Introduction to network functions in C

Last modified: 23rd June 2004


Simon Amor <>


In this tutorial, I will attempt to explain the use and syntax of some of the basic UNIX networking functions in C. If you want to know more about Windows Sockets programming, I'm afraid the WinSock resources at StarDust have disappeared now - any pointers to similar pages would be appreciated!

You might also want to check out the
Internet programming crash course

There are some example programs which I will explain how to write from the beginning. If you would like to know more about a function, you might have on-line manual pages (use 'man function') or your system administrators may be able to provide manuals.

You can view or download the source code but I cannot guarantee that it will work on all systems so if you find any code that doesn't work, please email me with details of your system (Operating System, Architecture etc) and what doesn't work and I'll see if I can fix it.

I would also appreciate email telling me if it DOES work on your system (or if you managed to modify parts and get it working), if you could email the alterations to me, I can then incorporate your modifications into the source code.
Some platforms that have been tested:

With some versions of UNIX, the example programs will not link, complaining about things such as undefined symbols bind, accept, listen and socket.

Such errors will probably look like this:

Undefined                       first referenced
 symbol                             in file
socket                              /var/tmp/ccLkjMcu.o
accept                              /var/tmp/ccLkjMcu.o
bind                                /var/tmp/ccLkjMcu.o
listen                              /var/tmp/ccLkjMcu.o
ld: fatal: Symbol referencing errors. No output written to a.out

This generally indicates that you should link in an additional library by adding -lsocket to the cc line.
For example: cc -o netapp netapp.c -lsocket

This tutorial assumes you know how to program in C.

If you find this tutorial of any interest whatsoever, please email and let me know. It's nowhere near complete as yet, but mail me anyway even if it's only to say "Hey! get that tutorial finished" :-)
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Creating a socket

The first thing to do is to create a socket which you can then manipulate in many ways. To create a socket, you can use the socket() function.


#include <sys/types.h>
#include <sys/socket.h> 


int socket(int af, int type, int protocol);

C source:

int socket_desc;

  if (socket_desc==-1)
    perror("Create socket");

socket() returns a socket descriptor which can be used in other network commands. This will create a socket which uses DARPA Internet addresses, and the method of connection is a byte stream which is similar to a pipe. An alternative to byte streams is a datagram but this tutorial does not cover them.
With servers, the first socket created is often known as a " master socket". Before the socket can send or receive data, it must be connected to another socket. If acting as a master socket, it must be bound to a port number so that clients can know where to "find" the socket and connect to it.
If successful, socket() returns a valid socket descriptor; otherwise it returns -1 and sets errno to indicate the error. perror() or strerror() can be used to turn the errno value into a human readable string.

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Binding a socket to a port

To set up a master socket, you need to bind the socket descriptor to a in many ways. To create a socket, you can use the socket() function as described above in Creating a socket.


#include <sys/socket.h>
#include <netinet/in.h>


int bind(int s, struct sockaddr *addr, int addrlen);

C source:

struct sockaddr_in address;

/* type of socket created in socket() */
  address.sin_family = AF_INET;
  address.sin_addr.s_addr = INADDR_ANY;
/* 7000 is the port to use for connections */
  address.sin_port = htons(7000);
/* bind the socket to the port specified above */
  bind(socket_desc,(struct sockaddr *)&address,sizeof(address));

If successful, bind() returns 0; otherwise it returns -1 and sets errno to indicate the error.
The port specified in the source code above (port 7000) is where the server can be connected to. To test this, compile the program `sockbind' from the source code directory and run it. While it is running, type:

telnet localhost 7000

and you should get


for a few seconds and then

telnet: Unable to connect to remote host: Connection refused

as the server program finishes. This indicates the server was ok. If the connection is refused immediately, there is probably a problem with the server.

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Listening for connections

Before any connections can be accepted, the socket must be told to listen for connections and also the maximum number of pending connections using listen()


#include <sys/socket.h>


int listen(int s, int backlog);

C source:


the above line specifies that there can be up to 3 connections pending. If a connection request arrives when there are already 3 connections pending, the client receives a timeout error.

listen() applies only to unconnected sockets of type SOCK_STREAM. If the socket has not been bound to a local port before listen() is invoked, the system automatically binds a local port for the socket to listen on.
If successful, listen() returns 0; otherwise it returns -1 and sets errno to indicate the error.

