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发信人: netiscpu (说不如做), 信区: Linux
标  题: Linux Network Programming (Chapter 54)
发信站: 紫丁香 (Thu Jul 23 09:14:28 1998), 转信

           o Ports and Sockets
               o Socket Programming
                    # socket()
                    # The bind() System Call
                    # The listen() System Call
                    # The accept() System Call
                    # The connect() System Call
                    # Connectionless Socket Programming
               o Record and File Locking
               o Interprocess Communications
               o Summary

     _________________________________________________________________

   54


   Network Programming


   This chapter will look at the basic concepts you need for network
   programming:
     * Ports and sockets

     * Record and file locking

     * Interprocess communications

   It is impossible to tell you how to program applications for a network
   in just a few pages. Indeed, the best reference to network programming
   available takes almost 800 pages in the first volume alone! If you
   really want to do network programming, you need a lot of experience
   with compilers, TCP/IP, network operating systems, and a great deal of
   patience.

   For information on details of TCP/IP, check the book Teach Yourself
   TCP/IP in 14 Days by Tim Parker (Sams).

   Ports and Sockets


   Network programming relies on the use of sockets to accept and
   transmit information. Although there is a lot of mystique about
   sockets, the concept is actually very simple to understand.

   Most applications that use the two primary network protocols,
   Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)
   have a port number that identifies the application. A port number is
   used for each different application the machine is handling, so it can
   keep track of them by numbers instead of names. The port number makes
   it easier for the operating system to know how many applications are
   using the system and which services are available.

   In theory, port numbers can be assigned on individual machines by the
   system administrator, but some conventions have been adopted to allow
   better communications. This convention enables the port number to
   identify the type of service that one system is requesting from
   another. For this reason, most systems maintain a file of port numbers
   and their corresponding services.

   Port numbers are assigned starting from the number 1. Normally, port
   numbers above 255 are reserved for the private use of the local
   machine, but numbers between 1 and 255 are used for processes
   requested by remote applications or for networking services.

   Each network communications circuit that goes into and out of the host
   computer's TCP application layer is uniquely identified by a
   combination of two numbers, together called the socket. The socket is
   composed of the IP address of the machine and the port number used by
   the TCP software.

   Because there are at least two machines involved in network
   communications, there will be a socket on both the sending and
   receiving machine. The IP address of each machine is unique, and the
   port numbers are unique to each machine, so socket numbers will also
   be unique across the network. This enables an application to talk to
   another application across the network based entirely on the socket
   number.

   The sending and receiving machines maintain a port table that lists
   all active port numbers. The two machines involved have reversed
   entries for each session between the two, a process called binding. In
   other words, if one machine has the source port number 23 and the
   destination port number set at 25, then the other machine will have
   its source port number set at 25 and the destination port number set
   at 23.

   Socket Programming


   Linux supports BSD style socket programming. Both connection-oriented
   and connectionless types of sockets are supported. In
   connection-oriented communication, the server and client establish a
   connection before any data is exchanged. In connectionless
   communication, data is exchanged as part of a message. In either case,
   the server always starts up first, binds itself to a socket, and
   listens to messages. How the server attempts to listen depends on the
   type of connection for which you have programmed it.

   You need to know about only a few system calls:
     * socket()

     * bind()

     * accept()

     * listen()

     * connect()

     * sendto()

     * recvfrom()

   We will cover these in the following examples.

   socket()


   The socket() system call creates a socket for the client or the
   server. The socket function is defined as follows:

#include<sys/types.h>
#include<sys/socket.h>
int socket(int family, int type, int protocol)

   For Linux, you will have family = AF_UNIX. The type is either
   SOCK_STREAM for reliable, though slower communications or SOCK_DGRAM
   for faster, but less reliable communications. The protocol should be
   IPPROTO_TCP for SOCK_STREAM and IPPROTO_UDP for SOCK_DGRAM.

   The return value from this function is -1 if there was an error;
   otherwise, it's a socket descriptor. You will use this socket
   descriptor to refer to this socket in all subsequent calls in your
   program.

   Sockets are created without a name. Clients use the name of the socket
   in order to read or write to it. This is where the bind function comes
   in.

   The bind() System Call


   The bind() system call assigns a name to an unnamed socket.

#include<sys/types.h>
#include<sys/socket.h>
int bind(int sockfd, struct sockaddr *saddr, int addrlen)

   The first item is a socket descriptor. The second is a structure with
   the name to use, and the third item is the size of the structure.

   Now that you have bound an address for your server or client, you can
   connect() to it or listen on it. If your program is a server, then it
   sets itself up to listen and accept connections. Let's look at the
   function available for such an endeavor.

   The listen() System Call


   The listen() system call is used by the server. It is defined as
   follows:

#include<sys/types.h>
#include<sys/socket.h>
int listen(int sockfd, int backlog);

   The sockfd is the descriptor of the socket. The backlog is the number
   of waiting connections at one time before rejecting any. Use the
   standard value of 5 for backlog. A returned value of less than 1
   indicates an error.

