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Beej's Guide to Network Programming
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3.    structs and Data Handling

Well, we're finally here. It's time to talk about programming. In this section, I'll cover various data types used by the sockets interface, since some of them are a real bear to figure out.

First the easy one: a socket descriptor. A socket descriptor is the following type:
  
    int 

Just a regular int.

Things get weird from here, so just read through and bear with me. Know this: there are two byte orderings: most significant byte (sometimes called an "octet") first, or least significant byte first. The former is called "Network Byte Order". Some machines store their numbers internally in Network Byte Order, some don't. When I say something has to be in Network Byte Order, you have to call a function (such as htons()) to change it from "Host Byte Order". If I don't say "Network Byte Order", then you must leave the value in Host Byte Order.

(For the curious, "Network Byte Order" is also know as "Big-Endian Byte Order".)

My First StructTM--struct sockaddr. This structure holds socket address information for many types of sockets:
  
    struct sockaddr {
        unsigned short    sa_family;    // address family, AF_xxx
        char              sa_data[14];  // 14 bytes of protocol address
    }; 

sa_family can be a variety of things, but it'll be AF_INET for everything we do in this document. sa_data contains a destination address and port number for the socket. This is rather unwieldy since you don't want to tediously pack the address in the sa_data by hand.

To deal with struct sockaddr, programmers created a parallel structure: struct sockaddr_in ("in" for "Internet".)
  
    struct sockaddr_in {
        short int          sin_family;  // Address family
        unsigned short int sin_port;    // Port number
        struct in_addr     sin_addr;    // Internet address
        unsigned char      sin_zero[8]; // Same size as struct sockaddr
    }; 

This structure makes it easy to reference elements of the socket address. Note that sin_zero (which is included to pad the structure to the length of a struct sockaddr) should be set to all zeros with the function memset(). Also, and this is the important bit, a pointer to a struct sockaddr_in can be cast to a pointer to a struct sockaddr and vice-versa. So even though socket() wants a struct sockaddr*, you can still use a struct sockaddr_in and cast it at the last minute! Also, notice that sin_family corresponds to sa_family in a struct sockaddr and should be set to "AF_INET". Finally, the sin_port and sin_addr must be in Network Byte Order!

"But," you object, "how can the entire structure, struct in_addr sin_addr, be in Network Byte Order?" This question requires careful examination of the structure struct in_addr, one of the worst unions alive:
  
    // Internet address (a structure for historical reasons)
    struct in_addr {
        unsigned long s_addr;
    }; 

Well, it used to be a union, but now those days seem to be gone. Good riddance. So if you have declared ina to be of type struct sockaddr_in, then ina.sin_addr.s_addr references the 4-byte IP address (in Network Byte Order). Note that even if your system still uses the God-awful union for struct in_addr, you can still reference the 4-byte IP address in exactly the same way as I did above (this due to #defines.)

3.1. Convert the Natives!

We've now been lead right into the next section. There's been too much talk about this Network to Host Byte Order conversion--now is the time for action!

All righty. There are two types that you can convert: short (two bytes) and long (four bytes). These functions work for the unsigned variations as well. Say you want to convert a short from Host Byte Order to Network Byte Order. Start with "h" for "host", follow it with "to", then "n" for "network", and "s" for "short": h-to-n-s, or htons() (read: "Host to Network Short").

It's almost too easy...

You can use every combination if "n", "h", "s", and "l" you want, not counting the really stupid ones. For example, there is NOT a stolh() ("Short to Long Host") function--not at this party, anyway. But there are:

Now, you may think you're wising up to this. You might think, "What do I do if I have to change byte order on a char?" Then you might think, "Uh, never mind." You might also think that since your 68000 machine already uses network byte order, you don't have to call htonl() on your IP addresses. You would be right, BUT if you try to port to a machine that has reverse network byte order, your program will fail. Be portable! This is a Unix world! (As much as Bill Gates would like to think otherwise.) Remember: put your bytes in Network Byte Order before you put them on the network.

A final point: why do sin_addr and sin_port need to be in Network Byte Order in a struct sockaddr_in, but sin_family does not? The answer: sin_addr and sin_port get encapsulated in the packet at the IP and UDP layers, respectively. Thus, they must be in Network Byte Order. However, the sin_family field is only used by the kernel to determine what type of address the structure contains, so it must be in Host Byte Order. Also, since sin_family does not get sent out on the network, it can be in Host Byte Order.

3.2. IP Addresses and How to Deal With Them

Fortunately for you, there are a bunch of functions that allow you to manipulate IP addresses. No need to figure them out by hand and stuff them in a long with the << operator.

First, let's say you have a struct sockaddr_in ina, and you have an IP address "10.12.110.57" that you want to store into it. The function you want to use, inet_addr(), converts an IP address in numbers-and-dots notation into an unsigned long. The assignment can be made as follows:
  
    ina.sin_addr.s_addr = inet_addr("10.12.110.57"); 

Notice that inet_addr() returns the address in Network Byte Order already--you don't have to call htonl(). Swell!

Now, the above code snippet isn't very robust because there is no error checking. See, inet_addr() returns -1 on error. Remember binary numbers? (unsigned)-1 just happens to correspond to the IP address 255.255.255.255! That's the broadcast address! Wrongo. Remember to do your error checking properly.

Actually, there's a cleaner interface you can use instead of inet_addr(): it's called inet_aton() ("aton" means "ascii to network"):
  
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>

    int inet_aton(const char *cp, struct in_addr *inp); 

And here's a sample usage, while packing a struct sockaddr_in (this example will make more sense to you when you get to the sections on bind() and connect().)
  
    struct sockaddr_in my_addr;

    my_addr.sin_family = AF_INET;         // host byte order
    my_addr.sin_port = htons(MYPORT);     // short, network byte order
    inet_aton("10.12.110.57", &(my_addr.sin_addr));
    memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct 

inet_aton() returns 0 on success, and non-zero on failure, and the address is passed back in inp.

Unfortunately, not all platforms implement inet_aton() so, although its use is preferred, the older more common inet_addr() is used in this guide.

All right, now you can convert string IP addresses to their binary representations. What about the other way around? What if you have a struct in_addr and you want to print it in numbers-and-dots notation? In this case, you'll want to use the function inet_ntoa() ("ntoa" means "network to ascii") like this:
  
    printf("%s", inet_ntoa(ina.sin_addr)); 

That will print the IP address. Note that inet_ntoa() takes a struct in_addr as an argument, not a long. Also notice that it returns a pointer to a char. This points to a statically stored char array within inet_ntoa() so that each time you call inet_ntoa() it will overwrite the last IP address you asked for. For example:
  
    char *a1, *a2;
    .
    .
    a1 = inet_ntoa(ina1.sin_addr);  // this is 192.168.4.14
    a2 = inet_ntoa(ina2.sin_addr);  // this is 10.12.110.57
    printf("address 1: %s\n",a1);
    printf("address 2: %s\n",a2); 

will print:
  
    address 1: 10.12.110.57
    address 2: 10.12.110.57 

If you need to save the address, strcpy() it to your own character array.

That's all on this topic for now. Later, you'll learn to convert a string like "whitehouse.gov" into its corresponding IP address (see DNS, below.)

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