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Accepting a connection

To actually tell the server to accept a connection, you have to use the function accept()


#include <sys/socket.h>


int accept(int s, struct sockaddr *addr, int *addrlen);

C source:

int addrlen;
struct sockaddr_in address;

  addrlen = sizeof(struct sockaddr_in);
  new_socket = accept(socket_desc, (struct sockaddr *)&address, &addrlen);
  if (new_socket<0)
    perror("Accept connection");

accept() is used with connection based sockets such as streams. The parameters are the socket descriptor of the master socket followed by a sockaddr_in structure and the size of the structure. If successful, accept() returns a positive integer which is the socket descriptor for the accepted socket. If an error occurs, -1 is returned and errno is set to indicate the cause.

There is some example source code in the directory, the program to compile is called `accept' While it is running, type:

telnet localhost 7000

and you should get

Connected to localhost.
Escape character is '^]'.

and then 10 seconds later, it should close the connection. Once again, if the connection is refused immediately, there is probably a problem with the server.

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Closing connections

Probably one of the easiest things to do with a socket, is close it. This is done using close()


#include <unistd.h>


int close(int sockdes);

C source:


close() closes the socket descriptor indicated by sockdes.
Upon successful completion, close() returns a value of 0; otherwise, it returns -1 and sets errno to indicate the error.

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Sending data to a connection

Accepting a connection would not be any use without the means to send or receive data. Send without receive could be used for an information server which always returns a fixed message.


#include <sys/socket.h>


int send(int s, const void *msg, int len, int flags);

C source:

char *message="This is a message to send\n\r";


The message should have \n\r instead of just \n or \r because otherwise the text which appears on some clients may seem strange. e.g the text with just a \n would appear as follows:

This is a message
                 and this is the second line
                                            and the third.

instead of:

This is a message
and this is the second line
and the third.

send() is used to transmit a message to another socket and can be used only when the socket is in a connected state. The socket descriptor that specifies the socket on which the message will be sent is 's' in the syntax above. 'msg' points to the buffer containing the message and the length of the message is given by len, in bytes.
The supported values for flags are zero, or
MSG_OOB (to send out-of-band data) - a write() call made to a socket behaves in exactly the same way as send() with flags set to zero.
Upon successful completion, send() returns the number of bytes sent. Otherwise, it returns -1 and sets errno to indicate a locally-detected error. The `accept' program is modified to send a welcome message to the connection before it closes the socket. To see how this is done, have a look at the source code for the program `send'.

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Receiving data from a connection

Accepting a connection would not be any use without the means to send or receive data. Receive only could be used as a data collection method.


#include <sys/socket.h>


int recv(int s, void *msg, int len, int flags);

C source:

int bufsize=1024;        /* a 1K buffer */
char *buffer=malloc(bufsize);


The flags parameter can be set to MSG_PEEK, MSG_OOB, both, or zero. If it is set to MSG_PEEK, any data returned to the user still is treated as if it had not been read, i.e the next recv() re-reads the same data.
A read() call made to a socket behaves in exactly the same way as a recv() with flags set to zero.
If successful, recv() returns the number of bytes received, otherwise, it returns -1 and sets errno to indicate the error. recv() returns 0 if the socket is blocking and the connection to the remote node failed.

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Setting socket options

To allow certain socket operations requires manipulation of socket options using setsockopt()


#include <sys/socket.h>


  int setsockopt(int s, int level, int optname,
                 const void *optval, int optlen);

C source:

#define TRUE   1
#define FALSE  0

int socket_desc;     /* master socket returned by socket() */
int opt=TRUE;        /* option is to be on/TRUE or off/FALSE */

              (char *)&opt,sizeof(opt));

SOL_SOCKET specifies the option is a `socket level' option, these are defined in <sys/socket.h>
The socket is identified by the socket descriptor s.
The option SO_REUSEADDR is only valid for AF_INET sockets.
There are two kinds of options: boolean and non-boolean. Boolean options are either set or not set and also can use optval and optlen to pass information. Non-boolean options always use optval and optlen to pass information.

