"Linux Gazette...making Linux just a little more fun!"

Psst, wanna have some fun? Try client/server computing. It's like talking through two tin cans and a taut string, upgraded to the computer era. Linux has all the tools you need. You are already using client/server computing in applications such as Netscape, telnet, and ftp. And it's easy to write your own client/server apps, maybe even useful ones.

Client/server computing links two different programs (the client and the server) over a network. For practice you can even skip the network by letting Linux talk to itself. So read on even if you aren't attached to a network. (But your Linux installation needs to be configured for networking.)

A very common form of client/server computing uses BSD sockets. BSD stands for Berkeley Software Distribution, an early version of Unix. Logically, a BSD socket is a combination of IP address and port number. The IP address defines the computer, and the port number defines the logical communication channel in that computer. (In this usage a port is not a physical device. One physical device, e.g. an Ethernet card, can access all the ports in the computer.)

Linux Journal ran a nice three-part series on network programming by Ivan Griffin and John Nelson in the February, March, and April, 1998, issues. The February article contains the code to set up a skeleton client/server pair using BSD sockets; it includes all the plumbing needed to get started. You can download the code from SSC, then use this article to start playing with more content.

After downloading the file 2333.tgz, expand it with the command tar&nsbp;-xzvf 2333.tgz. Rename the resultant file 2333l1.txt to server.c, and the file 2333l2.txt to client.c. Edit server.c to delete the extraneous characters @cx: from the start of the first line, and either delete the last line or make it a comment by enclosing it between the characters /* and */. Similarly, delete the last line of client.c, or make it a comment. Compile server.c with the command gcc -oserver server.c; similarly compile client.c using gcc -oclient client.c.

The server runs on the local computer, so it only needs to know its port number to define a socket. The client runs on any computer, so it needs to know both its target server computer and the server's port number. You have thousands of port numbers to play with. Just don't use a port that is already taken. Your file /etc/services lists most of the ports in use. I found that port 1024 worked fine.

Now I said you didn't need to be connected to a network, but you do need to have your computer configured for networking to try this out. In fact, this code won't run for me if I use the generic name localhost; I have to give the explicit name of my computer. So assuming you are set up for networking, start the sever by typing

server 1024 &

client hostname 1024

Connection request from 192.168.1.1
14: Hello, World!

Example of Query-Respone Client/Server

/* local test of client-server code */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char name[256] = "";
char buffer[256] = "";

void client(char *buffer)
{
 printf("%s", buffer);
 fgets(buffer, 256, stdin);
}

 int main(int argc, char *argv[])
{
 int year, age;

 sprintf(buffer, "Please enter your name: ");

 client(buffer);

 strcpy(name, buffer);
 sprintf(buffer, "Hi, %sPlease enter your year of birth: ", name);

 client(buffer);

 year = atoi(buffer);
 age = 1998 - year;
 sprintf(buffer, "Your approximate age is %d.\nEnter q to quit: ", age);

 client(buffer);

 return(0);
}

Copy the above code into an editor and save it as localtest.c. Compile it with the command gcc -olocaltest localtest.c. When you run it you should get output like:

Please enter your name: joe
Hi, joe
Please enter your year of birth: 1960
Your approximate age is 38.
Enter q to quit: q

int main(int argc, char *argv[])
{
int i, year, age;
char name[256] = "";
char buffer[256] = "";
char null_buffer[256] = "";
    int serverSocket = 0,

/*
* Server application specific code goes here,
* e.g. perform some action, respond to client etc.
*/

sprintf(buffer, "Please enter your name: ");
write(slaveSocket, buffer, strlen(buffer));
for (i = 0; i <= 255; i++) buffer[i] = 0;

/* get name */
read(slaveSocket, buffer, sizeof(buffer));
strcpy(name, buffer);
sprintf(buffer, "Hi, %sPlease enter your year of birth: ", name);
write(slaveSocket, buffer, strlen(buffer));
for (i = 0; i <= 255; i++) buffer[i] = 0;

