by Jim Frost
 February 13, 1990

As you delve into the mysteries of UNIX, you find more and more things that are difficult to understand
immediately. One of these things, at least for most people, is the BSD socket concept. This is a short
tutorial that explains what they are, how they work, and gives sample code showing how to use them.

 The Analogy
 (or: What *IS* a socket, anyway?)

The socket is the BSD method for accomplishing interprocess communication (IPC). What this means is
a socket is used to allow one process to speak to another, very much like the telephone is used to allow
one person to speak to another.

The telephone analogy is a very good one, and will be used repeatedly to describe socket behavior.

 Installing Your New Phone
 (or: How to listen for socket connections)

In order for a person to receive telephone calls, he must first have a telephone installed. Likewise you
must create a socket to listen for connections. This process involves several steps. First you must make a
new socket, which is similar to having a telephone line installed. The socket() command is used to do
this.

Since sockets can have several types, you must specify what type of socket you want when you create one.
One option that you have is the addressing format of a socket. Just as the mail service uses a different
scheme to deliver mail than the telephone company uses to complete calls, so can sockets differ. The two
most common addressing schemes are AF_UNIX and AF_INET. AF_UNIX addressing uses UNIX
pathnames to identify sockets; these sockets are very useful for IPC between processes on the same
machine. AF_INET addressing uses Internet addresses which are four byte numbers usually written as
four decimal numbers separated by periods (such as 192.9.200.10). In addition to the machine address,
there is also a port number which allows more than one AF_INET socket on each machine. AF_INET
addresses are what we will deal with here.

Another option which you must supply when creating a socket is the type of socket. The two most
common types are SOCK_STREAM and SOCK_DGRAM. SOCK_STREAM indicates that data will
come across the socket as a stream of characters, while SOCK_DGRAM indicates that data will come in
bunches (called datagrams). We will be dealing with SOCK_STREAM sockets, which are very common.

After creating a socket, we must give the socket an address to listen to, just as you get a telephone
number so that you can receive calls. The bind() function is used to do this (it binds a socket to an
address, hence the name).

SOCK_STREAM type sockets have the ability to queue incoming connection requests, which is a lot like
having "call waiting" for your telephone. If you are busy handling a connection, the connection request
will wait until you can deal with it. The listen() function is used to set the maximum number of requests
(up to a maximum of five, usually) that will be queued before requests start being denied. While it is not
necessary to use the listen() function, it's good practice.

The following function shows how to use the socket(), bind(), and listen() functions to establish a socket
which can accept calls:

 /* code to establish a socket; originally from bzs@bu-cs.bu.edu
 */

 int establish(portnum)
 u_short portnum;
 { char myname[MAXHOSTNAME+1];
 int s;
 struct sockaddr_in sa;
 struct hostent *hp;

 bzero(&sa,sizeof(struct sockaddr_in)); /* clear our address */
 gethostname(myname,MAXHOSTNAME); /* who are we? */
 hp= gethostbyname(myname); /* get our address info */
 if (hp == NULL) /* we don't exist !? */
 return(-1);
 sa.sin_family= hp->h_addrtype; /* this is our host address */
 sa.sin_port= htons(portnum); /* this is our port number */
 if ((s= socket(AF_INET,SOCK_STREAM,0)) < 0) /* create socket */
 return(-1);
 if (bind(s,&sa,sizeof sa,0) < 0) {
 close(s);
 return(-1); /* bind address to socket */
 }
 listen(s, 3); /* max # of queued connects */
 return(s);
 }

After you create a socket to get calls, you must wait for calls to that socket. The accept() function is
used to do this. Calling accept() is analogous to picking up the telephone if it's ringing. Accept() returns
a new socket which is connected to the caller.

The following function can be used to accept a connection on a socket that has been created using the
establish() function above:

 int get_connection(s)
 int s; /* socket created with establish() */
 { struct sockaddr_in isa; /* address of socket */
 int i; /* size of address */
 int t; /* socket of connection */

 i = sizeof(isa); /* find socket's address */
 getsockname(s,&isa,&i); /* for accept() */

 if ((t = accept(s,&isa,&i)) < 0) /* accept connection if there is one */
 return(-1);
 return(t);
 }

Unlike with the telephone, you may still accept calls while processing previous connections. For this
reason you usually fork off jobs to handle each connection. The following code shows how to use
establish() and get_connection() to allow multiple connections to be dealt with:

 #include  /* obligatory includes */
 #include 
 #include 
 #include 
 #include 
 #include 
 #include 
 #include 

 #define PORTNUM 50000 /* random port number, we need something */

 void fireman(), do_something();

 main()
 { int s, t;

 if ((s= establish(PORTNUM)) < 0) { /* plug in the phone */
 perror("establish");
 exit(1);
 }

 signal(SIGCHLD, fireman); /* this eliminates zombies */

 for (;;) { /* loop for phone calls */
 if ((t= get_connection(s)) < 0) { /* get a connection */
 if (errno == EINTR) /* EINTR might happen on accept(), */
 continue; /* try again */
 perror("accept"); /* bad */
 exit(1);
 }
 switch(fork()) { /* try to handle connection */
 case -1 : /* bad news. scream and die */
 perror("fork");
 close(s);
 close(t);
 exit(1);
 case 0 : /* we're the child, do something */
 do_something(t);
 exit(0);
 default : /* we're the parent so look for */
 close(t); /* another connection */
 continue;
 }
 }
 }

