BSD Sockets: A Quick And Dirty Primer
 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 th at 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