Sockets Tutorial

1. Create a socket with the socket() system call
   
2. Connect the socket to the address of the server using the connect() system call
   
3. Send and receive data. There are a number of ways to do this, but the simplest is to use the read() and write() system calls.

The steps involved in establishing a socket on the server side are as follows:

1. Create a socket with the socket() system call
   
2. Bind the socket to an address using the bind() system call. For a server socket on the Internet, an address consists of a port number on the host machine.
   
3. Listen for connections with the listen() system call
   
4. Accept a connection with the accept() system call. This call typically blocks until a client connects with the server.
   
5. Send and receive data

Socket Types

When a socket is created, the program has to specify the address domain and the socket type. Two processes can communicate with each other only if their sockets are of the same type and in the same domain.

There are two widely used address domains, the unix domain, in which two processes which share a common file system communicate, and the Internet domain, in which two processes running on any two hosts on the Internet communicate. Each of these has its own address format.

The address of a socket in the Unix domain is a character string which is basically an entry in the file system.

The address of a socket in the Internet domain consists of the Internet address of the host machine (every computer on the Internet has a unique 32 bit address, often referred to as its IP address).
In addition, each socket needs a port number on that host.
Port numbers are 16 bit unsigned integers.
The lower numbers are reserved in Unix for standard services. For example, the port number for the FTP server is 21. It is important that standard services be at the same port on all computers so that clients will know their addresses.
However, port numbers above 2000 are generally available.

There are two widely used socket types, stream sockets, and datagram sockets. Stream sockets treat communications as a continuous stream of characters, while datagram sockets have to read entire messages at once. Each uses its own communciations protocol.

Stream sockets use TCP (Transmission Control Protocol), which is a reliable, stream oriented protocol, and datagram sockets use UDP (Unix Datagram Protocol), which is unreliable and message oriented.

The examples in this tutorial will use sockets in the Internet domain using the TCP protocol.

Sample code

C code for a very simple client and server are provided for you. These communicate using stream sockets in the Internet domain. The code is described in detail below. However, before you read the descriptions and look at the code, you should compile and run the two programs to see what they do.

server.c
client.c

Download these into files called server.c and client.c and compile them separately into two executables called server and client.

They probably won't require any special compiling flags, but on some solaris systems you may need to link to the socket library by appending -lsocket to your compile command.

Ideally, you should run the client and the server on separate hosts on the Internet. Start the server first. Suppose the server is running on a machine called cheerios. When you run the server, you need to pass the port number in as an argument. You can choose any number between 2000 and 65535. If this port is already in use on that machine, the server will tell you this and exit. If this happens, just choose another port and try again. If the port is available, the server will block until it receives a connection from the client. Don't be alarmed if the server doesn't do anything;

It's not supposed to do anything until a connection is made.

Here is a typical command line:

server 51717

To run the client you need to pass in two arguments, the name of the host on which the server is running and the port number on which the server is listening for connections.

Here is the command line to connect to the server described above:

client cheerios 51717

You can simulate this on a single machine by running the server in one window and the client in another. In this case, you can use the keyword localhost as the first argument to the client.

The server code uses a number of ugly programming constructs, and so we will go through it line by line.

#include <stdio.h>

This header file contains declarations used in most input and output and is typically included in all C programs.

#include <sys/types.h>

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.

#include <sys/socket.h>

The header file socket.h includes a number of definitions of structures needed for sockets.

#include <netinet/in.h>

The header file in.h contains constants and structures needed for internet domain addresses.

void error(char *msg)

{
  perror(msg);
  exit(1);
}

int main(int argc, char *argv[])
{
  int sockfd, newsockfd, portno, clilen, n;

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.

char buffer[256];

struct sockaddr_in serv_addr, cli_addr;

Here is the definition:

struct sockaddr_in
{
  short   sin_family; /* must be AF_INET */
  u_short sin_port;
  struct  in_addr sin_addr;
  char    sin_zero[8]; /* Not used, must be zero */
};

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.

 if (argc < 2)
 {
   fprintf(stderr,"ERROR, no port provided
");
   exit(1);
 }

sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
  error("ERROR opening 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.

bzero((char *) &serv_addr, sizeof(serv_addr));

portno = atoi(argv[1]);

serv_addr.sin_family = AF_INET;

serv_addr.sin_port = htons(portno);

serv_addr.sin_addr.s_addr = INADDR_ANY;

if (bind(sockfd, (struct sockaddr *) &serv_addr,                   sizeof(serv_addr)) < 0)
  error("ERROR on binding");

listen(sockfd,5);

clilen = sizeof(cli_addr);
newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
if (newsockfd < 0)
  error("ERROR on accept");

bzero(buffer,256);
n = read(newsockfd,buffer,255);
if (n < 0) error("ERROR reading from socket");
printf("Here is the message: %s
",buffer);

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.

n = write(newsockfd,"I got your message",18);
if (n < 0) error("ERROR writing to socket");

  return 0;
}

Client code

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

void error(char *msg)
{
  perror(msg);
  exit(0);
}
int main(int argc, char *argv[])
{
  int sockfd, portno, n;
  struct sockaddr_in serv_addr;
  struct hostent *server;

The variable server is a pointer to a structure of type hostent. This structure is defined in the header file netdb.h as follows:

struct  hostent
{
  char    *h_name;        /* official name of host */
  char    **h_aliases;    /* alias list */
  int     h_addrtype;     /* host address type */
  int     h_length;       /* length of address */
  char    **h_addr_list;  /* list of addresses from name server */
  #define h_addr  h_addr_list[0]  /* address, for backward compatiblity */
};

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.

