Dec 202017
Article explains sockets, which is the BSD and other Unixs implement interprocess communications. Code examples provided.
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Article explains sockets, which is the BSD and other Unixs implement interprocess communications. Code examples provided.
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Contents of the SOCKETS.DOC file

BSD Sockets: A Quick And Dirty Primer
by Jim Frost
June 8, 1991

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 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 [email protected]

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 !? */
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 */
if (bind(s,&sa,sizeof sa,0) < 0) {
return(-1); /* bind address to socket */
listen(s, 3); /* max # of queued connects */

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(struct sockaddr_in);
if ((t = accept(s,&isa,&i)) < 0) /* accept connection if there is one */

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 */

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

void fireman(), do_something();

{ int s, t;

if ((s= establish(PORTNUM)) < 0) { /* plug in the phone */

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 */
switch(fork()) { /* try to handle connection */
case -1 : /* bad news. scream and die */
case 0 : /* we're the child, do something */
default : /* we're the parent so look for */
close(t); /* another connection */

/* 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

(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;
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 */

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 */

if (connect(s,&sa,sizeof sa) < 0) { /* connect */

This function returns a connected socket through which data can

(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 */
else if (br < 0) /* signal an error to the caller */

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),
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 htonl() function:

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

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

read_data(s, &i, sizeof(i));
i= ntohl(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. While there are not a lot of books describing BSD
sockets, one good reference is Unix Network Programming by W.
Richard Stevens (Prentice-Hall 1990, ISBN 0-13-949876-1). In
addition, you should look at some of the many public-domain
applications which make use of sockets, since real applications are
the best teachers.

Beware that the examples given here leave out a lot of error
checking which should be used in a real application. You should
check the manual pages for each of the functions discussed here for
further information. If you have further questions regarding
sockets, please feel free to ask me at email address
[email protected]

Jim Frost
CenterLine Software
(617) 498-3254
[email protected]

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