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Handling more than one connection

To enable a socket to be read without waiting if there is no input, the socket must be set non-blocking using the following snippet of code.

fcntl(mastersocket, F_SETFL, FNDELAY);


fcntl(mastersocket, F_SETFL, O_NONBLOCK);

If the above returns a non-zero result, the operation failed and errno should be set to an appropriate value.

Using select to monitor a number of sockets (or just one) is fairly straightforward and is shown in the code below. Please note this is incomplete code as the creation of the master socket is not included (see previous details).

fd_set readfds;

/* create a list of sockets to check for activity */

/* specify mastersocket - ie listen for new connections */
FD_SET(mastersocket, &readfds);

/* wait for connection, forever if have to */
new_conns=select(max_conns, readfds, NULL, NULL, NULL);

if ((new_conns<0) && (errno!=EINTR)) {
  /* there was an error with select() */
if (FD_ISSET(mastersocket,&readfds)) {
  /* Open the new socket */

Of course, the above will only wait for activity on the master socket. What you need to do is run it inside a loop which repeats until the server is shut down.
Any newly created sockets will need to be monitored as well (unless the connections accepted are closed after outputing a message).

For an example of this, see the sample program multi.c which accepts up to three connections and relays data from one socket to the others. It's almost a very basic chat server.

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Converting a hostname into a network address


#include <netdb.h>


  struct hostent *gethostbyname(const char *name);

C source:

  struct hostent *hent;

  hent = gethostbyname("");

A hostent structure:

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 */

Some of the network functions require a structure containing the network address and sometimes the port number to connect to or from. The easiest way to convert a hostname to a network address is to use the gethostbyname() function.

gethostbyname() returns a structure of type hostent - this structure contains the name of the host, an array of alternative names and also an array of network addresses (in network byte order).

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Establishing an outgoing connection

To establish a connection to another socket (similar to telnet), use the function connect().

#include <sys/types.h>
#include <sys/socket.h>

int  connect(int  sockfd, struct sockaddr *serv_addr, int addrlen );

Create the socket using socket(), convert the hostname to an IP address using gethostbyname() and then issue the connect() call passing the relevant structures containing the IP address and port to connect to.

  struct hostent     *he;
  struct sockaddr_in  server;
  int                 sockfd;

/* resolve localhost to an IP (should be */
  if ((he = gethostbyname("localhost")) == NULL) {
    puts("error resolving hostname..");

 * copy the network address part of the structure to the 
 * sockaddr_in structure which is passed to connect() 
  memcpy(&server.sin_addr, he->h_addr_list[0], he->h_length);
  server.sin_family = AF_INET;
  server.sin_port = htons(7000);

/* connect */
  if (connect(sockfd, (struct sockaddr *)&server, sizeof(server)) {
    puts("error connecting..");

Using connect() ... more to come when/if I get around to it.

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23rd June 2004 : Added information about getting it to work on Mac OSX 10.3 (due to a query regarding this from Nicolas). Tweaked the source to netlib slightly - main bugfix was to correct filenames in the Makefile.

15th November 2003 : Removed link to StarDust WinSock resources as this site has disappeared.

20th February 2003 : In Creating a socket, fixed comparison of socket() return value to -1 to indicate error instead of 0 (thanks to Vince for pointing this out).

16th September 2002 : Converted to be XHTML 1.0 Strict compliant and use CSS properly. Also added link to glossary page.

14th September 2002 : Added url to Internet programming crash course (thanks to Kelly Mandrake).

20th March 2001 : Improved navigation

2nd December 2000 : Updated locations and email addresses

29th May 1999 : Added information about -lsocket.

4th May 1998 : Added an extra include for socket() as per the man page.

13th April 1998 : Added links to downloadable source for netlib.

18th March 1998 : Added a link to downloadable archive for offline reading.

9th October 1997 : Corrected he->h_addr[0] to he->h_addr_list[0] under the
connect section

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