问题描述
我正在与使用C插座某些病人监护仪沟通的程序。我使用无连接插座（UDP）与所述设备进行通信。但是，我的计算机和设备，到目前为止，我这样做是为了获得病人监护仪解析响应之间的字节顺序不匹配：
I'm making a program that communicate with certain patient monitor using C sockets. I'm using connection-less sockets (UDP) to communicate with the device. But there is endianness mis-match between my computer and device and so far I was doing this to get parse response from the patient monitor:
recvfrom(int socket, char *buffer, size_t length, int flags,
struct sockaddr *address, socklen_t *address_len);
然后我被铸造缓冲区直接的结构，用ntohs和和ntohl改变字节顺序，例如：
Then I was casting the buffer directly to structure and using the ntohs and ntohl to change the byte ordering, for example:
struct A * a = (struct A *)buffer;
Struct A b;
b.v1 = ntohs(a->v1);
b.v2 = ntohl(a->v2);
在网上阅读几个例子之后，我想通了，这可能是错误的做法，因为编译器相关的填充。但我不知道这件事。我需要简单的方法来取消马歇尔缓冲区C结构具有正确的字节序。那我收到的结构可以取消predictable长度和复杂一些的为好。我需要快速且简单的方法做未编组。
After reading few examples over internet, I figured out that this may be wrong approach due to compiler dependent padding. But I'm not sure about it. I need simple way to un-marshall a buffer to a C structure with correct endiannes. The structure that I'm receiving can be of unpredictable length and little complex as well. I need fast and easy approach to do the un-marshalling.
我不控制发件人。发件人在网络字节顺序。我的问题仅仅是： -
是否是安全蒙上了缓冲结构，然后用ntohs和和ntohl在此铸造结构，使得这个结构的主机字节顺序翻版？它是一个最快的方法？如果没有，那么什么可以最快的方法？
I don't control sender. Sender is in network byte order. My question is only that:-Is is safe to cast a buffer to a structure and then use ntohs and ntohl on this casted structure to make host-byte order replica of this structure? Is it a fastest approach? If not, then what can be the fastest approach?
推荐答案
首先，因为 A 是apointer，你的code应该以最低的做到这一点。 ..
First of all, since a is apointer, your code should at minimum do this...
b.v1 = ntohs(a->v1);
b.v2 = ntohl(a->v2);
所有的二，答案取决于情况和病人监护仪的规范。谁写code为病人监护仪？是否有它的规范？什么机器架构是使用（你知道的情况下），你处理的只是一个模型，还是有显示器的许多版本 - 你可以改变显示器等等等等
Second of all, the answer depends on circumstances and specification of the patient monitor. Who writes the code for the patient monitor? Is there a specification for it? What machine architecture is it using(in case you know), are you dealing with just one model or are there many versions of the monitor -- can you change the monitor, etc. etc.
我要去假设你的无法的改变显示器，而字节顺序在某处记录 - 你的可能的必须创建自己的解包/包例程，按字节寻址和位操作这样做 - 那就是，除非你知道的格式完全一致，即网络命令 - 和结构的填充是在网络缓冲区相同
Im going to assume that you cannot change the monitor, and that the byte order is documented somewhere -- and you may have to create your own unpack/pack routines, by doing byte addressing and bit manipulation -- that is unless you know that the format exactly matches that of "network" order -- and that padding of structs are the same in the network buffer.
因此，像;
void unpack(struct *b, unsigned char *buffer)
{
b->v1 = (buffer[0] <<8)|(buffer[1]);
b->v2 = (buffer[2] <<24)|(buffer[3] <<16)|(buffer[4] <<8)|(buffer[5]);
etc....
}
或者类似这样的，如果你preFER你ntohX;
or like this if you prefer to you ntohX;
void unpack(struct *b, unsigned char *buffer)
{
b->v1 = ntohs(buffer+0);
b->v2 = ntohl(buffer+2);
etc....
}
然而，如果你的做的控制监视器code，然后使用类似的将摆脱做位操作的所有复杂性和担心字节顺序....
However if you do control the monitor code, then using a tool like protocol buffers would get rid of all the complexity of doing bit manipulation and worry about byte orders....
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