char buffer[256];
if (argc < 3)
{
  fprintf(stderr,"usage %s hostname port
", argv[0]);
  exit(0);
}
portno = atoi(argv[2]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
  error("ERROR opening socket");

server = gethostbyname(argv[1]);
if (server == NULL)
{
  fprintf(stderr,"ERROR, no such host
");
  exit(0);
}

 struct hostent *gethostbyname(char *name)

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.

bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr,
      (char *)&serv_addr.sin_addr.s_addr,
      server->h_length);
serv_addr.sin_port = htons(portno);

void bcopy(char *s1, char *s2, int length)

if (connect(sockfd,&serv_addr,sizeof(serv_addr)) < 0)
  error("ERROR connecting");

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.

  printf("Please enter the message: ");
  bzero(buffer,256);
  fgets(buffer,255,stdin);
  n = write(sockfd,buffer,strlen(buffer));
  if (n < 0)
    error("ERROR writing to socket");
  bzero(buffer,256);
  n = read(sockfd,buffer,255);
  if (n < 0)
    error("ERROR reading from socket");
  printf("%s
",buffer);
  return 0;
}

Enhancements to the server code

The sample server code above has the limitation that it only handles one connection, and then dies. A "real world" server should run indefinitely and should have the capability of handling a number of simultaneous connections, each in its own process. This is typically done by forking off a new process to handle each new connection.

The following code has a dummy function called dostuff(int sockfd).

This function will handle the connection after it has been established and provide whatever services the client requests. As we saw above, once a connection is established, both ends can use read and write to send information to the other end, and the details of the information passed back and forth do not concern us here.

To write a "real world" server, you would make essentially no changes to the main() function, and all of the code which provided the service would be in dostuff().

To allow the server to handle multiple simultaneous connections, we make the following changes to the code:

1. Put the accept statement and the following code in an infinite loop.
   
2. After a connection is established, call fork()#### to create a new process.
   
3. The child process will close sockfd#### and call #dostuff#####, passing the new socket file descriptor as an argument. When the two processes have completed their conversation, as indicated by dostuff()#### returning, this process simply exits.
   
4. The parent process closes newsockfd####. Because all of this code is in an infinite loop, it will return to the accept statement to wait for the next connection.

Here is the code.

 while (1)
 {
   newsockfd = accept(sockfd,
               (struct sockaddr *) &cli_addr, &clilen);
   if (newsockfd < 0)
     error("ERROR on accept");
   pid = fork();
   if (pid < 0)
     error("ERROR on fork");
   if (pid == 0)
   {
     close(sockfd);
     dostuff(newsockfd);
     exit(0);
   }
   else
     close(newsockfd);
 } /* end of while */

The zombie problem

The above code has a problem; if the parent runs for a long time and accepts many connections, each of these connections will create a zombie when the connection is terminated. A zombie is a process which has terminated but but cannot be permitted to fully die because at some point in the future, the parent of the process might execute a wait and would want information about the death of the child. Zombies clog up the process table in the kernel, and so they should be prevented. Unfortunately, the code which prevents zombies is not consistent across different architectures. When a child dies, it sends a SIGCHLD signal to its parent. On systems such as AIX, the following code in main() is all that is needed.

signal(SIGCHLD,SIG_IGN);

void *SigCatcher(int n)
{
  wait3(NULL,WNOHANG,NULL);
}
...
int main()
{
  ...
  signal(SIGCHLD,SigCatcher);
  ...

Alternative types of sockets

This example showed a stream socket in the Internet domain. This is the most common type of connection. A second type of connection is a datagram socket. You might want to use a datagram socket in cases where there is only one message being sent from the client to the server, and only one message being sent back. There are several differences between a datagram socket and a stream socket.

1. Datagrams are unreliable, which means that if a packet of information gets lost somewhere in the Internet, the sender is not told (and of course the receiver does not know about the existence of the message). In contrast, with a stream socket, the underlying TCP protocol will detect that a message was lost because it was not acknowledged, and it will be retransmitted without the process at either end knowing about this.
   
2. Message boundaries are preserved in datagram sockets. If the sender sends a datagram of 100 bytes, the receiver must read all 100 bytes at once. This can be contrasted with a stream socket, where if the sender wrote a 100 byte message, the receiver could read it in two chunks of 50 bytes or 100 chunks of one byte.
   
3. The communication is done using special system calls sendto()#### and receivefrom()#### rather than the more generic read()#### and write()####.
   
4. There is a lot less overhead associated with a datagram socket because connections do not need to be established and broken down, and packets do not need to be acknowledged. This is why datagram sockets are often used when the service to be provided is short, such as a time-of-day service.

Click here for the server code using a datagram socket.

Click here for the client code using a datagram socket.

These two programs can be compiled and run in exactly the same way as the server and client using a stream socket.

Most of the server code is similar to the stream socket code. Here are the differences.

sock=socket(AF_INET, SOCK_DGRAM, 0);

fromlen = sizeof(struct sockaddr_in);
while (1)
{
  n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
  if (n < 0) error("recvfrom");

  n = sendto(sock,"Got your message
",17,
             0,(struct sockaddr *) &from,fromlen);
  if (n  < 0)
    error("sendto");
  }
}

The client code for a datagram socket client is the same as that for a stream socket with the following differences.

• the socket system call has SOCK_DGRAM instead of SOCK_STREAM as its second argument.
  
• there is no connect()**** system call
  
• instead of read**** and write****, the client uses recvfrom**** and sendto **** which are described in detail above.

Sockets in the Unix Domain

Here is the code for a client and server which communicate using a stream socket in the Unix domain.

U_server.c

U_client

The only difference between a socket in the Unix domain and a socket in the Internet domain is the form of the address. Here is the address structure for a Unix Domain address, defined in the header file.

struct	sockaddr_un
{
 short	sun_family;	/* AF_UNIX */
 char	sun_path[108];	/* path name (gag) */
};

Designing servers

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.