小尾数字节顺序（iOS，BLE扫描响应） [英] Little-Endian Byte Order (iOS, BLE scan response)
问题描述
基本而言，我正在将nativescript用于跨平台应用程序。该应用程序与BLE设备进行交互。 BLE设备在广告包之后发送扫描响应，我可以在iOS端将其作为NSData对象进行检索。
In basic terms, I am using nativescript for a cross platform application. This application interacts with BLE devices. The BLE devices send a scan response after the advertisement packet, which I am able to retrieve as an NSData object on the iOS side.
这是来自的（伪）输出description属性：< 54fca267 0b00>。
Here is the (pseudo) output from the description property: <54fca267 0b00>.
输出代表外围设备的硬件地址。
The output represents the hardware address of the peripheral.
硬件人员告诉我，这几乎没有-endian，我知道它是一个48位字符串。
The hardware guys are telling me this is in little-endian, and I know it is a 48-bit string. It is coming from the peripheral, which has its firmware written in C.
我已经尝试了几天，将其转换为字符串（具有正确的字节序）。这样我就可以将硬件地址存储在后端，但是无法提出一种优雅的解决方案。
I have been trying for a few days to convert this into a string (with the correct endianness) so that I can store the hardware address on our back-end, but am unable to come up with an elegant solution.
我想出的一个解决方案是读取NSData对象的：description属性，然后手动（使用javascript）对其进行解码。我只是觉得这种解决方案不适用于生产环境，而是想使用本机实用程序。
One solution I did come up with was reading the :description property of the NSData object, and then manually (in javascript) 'decoding' it. I just do not feel like this solution is proper enough for a production environment, and would like to use native utilities instead.
我尝试使用：
NSString.alloc().initWithDataEncoding(mac, NSUTF8StringEncoding);
..每种编码类型均来自此页面页。
..with every encoding type from THIS page of the documentation. I am either getting nil in return, or incorrect characters.
就像我之前说过的那样，这是用NativeScript编写的。但是，我对目标C有一个不错的了解，因此我应该能够转换以该语言提供的任何示例。
Like I previously stated, this is being written in NativeScript; however, I have a decent understanding of objective C, so I should be able to convert any examples provided in that language.
任何输入将不胜感激。
Any input would be greatly appreciated. Thanks in advance!
推荐答案
CRD的答案是正确的，但我想这不是您要的内容。您想要一个看起来像 54：fc：a2：67：0b：00（蓝牙/ MAC地址）的结果，对吗？这是一种简单的蛮力方法（您可以循环执行类似操作，但不值得IMO）。
CRD's answer is on the right track, but I'm guessing is not what you're asking for. You want a result that looks like "54:fc:a2:67:0b:00" (a Bluetooth/MAC address), right? Here's a simple, brute force approach (you could do this in a loop or the like, but not worth it IMO).
// Just for testing; you already have data.
uint8_t address[6] = { 0x54, 0xfc, 0xa2, 0x67, 0x0b, 0x00 };
NSData *data = [NSData dataWithBytes:address length:6];
// Check the length before proceeding (maybe return an error rather than crash :D)
assert([data length] == 6);
// Grab the bytes
const uint8_t *bytes = data.bytes;
// Format them
NSString *addressString = [NSString stringWithFormat:@"%02x:%02x:%02x:%02x:%02x:%02x",
bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5]];
// 3. Profit
NSLog(@"%@", addressString);
但是....
I我不确定工程师在这里所说的小尾数是什么意思。 54：fc：a2 不是分配的制造商代码。通常，我会假设 little-endian表示它们已经将16位字（我在CSR平台上遇到过）进行了字节交换，因此应为 fc：54：67 ，但这两者均未分配。完全将整个东西视为 倒向我们 00：0b：67 分配给了制造蓝牙摄像头的TopView，因此我敢打赌这就是小端的含义（这很有意义，因为在线路上写入MAC地址的正常方法恰好相反。并且是 big-endian。）
I'm not quite certain what the engineer means here by "little-endian." 54:fc:a2 isn't an assigned manufacturer code. Typically, I would assume "little-endian" means that they've byte-swapped the 16-bit words (which I've run into on CSR platforms), and so this should be fc:54:67, but that isn't assigned either. Treating the whole thing as completely backwards gives us 00:0b:67 which is assigned to TopView, who makes Bluetooth cameras, so I'm betting that's what "little-endian" means here (which makes sense, because the normal way to write MAC addresses on the wire is exactly backwards of that, and is "big-endian").
因此这会将代码更改为：
So that would change the code to:
NSString *addressString = [NSString stringWithFormat:@"%02x:%02x:%02x:%02x:%02x:%02x",
bytes[5], bytes[4], bytes[3], bytes[2], bytes[1], bytes[0]];
地址是00：b0：67：a2：fc：54吗？
Is the address 00:b0:67:a2:fc:54?
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