Topic : Using Internet Sockets
Author : Beej
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     sockfd = socket(AF_INET, SOCK_STREAM, 0); // do some error checking!

        dest_addr.sin_family = AF_INET;          // host byte order
        dest_addr.sin_port = htons(DEST_PORT);   // short, network byte order
        dest_addr.sin_addr.s_addr = inet_addr(DEST_IP);
        memset(&(dest_addr.sin_zero), '\0', 8);  // zero the rest of the struct

        // don't forget to error check the connect()!
        connect(sockfd, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr));

Again, be sure to check the return value from connect()--it'll return -1 on error and set the variable errno.

Also, notice that we didn't call bind(). Basically, we don't care about our local port number; we only care where we're going (the remote port). The kernel will choose a local port for us, and the site we connect to will automatically get this information from us. No worries.

4.4. listen()--Will somebody please call me?
Ok, time for a change of pace. What if you don't want to connect to a remote host. Say, just for kicks, that you want to wait for incoming connections and handle them in some way. The process is two step: first you listen(), then you accept() (see below.)

The listen call is fairly simple, but requires a bit of explanation:

    int listen(int sockfd, int backlog);

sockfd is the usual socket file descriptor from the socket() system call. backlog is the number of connections allowed on the incoming queue. What does that mean? Well, incoming connections are going to wait in this queue until you accept() them (see below) and this is the limit on how many can queue up. Most systems silently limit this number to about 20; you can probably get away with setting it to 5 or 10.

Again, as per usual, listen() returns -1 and sets errno on error.

Well, as you can probably imagine, we need to call bind() before we call listen() or the kernel will have us listening on a random port. Bleah! So if you're going to be listening for incoming connections, the sequence of system calls you'll make is:

    /* accept() goes here */

I'll just leave that in the place of sample code, since it's fairly self-explanatory. (The code in the accept() section, below, is more complete.) The really tricky part of this whole sha-bang is the call to accept().

4.5. accept()--"Thank you for calling port 3490."

Get ready--the accept() call is kinda weird! What's going to happen is this: someone far far away will try to connect() to your machine on a port that you are listen()ing on. Their connection will be queued up waiting to be accept()ed. You call accept() and you tell it to get the pending connection. It'll return to you a brand new socket file descriptor to use for this single connection! That's right, suddenly you have two socket file descriptors for the price of one! The original one is still listening on your port and the newly created one is finally ready to send() and recv(). We're there!

The call is as follows:

     #include <sys/socket.h>

     int accept(int sockfd, void *addr, int *addrlen);

sockfd is the listen()ing socket descriptor. Easy enough. addr will usually be a pointer to a local struct sockaddr_in. This is where the information about the incoming connection will go (and with it you can determine which host is calling you from which port). addrlen is a local integer variable that should be set to sizeof(struct sockaddr_in) before its address is passed to accept(). Accept will not put more than that many bytes into addr. If it puts fewer in, it'll change the value of addrlen to reflect that.

Guess what? accept() returns -1 and sets errno if an error occurs. Betcha didn't figure that.

Like before, this is a bunch to absorb in one chunk, so here's a sample code fragment for your perusal:

    #include <string.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>

    #define MYPORT 3490    // the port users will be connecting to

    #define BACKLOG 10     // how many pending connections queue will hold

        int sockfd, new_fd;  // listen on sock_fd, new connection on new_fd
        struct sockaddr_in my_addr;    // my address information
        struct sockaddr_in their_addr; // connector's address information
        int sin_size;

        sockfd = socket(AF_INET, SOCK_STREAM, 0); // do some error checking!

        my_addr.sin_family = AF_INET;         // host byte order
        my_addr.sin_port = htons(MYPORT);     // short, network byte order
        my_addr.sin_addr.s_addr = INADDR_ANY; // auto-fill with my IP
        memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct

        // don't forget your error checking for these calls:
        bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr));

        listen(sockfd, BACKLOG);

        sin_size = sizeof(struct sockaddr_in);
        new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);

Again, note that we will use the socket descriptor new_fd for all send() and recv() calls. If you're only getting one single connection ever, you can close() the listening sockfd in order to prevent more incoming connections on the same port, if you so desire.

4.6. send() and recv()--Talk to me, baby!
These two functions are for communicating over stream sockets or connected datagram sockets. If you want to use regular unconnected datagram sockets, you'll need to see the section on sendto() and recvfrom(), below.

The send() call:

    int send(int sockfd, const void *msg, int len, int flags);

sockfd is the socket descriptor you want to send data to (whether it's the one returned by socket() or the one you got with accept().) msg is a pointer to the data you want to send, and len is the length of that data in bytes. Just set flags to 0. (See the send() man page for more information concerning flags.)

Some sample code might be:

    char *msg = "Beej was here!";
    int len, bytes_sent;
    len = strlen(msg);
    bytes_sent = send(sockfd, msg, len, 0);

send() returns the number of bytes actually sent out--this might be less than the number you told it to send! See, sometimes you tell it to send a whole gob of data and it just can't handle it. It'll fire off as much of the data as it can, and trust you to send the rest later. Remember, if the value returned by send() doesn't match the value in len, it's up to you to send the rest of the string. The good news is this: if the packet is small (less than 1K or so) it will probably manage to send the whole thing all in one go. Again, -1 is returned on error, and errno is set to the error number.

The recv() call is similar in many respects:

    int recv(int sockfd, void *buf, int len, unsigned int flags);

sockfd is the socket descriptor to read from, buf is the buffer to read the information into, len is the maximum length of the buffer, and flags can again be set to 0. (See the recv() man page for flag information.)

recv() returns the number of bytes actually read into the buffer, or -1 on error (with errno set, accordingly.)

Wait! recv() can return 0. This can mean only one thing: the remote side has closed the connection on you! A return value of 0 is recv()'s way of letting you know this has occurred.

There, that was easy, wasn't it? You can now pass data back and forth on stream sockets!

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