Socket send question

Question

Is there any reason why this shouldn't work?

[PseudoCode]
    main() {
       for (int i = 0; i < 10000; ++i) {
          send(i, "abc", 3, 0);
       }
    }

I mean, to send "abc" through every number from 0 to 10000, aren't we passing in theory by a lot of different sockets? Most numbers between 0 and 10000 will not correspond to any socket, but some will. Is this correct?

edit: The desired goal is to have "abc" sent through every application that has an open socket.

Answer 1

This won't work. File descriptor 0 in your process won't give you access to file descriptor 0 in some other application's process.

To answer your followup questions: Socket IDs are local to each process. They behave a lot like file descriptors -- there are many processes running at once, and of course the operating system keeps track of which process has which files open. But within each process, file descriptors 0, 1, and 2 will refer to its own, private, stdin, stdout, and stderr streams respectively. When a socket is created, the file descriptor it's assigned to is also only accessible from within that process.

Answer 2

That will never work. File descriptors are useful only within the same process (and its children).

You have to create a socket (this will get you a file descriptor you own and can use), connect it to an end point (which of course has to be open and listening) and only then you can send something through it.

For example:

struct sockaddr_in pin;
struct hostent *hp;

/* go find out about the desired host machine */
if ((hp = gethostbyname("foobar.com")) == 0) {
 exit(1);
}

/* fill in the socket structure with host information */
memset(&pin, 0, sizeof(pin));
pin.sin_family = AF_INET;
pin.sin_addr.s_addr = ((struct in_addr *)(hp->h_addr))->s_addr;
pin.sin_port = htons(PORT);

/* grab an Internet domain socket: sd is the file descriptor */
if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
 exit(1);
}

/* connect to PORT on HOST */
if (connect(sd,(struct sockaddr *)  &pin, sizeof(pin)) == -1) {
 exit(1);
}

/* send a message to the server PORT on machine HOST */
if (send(sd, argv[1], strlen(argv[1]), 0) == -1) {
 exit(1);
}

The other side of the coin is to create a listening socket (what servers do) which will receive connections. The process is similar but the calls change, they are socket(), bind(), listen(), accept(). Still, you have to create a socket to get the file descriptor in your own process and know where would you want to listen or connect to.

Answer 3

So, based on your replies to other people...

You have program A running on your machine which has opened a socket connection to some other program B, which could be running anywhere. But neither of these programs are the one you're trying to write here. And so you want your program to be able to send data through program A's socket connection to program B.

If this is roughly what you're trying to do, then no, you probably cannot do this. At least not without dll injection to get into the process of program A.

Furthermore, even if you could find a way to send through program A's socket, you would have to know the exact details of the communication protocol that program A and B are using. If you don't, then you'll run the risk of sending data to program B that it doesn't expect, in which case it could terminate the connection, crash, or do any number of bad things depending on how it was written.

And if you are really trying to send a particular piece of data not just through a single program A but through every program on the computer with a socket connection open, then you are highly likely to encounter what I just described. Even if the data you want to send would work for one particular program, other programs are almost certainly using entirely different communication protocols and thus will most likely have problems handling your data.

Without knowing what you're really trying to achieve, I can't say whether your goal is just going to be complicated and time-consuming to accomplish or if it is simply a bad idea that you shouldn't ever be trying to do. But whatever it is, I would suggest trying to find a different and better way than trying to send data through another program's socket.