Sample read/write handling of packets in C

Question

I'm a bit new to C, but I've done my homework (some tutorials, books, etc.) and I need to program a simple server to handle requests from clients and interact with a db. I've gone through Beej's Guide to Network programming, but I'm a bit unsure how to piece together and handle different parts of the data getting sent back and forth. For instance, say the client is sending some information that the server will put in multiple fields. How do I piece together that data to be sent and then break it back up on the server side? Thanks, Eric

Accepted Answer

If I understand correctly, you're asking, "how does the server understand the information the client sends it"?

If that's what you're asking, the answer is simple: it's mutually agreed upon ahead of time that the data structures each uses will be compatible. I.e. you decide upon what your communication protocol will be ahead of time.

So, for example, if I have a client-server application where the client connects and can ask for things such as "time", "date" and can say "settime " and "setdate ", I need to write my server in such a way that it will understand those commands.

Obviously, in the above case it's trivial, since it'd just be a text-based protocol. But let's say you're writing an application that will return a struct of information, i.e.

```
struct Person {
    char* name;
    int age;
    int heightInInches;
    // ... other fields ...
};
```

You might write the entire struct out from the server/client. In this case there are a few things to be aware of:

- You need to hton/ntoh properly

- You need to make sure that your client and server both can understand the struct in question.

- You may or may not have to align on a 4B boundary (because if you don't, different C compilers may do different things, which may burn you between the client and the server, or it may not).

In general, though, when writing a client/server app, the most important thing to get right is the communication protocol.

I'm not sure if this quite answers your question, though. Is this what you were after, or were you asking more about how, exactly, do you use the send/recv functions?

Sample read/write handling of packets in C

Solution

Problem