\documentstyle[a4]{article}
\begin{document}
\title{BSD Sockets: A Quick And Dirty Primer}
\author{Jim Frost}
\date{August 7, 1990}

\maketitle
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.
\section{The Analogy {\normalsize (or:  What *IS* a socket, anyway?)}}
The {\em 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.
\section{Installing Your New Phone {\normalsize(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 {\em 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 {\em AF\_UNIX} and {\em 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 {\em SOCK\_STREAM} and
{\em 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 {\em 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 {\em 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:
\begin{verbatim}
  /*
   *  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);
  }
\end{verbatim}
After you create a socket to get calls, you must wait for calls to
that socket.  The {\em 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:

\begin{verbatim}
  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(struct sockaddr_in);
    if ((t = accept(s,&isa,&i)) < 0)   /* accept connection if there is one */
      return(-1);
    return(t);
  }
\end{verbatim}
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:

\begin{verbatim}
  #include <errno.h>       /* obligatory includes */
  #include <signal.h>
  #include <stdio.h>
  #include <sys/types.h>
  #include <sys/socket.h>
  #include <sys/wait.h>
  #include <netinet/in.h>
  #include <netdb.h>

  #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
        :
        :
     */
  }
\end{verbatim}

\section{Dialing {\normalsize (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 {\em connect()} function to try to connect to a
listening socket.  The following function calls a particular port
number on a particular host:

\begin{verbatim}
  int call_socket(hostname, portnum)
  char *hostname;
  u_short portnum;
  { 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);
  }
\end{verbatim}
This function returns a connected socket through which data can flow.

\section{Conversation {\normalsize (or:  How to talk between sockets)}}
Now that you have a connection between sockets you want to send data
between them.  The {\em read()} and {\em 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:

\begin{verbatim}
  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 */
      }
      else if (br < 0)               /* signal an error to the caller */
        return(-1);
    }
    return(bcount);
  }
\end{verbatim}
A very similar function should be used to write data; we leave that
function as an exercise to the reader.

\section{Hanging Up {\normalsize (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
{\em 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.

\section{Speaking The Language {\normalsize (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 {\em htons()} (host to network
short integer), {\em ntohs()} (network to host short integer),
{\em htonl()} (host to network long integer), and {\em ntohl()}
(network to host long integer).  Before sending an integer through a
socket, you should first massage it with the htonl() function:

\begin{verbatim}
  i= htonl(i);
  write_data(s, &i, sizeof(i));
\end{verbatim}
and after reading data you should convert it back with ntohl():

\begin{verbatim}
  read_data(s, &i, sizeof(i));
  i= ntohl(i);
\end{verbatim}
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.

\section{The Future Is In Your Hands {\normalsize (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
\vspace*{1cm}
\begin{center}
\begin{tabular}{|c|} \hline
Jim Frost \\
Saber Software \\
(617) 876-7636 \\
jimf@saber.com \\ \hline
\end{tabular}
\end{center}
\end{document}