   If this call is successful, you can accept connections.

   The accept() System Call


   The accept() system call is used by a server to accept any incoming
   messages from clients' connect() calls. Be aware that this function
   will not return if no connections are received.

#include<sys/types.h>
#include<sys/socket.h>
int accept(int sockfd, struct sockaddr *peeraddr, int addrlen)

   The parameters are the same as that for the bind call, with the
   exception that the peeraddr points to information about the client
   that is making a connection request. Based on the incoming message,
   the fields in the structure pointed at by peeraddr are filled out.

   So how does a client connect to a server. Let's look at the connect()
   call.

   The connect() System Call


   The connect() system call is used by clients to connect to a server in
   a connection-oriented system. This connect() call should be made after
   the bind() call.

#include<sys/types.h>
#include<sys/socket.h>
int connect(int sockfd, struct sockaddr *servsaddr, int addrlen)

   The parameters are the same as that for the bind call, with the
   exception that the servsaddr points to information about the server
   that the client is connecting to. The accept call creates a new socket
   for the server to work with the request. This way the server can
   fork() off a new process and wait for more connections. On the server
   side of things, you would have code that looks like that shown in
   Listing 54.1.

   Listing 54.1. Server side for socket-oriented protocol.

#include <sys/types.h>
#include <sys/socket.h>
#include <linux/in.h>
#include <linux/net.h>
#define MY_PORT 6545
main(int argc, char *argv[])
{
int sockfd, newfd;
int cpid; /* child id */
struct sockaddr_in servaddr;
struct sockaddr_in clientInfo;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0) < 0)
{
myabort("Unable to create socket");
}
bzero((char *)&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_family = htons(MY_PORT);
/*
* The htonl(for a long integer) and htons(for short integer) convert
* a host oriented byte order * into a network order.
*/
if (bind(sockfd,(struct sockaddr *)&servaddr,sizeof(struct sockaddr)) < 0)
{
myabort("Unable to bind socket");
}
listen(sockfd,5);
for (;;)
{
/* wait here */
newfd=accept(sockfd,(struct sockaddr *)&clientInfo,
sizeof(struct sockaddr);
if (newfd < 0)
{
myabort("Unable to accept on socket");
}
if ((cpid = fork()) < 0)
{
myabort("Unable to fork on accept");
}
else if (cpid == 0) { /* child */
close(sockfd); /* no need for original */
do_your_thing(newfd);
exit(0);
}
close(newfd); /* in the parent */
}
}

   In the case of connection-oriented protocols, the server performs the
   following functions:
     * Creates a socket with a call to the socket() function.

     * Binds itself to an address with the bind() function call.

     * Listens for connections with the listen() function call.

     * Accepts any incoming requests with the accept() function call.

     * Gets incoming messages with the read() function and replies back
       with the write() call.

   Now let's look at the client side of things in Listing 54.2.

   Listing 54.2. Client side function.

#include <sys/types.h>
#include <sys/socket.h>
#include <linux/in.h>
#include <linux/net.h>
#define MY_PORT 6545
#define MY_HOST_ADDR "204.25.13.1"
int getServerSocketId()
{
int fd, len;
struct sockaddr_in unix_addr;
/* create a Unix domain stream socket */
if ( (fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)
{
return(-1);
}
/* fill socket address structure w/our address */
memset(&unix_addr, 0, sizeof(unix_addr));
unix_addr.sin_family = AF_INET;
/* convert internet address to binary value*/
unix_addr.sin_addr.s_addr = inet_addr(MY_HOST_ADDR);
unix_addr.sin_family = htons(MY_PORT);
if (bind(fd, (struct sockaddr *) &unix_addr, len) < 0)
return(-2);
memset(&unix_addr, 0, sizeof(unix_addr));
if (connect(fd, (struct sockaddr *) &unix_addr, len) < 0)
return(-3);
return(fd);
}

   The client for connection-oriented communication also takes the
   following steps:
     * Creates a socket with a call to the socket() function.

     * Attempts to connect to the server with a connect() call.

     * If a connection is made, requests for data with the write() call,
       and reads incoming replies with the read() function.


   Connectionless Socket Programming


   Now let's consider the case of a connectionless exchange of
   information. The principle on the server side is different from the
   connection-oriented server side in that the server calls recvfrom()
   instead of the listen and accept calls. Also, to reply to messages,
   the server uses the sendto() function call. See Listing 54.3 for the
   server side.

   Listing 54.3. The server side.

