本文参考: http://www.linuxhowtos.org/C_C++/socket.htm， 仅供学习用途
有两种十分常见的Socket类型，他们就是流类型(stream sockets)和数据报类型(datagram socekts)。流类型用于可以实现连续信息的传递，使用TCP连接，而数据报类型用于单个信息的传递，使用UDP连接。
./client localhost 51717
This header file contains declarations used in most input and output and is typically included in all C programs.
This header file contains definitions of a number of data types used in system calls. These types are used in the next two include files.
The header file socket.h includes a number of definitions of structures needed for sockets.
The header file in.h contains constants and structures needed for internet domain addresses.
This function is called when a system call fails. It displays a message about the error on stderr and then aborts the program. The perror man page gives more information.
sockfd and newsockfd are file descriptors, i.e. array subscripts into the file descriptor table . These two variables store the values returned by the socket system call and the accept system call.
portno stores the port number on which the server accepts connections.
clilen stores the size of the address of the client. This is needed for the accept system call.
n is the return value for the read() and write() calls; i.e. it contains the number of characters read or written.
The server reads characters from the socket connection into this buffer.
A sockaddr_in is a structure containing an internet address. This structure is defined in netinet/in.h.
Here is the definition:
An in_addr structure, defined in the same header file, contains only one field, a unsigned long called s_addr.
The variable serv_addr will contain the address of the server, and cli_addr will contain the address of the client which connects to the server.
The user needs to pass in the port number on which the server will accept connections as an argument. This code displays an error message if the user fails to do this.
The socket() system call creates a new socket. It takes three arguments. The first is the address domain of the socket.
Recall that there are two possible address domains, the unix domain for two processes which share a common file system, and the Internet domain for any two hosts on the Internet. The symbol constant AF_UNIX is used for the former, and AF_INET for the latter (there are actually many other options which can be used here for specialized purposes).
The second argument is the type of socket. Recall that there are two choices here, a stream socket in which characters are read in a continuous stream as if from a file or pipe, and a datagram socket, in which messages are read in chunks. The two symbolic constants are SOCK_STREAM and SOCK_DGRAM.
The third argument is the protocol. If this argument is zero (and it always should be except for unusual circumstances), the operating system will choose the most appropriate protocol. It will choose TCP for stream sockets and UDP for datagram sockets.
The socket system call returns an entry into the file descriptor table (i.e. a small integer). This value is used for all subsequent references to this socket. If the socket call fails, it returns -1.
In this case the program displays and error message and exits. However, this system call is unlikely to fail.
This is a simplified description of the socket call; there are numerous other choices for domains and types, but these are the most common. The socket() man page has more information.
The function bzero() sets all values in a buffer to zero. It takes two arguments, the first is a pointer to the buffer and the second is the size of the buffer. Thus, this line initializes serv_addr to zeros. —-
The port number on which the server will listen for connections is passed in as an argument, and this statement uses the atoi() function to convert this from a string of digits to an integer.
The variable serv_addr is a structure of type struct sockaddr_in. This structure has four fields. The first field is short sin_family, which contains a code for the address family. It should always be set to the symbolic constant AF_INET.
The second field of serv_addr is unsigned short sin_port, which contain the port number. However, instead of simply copying the port number to this field, it is necessary to convert this to network byte order using the function htons() which converts a port number in host byte order to a port number in network byte order.
The third field of sockaddr_in is a structure of type struct in_addr which contains only a single field unsigned long s_addr. This field contains the IP address of the host. For server code, this will always be the IP address of the machine on which the server is running, and there is a symbolic constant INADDR_ANY which gets this address.
The bind() system call binds a socket to an address, in this case the address of the current host and port number on which the server will run. It takes three arguments, the socket file descriptor, the address to which is bound, and the size of the address to which it is bound. The second argument is a pointer to a structure of type sockaddr, but what is passed in is a structure of type sockaddr_in, and so this must be cast to the correct type. This can fail for a number of reasons, the most obvious being that this socket is already in use on this machine. The bind() manual has more information.
The listen system call allows the process to listen on the socket for connections. The first argument is the socket file descriptor, and the second is the size of the backlog queue, i.e., the number of connections that can be waiting while the process is handling a particular connection. This should be set to 5, the maximum size permitted by most systems. If the first argument is a valid socket, this call cannot fail, and so the code doesn’t check for errors. The listen() man page has more information.