 /* as children die we should get catch their returns or else we get
 * zombies, A Bad Thing. fireman() catches falling children.
 */

 void fireman()
 { union wait wstatus;

 while(wait3(&wstatus,WNOHANG,NULL) > 0)
 ;
 }

 /* this is the function that plays with the socket. it will be called
 * after getting a connection.
 */

 void do_something(s)
 int s;
 {
 /* do your thing with the socket here
 :
 :
 */
 }

 Dialing
 (or: How to call a socket)

You now know how to create a socket that will accept incoming calls.
So how do you call it? As with the telephone, you must first have the
phone before using it to call. You use the socket() function to do
this, exactly as you establish a socket to listen to.

After getting a socket to make the call with, and giving it an
address, you use the connect() function to try to connect to a
listening socket. The following function calls a particular port
number on a particular host:

 int call_socket(hostname, portnum)
 char *hostname;
 { struct sockaddr_in sa;
 struct hostent *hp;
 int a, s;

 if ((hp= gethostbyname(hostname)) == NULL) { /* do we know the host's */
 errno= ECONNREFUSED; /* address? */
 return(-1); /* no */
 }

 bzero(&sa,sizeof(sa));
 bcopy(hp->h_addr,(char *)&sa.sin_addr,hp->h_length); /* set address */
 sa.sin_family= hp->h_addrtype;
 sa.sin_port= htons((u_short)portnum);

 if ((s= socket(hp->h_addrtype,SOCK_STREAM,0)) < 0) /* get socket */
 return(-1);
 if (connect(s,&sa,sizeof sa) < 0) { /* connect */
 close(s);
 return(-1);
 }
 return(s);
 }

This function returns a connected socket through which data can flow.

 Conversation
 (or: How to talk between sockets)

Now that you have a connection between sockets you want to send data between them. The read() and
write() functions are used to do this, just as they are for normal files. There is only one major difference
between socket reading and writing and file reading and writing: you don't usually get back the same
number of characters that you asked for, so you usually loop until you have read the number of
characters that you want. A simple function to read a given number of characters into a buffer is:

 int read_data(s,buf,n)
 int s; /* connected socket */
 char *buf; /* pointer to the buffer */
 int n; /* number of characters (bytes) we want */
 { int bcount, /* counts bytes read */
 br; /* bytes read this pass */

 bcount= 0;
 br= 0;
 while (bcount < n) { /* loop until full buffer */
 if ((br= read(s,buf,n-bcount)) > 0) {
 bcount += br; /* increment byte counter */
 buf += br; /* move buffer ptr for next read */
 }
 if (br < 0) /* signal an error to the caller */
 return(-1);
 }
 return(bcount);
 }

A very similar function should be used to write data; we leave that function as an exercise to the reader.

 Hanging Up
 (or: What to do when you're done with a socket)

Just as you hang up when you're through speaking to someone over the telephone, so must you close a
connection between sockets. The normal close() function is used to close each end of a socket
connection. If one end of a socket is closed and the other tries to write to its end, the write will return an
error.

 Speaking The Language
 (or: Byte order is important)

Now that you can talk between machines, you have to be careful what you say. Many machines use
differing dialects, such as ASCII versus (yech) EBCDIC. More commonly there are byte-order problems.
Unless you always pass text, you'll run up against the byte-order problem. Luckily people have already
figured out what to do about it.

Once upon a time in the dark ages someone decided which byte order was "right". Now there exist
functions that convert one to the other if necessary. Some of these functions are htons() (host to network
short integer), ntohs() (network to host short integer), htoni() (host to network integer), ntohi() (network
to host integer), htonl() (host to network long integer), and ntohl() (network to host long integer).
Before sending an integer through a socket, you should first massage it with the htoni() function:

 i= htoni(i);
 write_data(s, &i, sizeof(i));

and after reading data you should convert it back with ntohi():

 read_data(s, &i, sizeof(i));
 i= ntohi(i);

If you keep in the habit of using these functions you'll be less likely to goof it up in those circumstances
where it is necessary.

 The Future Is In Your Hands
 (or: What to do now)

Using just what's been discussed here you should be able to build your own programs that communicate
with sockets. As with all new things, however, it would be a good idea to look at what's already been
done. Many public domain programs exist which make use of the socket concept, and many books exist
which go into much more depth than I have here. In addition I've deliberately left out a lot of details
such as what kinds of things can go wrong; the manual pages for each of the functions should be
consulted for this information.

If you have further questions about sockets or this primer, please feel free to ask me at email address
jimf@saber.com

Jim Frost
Saber Software
(617) 876-7636
jimf@saber.com