#include <sys/types.h>
#include <sys/socket.h>
#include <linux/in.h>
#include <linux/net.h>
#define MY_PORT 6545
#define MAXM 4096
char mesg[MAXM];
main(int argc, char *argv[])
{
int sockfd, newfd;
int cpid; /* child id */
struct sockaddr_in servaddr;
struct sockaddr_in clientInfo;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0) < 0)
{
myabort("Unable to create socket");
}
bzero((char *)&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_family = htons(MY_PORT);
/*
* The htonl(for a long integer) and htons(for short integer) convert
* a host oriented byte order * into a network order.
*/
if (bind(sockfd,(struct sockaddr *)&servaddr,sizeof(struct sockaddr)) < 0)
{
myabort("Unable to bind socket");
}
for (;;)
{
/* wait here */
n = recvfrom(sockfd, mesg, MAXM, 0,
(struct sockaddr *)&clientInfo,
sizeof(struct sockaddr));
doSomethingToIt(mesg);
sendto(sockfd,mesg,n,0,
(struct sockaddr *)&clientInfo,
sizeof(struct sockaddr));
}
}

   As you can see, the two function calls to process each message make
   this an easier implementation than a connection-oriented one. However,
   you have to process each message one at a time because messages from
   multiple clients can be multiplexed together. In a connection-oriented
   scheme, the child process always knows where each message originated.

   The client does not have to call the connect() system call either.
   Instead, the client can call the sendto() function directly. The
   client side is identical to the server side, with the exception that
   the sendto call is made before the recvfrom() call.

#include <sys/types.h>
#include <sys/socket.h>
int sendto((int sockfd,
const void *message__, /* the pointer to message */
int length, /* of message */
unsigned int type, /* of routing, leave 0 *
const struct sockaddr * client, /* where to send it */
int length ); /* of sockaddr);

       ______________________________________________________________


     NOTE: If you are a BSD user, use the sendto() call, do not use
     sendmsg() call. The sendto() call is more efficient.


       ______________________________________________________________

   Any errors are indicated by a return value of -1. Only local errors
   are detected.

   The recvfrom() system call is defined as follows:

#include <sys/types.h>
#include <sys/socket.h>
int recvfrom(int sockfd,
const void *message__, /* the pointer to message */
int length, /* of message */
unsigned int flags, /* of routing, leave 0 *
const struct sockaddr * client, /* where to send it */
int length ); /* of sockaddr);

   If a message is too long to fit in the supplied buffer, the extra
   bytes are discarded. The call may return immediately or wait forever,
   depending on the type of the flag being set. You can even set time out
   values. Check the man pages for recvfrom for more information.

   There you have it: the very basics of how to program applications to
   take advantage of the networking capabilities under Linux. We have not
   even scratched the surface of all the intricacies of programming for
   networks. A good starting point for more detailed information would be
   UNIX Network Programming by W. Richard Stevens, published in 1990 by
   Prentice Hall. This book is a classic used in universities and is, by
   far, the most detailed book to date.

   Record and File Locking


   When two processes want to share a file, the danger exists that one
   process might affect the contents of the file, and thereby affect the
   other process. For this reason, most operating systems use a mutually
   exclusive principle: When one process has a file open, no other
   process can touch it. This is called file locking.

   The technique is simple to implement. What usually happens is that a
   "lock file" is created with the same name as the original file but
   with the extension .lock, which tells other processes that the file is
   unavailable. This is how many Linux spoolers, such as the print system
   and UUCP, implement file locking. It is a brute-force method, perhaps,
   but effective and easy to program.

   Unfortunately, this technique is not good when you must have several
   processes access the same information quickly because the delays
   waiting for file opening and closing can grow to be appreciable. Also,
   if one process doesn't release the file properly, other processes can
   hang there, waiting for access.

   For this reason, record locking is sometimes implemented. With record
   locking, a single part of a larger file is locked to prevent two
   processes from changing its contents at the same time. Record locking
   enables many processes to access the same file at the same time, each
   updating different records within the file, if necessary. The
   programming necessary to implement record locking is more complex than
   file locking, of course.

   Normally, to implement record locking, you use a file offset, or the
   number of characters from the beginning of the file. In most cases, a
   range of characters are locked, so the program has to note the start
   of the locking region and the length of it, and then store that
   information somewhere other processes can examine it.

   Writing either file locking or record locking code requires a good
   understanding of the operating system, but is otherwise not difficult,
   especially because there are thousands of programs readily available
   from the Internet, in networking programming books, and on BBSes to
   examine for example code.

   Interprocess Communications


   Network programming always involves two or more processes talking to
   each other (interprocess communications), so the way in which
   processes communicate is vitally important to network programmers.
   Network programming differs from the usual method of programming in a
   few important aspects. A traditional program can talk to different
   modules (or even other applications on the same machine) through
   global variables and function calls. That doesn't work across
   networks.

   A key goal of network programming is to ensure that processes don't
   interfere with each other. Otherwise, systems can get bogged down or
   lock up. Therefore, processes must have a clean and efficient method
   of communicating. UNIX is particularly strong in this regard, because
   many of the basic UNIX capabilities, such as pipes and queues, are
   used effectively across networks.

   Writing code for interprocess communications is quite difficult
   compared to single application coding. If you want to write this type
   of routine, you should study sample programs from a network
   programming book or a BBS site to see how they accomplish the task.

   Summary


   Few people need to write network applications, so the details of the
   process are best left to those who want them. Experience and lots of
   examples are the best way to begin writing network code, and mastering
   the skills can take many years.

--

                              Enjoy Linux!
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