The accept() system call causes the process to block until a client connects to the server. Thus, it wakes up the process when a connection from a client has been successfully established. It returns a new file descriptor, and all communication on this connection should be done using the new file descriptor. The second argument is a reference pointer to the address of the client on the other end of the connection, and the third argument is the size of this structure. The accept() man page has more information.
Note that we would only get to this point after a client has successfully connected to our server. This code initializes the buffer using the bzero() function, and then reads from the socket. Note that the read call uses the new file descriptor, the one returned by accept(), not the original file descriptor returned by socket(). Note also that the read() will block until there is something for it to read in the socket, i.e. after the client has executed a write().
It will read either the total number of characters in the socket or 255, whichever is less, and return the number of characters read. The read() man page has more information.
Once a connection has been established, both ends can both read and write to the connection. Naturally, everything written by the client will be read by the server, and everything written by the server will be read by the client. This code simply writes a short message to the client. The last argument of write is the size of the message. The write() man page has more information.
This terminates main and thus the program. Since main was declared to be of type int as specified by the ascii standard, some compilers complain if it does not return anything.
As before, we will go through the program client.c line by line.
The header files are the same as for the server with one addition. The file netdb.h defines the structure hostent, which will be used below.
The error() function is identical to that in the server, as are the variables sockfd, portno, and n. The variable serv_addr will contain the address of the server to which we want to connect. It is of type struct sockaddr_in.
The variable server is a pointer to a structure of type hostent. This structure is defined in the header file netdb.h as follows:
It defines a host computer on the Internet. The members of this structure are:
- h_name: Official name of the host.
- h_aliases: A zero terminated array of alternate names for the host.
- h_addrtype: The type of address being returned; currently always AF_INET.
- h_length: The length, in bytes, of the address.
- h_addr_list: A pointer to a list of network addresses for the named host. Host addresses are returned in network byte order.
Note that h_addr is an alias for the first address in the array of network addresses.
All of this code is the same as that in the server.
The variable argv contains the name of a host on the Internet, e.g. cs.rpi.edu. The function:
Takes such a name as an argument and returns a pointer to a hostent containing information about that host.
The field char *h_addr contains the IP address.
If this structure is NULL, the system could not locate a host with this name.
In the old days, this function worked by searching a system file called /etc/hosts but with the explosive growth of the Internet, it became impossible for system administrators to keep this file current. Thus, the mechanism by which this function works is complex, often involves querying large databases all around the country. The gethostbyname() man page has more information.
This code sets the fields in serv_addr. Much of it is the same as in the server. However, because the field server->h_addr is a character string, we use the function:
which copies length bytes from s1 to s2. —-
The connect function is called by the client to establish a connection to the server. It takes three arguments, the socket file descriptor, the address of the host to which it wants to connect (including the port number), and the size of this address. This function returns 0 on success and -1 if it fails. The connect() man page has more information.
Notice that the client needs to know the port number of the server, but it does not need to know its own port number. This is typically assigned by the system when connect is called.
The remaining code should be fairly clear. It prompts the user to enter a message, uses fgets to read the message from stdin, writes the message to the socket, reads the reply from the socket, and displays this reply on the screen.
There are a number of different ways to design servers. These models are discussed in detail in a book by Douglas E. Comer and David L. Stevens entiteld Internetworking with TCP/IP Volume III:Client Server Programming and Applications published by Prentice Hall in 1996. These are summarized here.
Concurrent, connection oriented servers
The typical server in the Internet domain creates a stream socket and forks off a process to handle each new connection that it receives. This model is appropriate for services which will do a good deal of reading and writing over an extended period of time, such as a telnet server or an ftp server. This model has relatively high overhead, because forking off a new process is a time consuming operation, and because a stream socket which uses the TCP protocol has high kernel overhead, not only in establishing the connection but also in transmitting information. However, once the connection has been established, data transmission is reliable in both directions.
Iterative, connectionless servers
Servers which provide only a single message to the client often do not involve forking, and often use a datagram socket rather than a stream socket. Examples include a finger daemon or a timeofday server or an echo server (a server which merely echoes a message sent by the client). These servers handle each message as it receives them in the same process. There is much less overhead with this type of server, but the communication is unreliable. A request or a reply may get lost in the Internet, and there is no built-in mechanism to detect and handle this.
Single Process concurrent servers
A server which needs the capability of handling several clients simultaneous, but where each connection is I/O dominated (i.e. the server spends most of its time blocked waiting for a message from the client) is a candidate for a single process, concurrent server. In this model, one process maintains a number of open connections, and listens at each for a message. Whenever it gets a message from a client, it replies quickly and then listens for the next one. This type of service can be done with the select system call.