beej's 网络编程 打包数据pack data

7.4. Serialization—How to Pack Data

It's easy enough to send text data across the network, you're finding, but what happens if you want to send some "binary" data like ints or floats? It turns out you have a few options.

1. Convert the number into text with a function like sprintf(), then send the text. The receiver will parse the text back into a number using a function like strtol().
2. Just send the data raw, passing a pointer to the data to send(). (因为send的原型：

   ssize_t send(int sockfd, const void *buf, size_t len, int flags);）

3. Encode the number into a portable binary form. The receiver will decode it.

Sneak preview! Tonight only!

[Curtain raises]

Beej says, "I prefer Method Three, above!"

[THE END]

(Before I begin this section in earnest,(认真的，诚挚的;正正经经)

I should tell you that there are libraries out there for doing this, and rolling your own自己来弄 and remaining portable and error-free is quite a challenge. So hunt around and do your homework before deciding to implement this stuff yourself. I include the information here for those curious about how things like this work.)

Actually all the methods, above, have their drawbacks and advantages, but, like I said, in general, I prefer the third method. First, though, let's talk about some of the drawbacks and advantages to the other two.

The first method, encoding the numbers as text before sending, has the advantage that you can easily print and read the data that's coming over the wire. Sometimes a human-readable protocol is excellent to use in a non-bandwidth-intensive situation, such as with Internet Relay Chat (IRC). However, it has the disadvantage that it is slow to convert, and the results almost always take up more space than the original number!

Method two: passing the raw data. This one is quite easy (but dangerous!): just take a pointer to the data to send, and call send with it.

double d = 3490.15926535;

send(s, &d, sizeof d, 0);  /* DANGER--non-portable! */

The receiver gets it like this:

double d;

recv(s, &d, sizeof d, 0);  /* DANGER--non-portable! */

Fast, simple—what's not to like? Well, it turns out that not all architectures represent a double (or int for that matter) with the same bit representation or even the same byte ordering! The code is decidedly non-portable. (Hey—maybe you don't need portability, in which case this is nice and fast.)

When packing integer types, we've already seen how the htons()-class of functions can help keep things portable by transforming the numbers into Network Byte Order, and how that's the Right Thing to do. Unfortunately, there are no similar functions for float types. Is all hope lost?

Fear not! (Were you afraid there for a second? No? Not even a little bit?) There is something we can do: we can pack (or "marshal", or "serialize", or one of a thousand million other names) the data into a known binary format that the receiver can unpack on the remote side.

What do I mean by "known binary format"? Well, we've already seen the htons() example, right? It changes (or "encodes", if you want to think of it that way) a number from whatever the host format is into Network Byte Order. To reverse (unencode) the number, the receiver calls ntohs().

But didn't I just get finished saying there wasn't any such function for other non-integer types? Yes. I did. And since there's no standard way in C to do this, it's a bit of a pickle (that a gratuitous pun there for you Python fans).

The thing to do is to pack the data into a known format and send that over the wire for decoding. For example, to pack floats, here's something quick and dirty with plenty of room for improvement:

http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html#serialization

pack模拟实现：