参数准备一个临时区域，并传递该区域的地址，该地址在调用后立即释放。不幸的是，此代码在某些情况下可能有效，但您知道在某些情况下有效的代码可能是一个难以修复的错误。

---

其次，在您的 Objective-C 代码中，您声明 data作为FOSDEC_DATA* -- 指向 FOSDEC_DATA 的指针-- 并将其初始化为 NULL .

为什么要尝试初始化 data 的 Swift 版本？与 &fosdecData ？没有 fosdecData在您的 Objective-C 代码中。

声明 data 的等效 Swift 代码应该是这样的:

var data: UnsafeMutablePointer<FOSDEC_DATA>? = nil

您应该声明data作为UnsafeMutablePointer<FOSDEC_DATA>? -- 指向 FOSDEC_DATA 的指针-- 并将其初始化为 nil .

另外还需要是var , 你的 FosSdk_GetVideoData会将其重写为指向 FOSDEC_DATA 的有效指针.

---

第三，需要通过UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!到 FosSdk_GetVideoData 的第二个参数，但是&data会生成一个 UnsafeMutablePointer<UnsafeMutablePointer<FOSDEC_DATA>?> 类型的指针.

要进行这种指针类型转换，需要使用withMemoryRebound<T, Result>(to:capacity:_:) .你需要调用withUnsafeMutablePointer<T, Result>(to:_:)获取指向 data 的指针.

let result = withUnsafeMutablePointer(to: &data) {
    pointerToFosdecDataPtr in
    //pointerToFosdecDataPtr: UnsafeMutablePointer<UnsafeMutablePointer<FOSDEC_DATA>?>
    pointerToFosdecDataPtr.withMemoryRebound(to: UnsafeMutablePointer<Int8>?.self, capacity: 1) {
        pointerToInt8Ptr in
        //pointerToInt8Ptr: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>
        //...
    }
}

---

因此，您的 Swift 代码相当于您的 Objective-C 代码将是这样的:

var data: UnsafeMutablePointer<FOSDEC_DATA>? = nil
var outlen: Int32 = 0

while mHandle != 0 {

    let result = withUnsafeMutablePointer(to: &data) {
        pointerToFosdecDataPtr in
        //pointerToFosdecDataPtr: UnsafeMutablePointer<UnsafeMutablePointer<FOSDEC_DATA>?>
        pointerToFosdecDataPtr.withMemoryRebound(to: UnsafeMutablePointer<Int8>?.self, capacity: 1) {
            pointerToInt8Ptr in
            //pointerToInt8Ptr: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>
            FosSdk_GetVideoData(mHandle, pointerToInt8Ptr, &outlen, FOSDECTYPE_RGB24)
            //Implicit `return` works here and the result is passed to `result`.
        }
    }
    if result == FOSCMDRET_OK && outlen > 0 {
        if let data = data, data.pointee.type == FOSMEDIATYPE_VIDEO {
            //...
            //self.imageFromAVPicture...
        } else {
        }
    }
    usleep(20*1000)
}

---

编辑

代码更新以反射(reflect)当前工作 Objective-C 代码的整体行为。 (在 Objective-C 代码中将 mhandle 重命名为 mHandle。)