Linux Kernel Architecture

Chapter 12: Networks

printf("\nBytes received: %u\n", bytes);

printf("Text: ’%s’\n", buf);

/* End communication (i.e. close socket) */

close(sockfd);

}

The Internet superdaemon (inetd,xinetd, or similar) normally uses a built-in echo server. Consequently,
the source code can be tested immediately after compilation.

wolfgang@meitner>./echo_client

Connect to 192.168.1.20

Numeric: 335653056

Send: ’Hello World’

Bytes received: 11

Text: ’Hello World’

The following steps are performed by the client:

1. An instance of thesockaddr_instructure is generated to define the address of the server to
   be contacted.AF_INETindicates that it is an Internet address and the target server is precisely
   defined by its IP address (192.168.1.20) and port number ( 7 ).
   Also, the data from the host are converted to the network byte order.htonsis used for the
   port number, and theinet_addrauxiliary function performs the conversion implicitly by
   translating the text string with a dotted decimal address into a number.


2. A socket is created in the kernel by means of thesocketfunction, which (as shown below)
   is based on thesocketcallsystem call of the kernel. The result returned is an integer num-
   ber that is interpreted as a file descriptor — and can therefore be processed by all functions
   available for regular files, as described in Chapter 8. In addition to these operations, there
   are other network-specific ways of handling the file descriptor; these permit exact setting of
   various transmission parameters not discussed here.


3. A connection is set up by invoking theconnectfunction in conjunction with the file descrip-
   tor and theservervariable that stores the server connection data (this function is also based
   on thesocketcallsystem call).


4. Actual communication is initiated by sending a text string (‘‘Hello World‘‘ — how could it
   be anything else?) to the server by means ofwrite. Writing data to a socket file descriptor
   is the equivalent of sending data. This step is totally independent of the server location and
   the protocol used to set up the connection. The network implementation ensures that the
   character string reaches its destination — no matter how this is done.


5. The server response is read byread, but a buffer must first be allocated to hold the data
   received. As a precaution, 1,000 bytes are reserved in memory, although we only expect the
   original string to be returned.readblocks until the server supplies a response, and it then
   returns the number of bytes received as an integer number.

Because strings in C are always null-terminated, 11 bytes are received, although the mes-

sage itself appears to be only 10 bytes long.