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<hr><p align="left"><small>寄信人: guru.bbs@bbs.gznet.edu.cn <br>
标 题: [转寄] Quick and Dirty guide to sockets <br>
注 意: 站外信件 <br>
日 期: Mon Aug 6 20:06:28 2001 <br>
发信人: Happy (欢乐幸福人), 信区: Socket <br>
标 题: Quick and Dirty guide to sockets <br>
发信站: 华南网木棉站 (Fri May 15 13:57:24 1998), 转信 <br>
BSD Sockets: A Quick And Dirty Primer <br>
by Jim Frost <br>
February 13, 1990 <br>
As you delve into the mysteries of UNIX, you find more and more things that <br>
are difficult to <br>
understand immediately. One of these things, at least for most people, is th <br>
e BSSD socket con- cept. <br>
This is a short tutorial that explains what they are, how they work, and giv <br>
es ssample code showing <br>
how to use them. <br>
The Analogy (or: What *IS* a socket, anyway?) <br>
The socket is the BSD method for accomplishing interprocess com- munication <br>
(IPC <br>
. What this <br>
means is a socket is used to allow one process to speak to another, very muc <br>
h li <br>
h li <br>
e the telephone is <br>
used to allow one person to speak to another. <br>
The telephone analogy is a very good one, and will be used re- peatedly to d <br>
escr <br>
be socket behavior. <br>
Installing Your New Phone (or: How to listen for socket <br>
connections) <br>
In order for a person to receive telephone calls, he must first have a telep <br>
hone <br>
installed. Likewise you <br>
must create a socket to listen for connections. This process involves severa <br>
l st <br>
ps. First you must <br>
make a new socket, which is similar to having a telephone line installed. Th <br>
e so <br>
ket() command is <br>
used to do this. <br>
Since sockets can have several types, you must specify what type of socket y <br>
ou w <br>
nt when you <br>
create one. One option that you have is the addressing format of a socket. J <br>
ust <br>
ust <br>
s the mail service <br>
uses a different scheme to deliver mail than the telephone com- pany uses to <br>
com <br>
lete calls, so can <br>
sockets differ. The two most common addressing schemes are AF_UNIX and AF_IN <br>
ET. <br>
F_UNIX <br>
ad- dressing uses UNIX pathnames to identify sockets; these sockets are very <br>
use <br>
ul for IPC <br>
between processes on the same machine. AF_INET addressing uses Internet addr <br>
esse <br>
which are <br>
four byte numbers usually written as four decimal numbers separated by perio <br>
ds ( <br>
uch as <br>
192.9.200.10). In addition to the machine ad- dress, there is also a port nu <br>
mber <br>
which allows more <br>
than one AF_INET socket on each machine. AF_INET addresses are what we will <br>
deal <br>
with here. <br>
with here. <br>
Another option which you must supply when creating a socket is the type of s <br>
ocke <br>
. The two most <br>
common types are SOCK_STREAM and SOCK_DGRAM. SOCK_STREAM indicates that data <br>
<br>
will come across the socket as a stream of characters, while SOCK_DGRAM indi <br>
cate <br>
that data will <br>
come in bunches (called datagrams). We will be dealing with SOCK_STREAM sock <br>
ets, <br>
which are <br>
very common. <br>
After creating a socket, we must give the socket an address to listen to, ju <br>
st a <br>
you get a telephone <br>
number so that you can re- ceive calls. The bind() function is used to do th <br>
is ( <br>
t binds a socket to an <br>
address, hence the name). <br>
SOCK_STREAM type sockets have the ability to queue incoming con- nection req <br>
uest <br>
, which is a <br>
, which is a <br>
lot like having "call waiting" for your telephone. If you are busy handling <br>
a co <br>
nection, the con- <br>
nection request will wait until you can deal with it. The listen() function <br>
is u <br>
ed to set the maximum <br>
number of requests (up to a maximum of five, usually) that will be queued be <br>
fore <br>
re- quests start <br>
being denied. While it is not necessary to use the listen() function, it's g <br>
ood <br>
ractice. <br>
The following function shows how to use the socket(), bind(), and listen() f <br>
unct <br>
ons to establish a <br>
socket which can accept calls: <br>
/* code to establish a socket; originally from bzs@bu-cs.bu.edu <br>
*/ <br>
int establish(portnum) <br>
u_short portnum; <br>
{ char myname[MAXHOSTNAME+1]; <br>
int s; <br>
int s; <br>
struct sockaddr_in sa; <br>
struct hostent *hp; <br>
bzero(&sa,sizeof(struct sockaddr_in)); /* clear our address */ <br>
gethostname(myname,MAXHOSTNAME); /* who are we? */ <br>
hp= gethostbyname(myname); /* get our address info */ <br>
if (hp == NULL) /* we don't exist !? */ <br>
return(-1); <br>
sa.sin_family= hp->h_addrtype; /* this is our host address <br>
*/ <br>
sa.sin_port= htons(portnum); /* this is our port number * <br>
/ <br>
if ((s= socket(AF_INET,SOCK_STREAM,0)) < 0) /* create socket */ <br>
return(-1); <br>
if (bind(s,&sa,sizeof sa,0) < 0) { <br>
close(s); <br>
return(-1); /* bind address to socket */ <br>
<br>
} <br>
listen(s, 3); /* max # of queued connects <br>
*/ <br>
return(s); <br>
} <br>
} <br>
After you create a socket to get calls, you must wait for calls to that sock <br>
et. <br>
he accept() function is <br>
used to do this. Cal- ling accept() is analogous to picking up the telephone <br>
if <br>
t's ringing. Accept() <br>
returns a new socket which is connected to the caller. <br>
The following function can be used to accept a connection on a socket that h <br>
as b <br>
en created using the <br>
establish() function above: <br>
int get_connection(s) <br>
int s; /* socket created with establish() */ <br>
{ struct sockaddr_in isa; /* address of socket */ <br>
int i; /* size of address */ <br>
int t; /* socket of connection */ <br>
i = sizeof(isa); /* find socket's address */ <br>
getsockname(s,&isa,&i); /* for accept() */ <br>
if ((t = accept(s,&isa,&i)) < 0) /* accept connection if there is one <br>
*/ <br>
return(-1); <br>
return(t); <br>
return(t); <br>
} <br>
Unlike with the telephone, you may still accept calls while pro- cessing pre <br>
viou <br>
connections. For <br>
this reason you usually fork off jobs to handle each connection. The followi <br>
ng c <br>
de shows how to use <br>
establish() and get_connection() to allow multiple connec- tions to be dealt <br>
wit <br>
: <br>
#include <errno.h> /* obligatory includes */ <br>
#include <signal.h> <br>
#include <stdio.h> <br>
#include <sys/types.h> <br>
#include <sys/socket.h> <br>
#include <sys/wait.h> <br>
#include <netinet/in.h> <br>
#include <netdb.h> <br>
#define PORTNUM 50000 /* random port number, we need something */ <br>
void fireman(), do_something(); <br>
main() <br>
{ int s, t; <br>
{ int s, t; <br>
if ((s= establish(PORTNUM)) < 0) { /* plug in the phone */ <br>
perror("establish"); <br>
exit(1); <br>
} <br>
signal(SIGCHLD, fireman); /* this eliminates zombies */ <br>
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