/* get year of birth */
read(slaveSocket, buffer, sizeof(buffer));
year = atoi(buffer);
age = 1998 - year;
sprintf(buffer, "Your approximate age is %d.\nEnter q to quit: ", age);
write(slaveSocket, buffer, strlen(buffer));

close(slaveSocket);
exit(0);

The client code is very simple. Insert declarations into client.c by changing the beginning statements of main to read

int main(int argc, char *argv[])
{
  int i;
  int clientSocket,

/*
* Client application specific code goes here
* e.g. receive messages from server, respond, etc.
* Receive and respond until server stops sending messages
*/

while (0 < (status = read(clientSocket, buffer, sizeof(buffer))))
  {
    printf("%s", buffer);
    for (i = 0; i <= 255; i++) buffer[i] = 0;
    fgets(buffer, 256, stdin);
    write(clientSocket, buffer, strlen(buffer));
  }
    close(clientSocket);
    return 0;
  }

Recompile server.c and client.c and run them again as before. This time the output should be something like:

Connection request from 192.168.1.1
Please enter your name: joe
Hi, joe.
Please enter your year of birth: 1960
Your approximate age is 38.
Enter q to quit: q

Chat Program as a Client/Server

Insert declarations into server.c by changing the beginning statements of main to read:

 int main(int argc, char *argv[])
{
  char buffer[256] = "";
  int i, serverquit = 1, clientquit = 1;
    int serverSocket = 0,

/*
* Server application specific code goes here,
* e.g. perform some action, respond to client etc.
*/

printf("Send q to quit.\n");
sprintf(buffer, "Hi, %s\nS: Please start chat. Send q to quit.\n", inet_ntoa(clientName.sin_addr));
write(slaveSocket, buffer, strlen(buffer));
for (i = 0; i <= 255; i++) buffer[i] = 0;

while (serverquit != 0 && clientquit != 0)
{
 status = 0;
 while (status == 0)
  status = read(slaveSocket, buffer, sizeof(buffer));
 clientquit = strcmp(buffer, "q\n");

 if (clientquit != 0)
 {
  printf("C: %s", buffer);
  for (i = 0; i <= 255; i++) buffer[i] = 0;

  printf("S: ");
  fgets(buffer, 256, stdin);
  serverquit  = strcmp(buffer, "q\n");
  write(slaveSocket, buffer, strlen(buffer));
  for (i = 0; i <= 255; i++) buffer[i] = 0;
 }
}
printf("Goodbye\n");
close(slaveSocket);
exit(0);

int main(int argc, char *argv[])
{
 int i, serverquit = 1, clientquit = 1;
    int clientSocket,

/*
* Client application specific code goes here
* e.g. receive messages from server, respond, etc.
*/

while (serverquit != 0 && clientquit != 0)
{
  status = 0;
  while (status == 0)
    status = read(clientSocket, buffer, sizeof(buffer));
  serverquit = strcmp(buffer, "q\n");

  if (serverquit != 0)
  {
    printf("S: %s", buffer);
    for (i = 0; i <= 255; i++) buffer[i] = 0;

    printf("C: ");
    fgets(buffer, 256, stdin);
    clientquit = strcmp(buffer, "q\n");
    write(clientSocket, buffer, strlen(buffer));
    for (i = 0; i <= 255; i++) buffer[i] = 0;
   }
 }
 printf("Goodbye\n");
 close(clientSocket);
 return 0;
 }

server 1024

client hostname 1024

Server and client code for this chat program are almost identical, and very similar to the previous example. There are two main differences. The first is the test to see whether either party has entered a "q" to quit. The flags serverquit and clientquit signal this. The second is the tight loop waiting for response from the other party. The function read returns the number of character read from the socket; this is stored into status. A non-zero number of characters indicates the other side has sent a message.

Here is an example session as printed by the server:

Connection request from 192.168.1.1
Send q to quit.
C: Hi server
S: Hi client
C: Bye server
S: Bye client
Goodbye

S: Hi, 192.168.1.1
S: Please start chat. Send q to quit.
C: Hi server
S: Hi client
C: Bye server
S: Bye client
C: q
Goodbye