Using sockets in linux

Socket programming in C on Linux – The Ultimate Guide for Beginners

TCP/IP socket programming in C

This is a quick tutorial on socket programming in c language on a Linux system. «Linux» because the code snippets shown over here will work only on a Linux system and not on Windows. The windows api to socket programming is called winsock and we shall go through it in another tutorial.

Sockets are the «virtual» endpoints of any kind of network communications done between 2 hosts over in a network. For example when you type www.google.com in your web browser, it opens a socket and connects to google.com to fetch the page and show it to you. Same with any chat client like gtalk or skype. Any network communication goes through a socket.

The socket api on linux is similar to bsd/unix sockets from which it has evolved. Although over time the api has become slightly different at few places. And now the newer official standard is posix sockets api which is same as bsd sockets.

This tutorial assumes that you have basic knowledge of C and pointers. You will need to have gcc compiler installed on your Linux system. An IDE along with gcc would be great. I would recommend geany as you can quickly edit and run single file programs in it without much configurations. On ubuntu you can do a sudo apt-get install geany on the terminal.

All along the tutorial there are code snippets to demonstrate some concepts. You can run those code snippets in geany rightaway and test the results to better understand the concepts.

1. Create a socket

This first thing to do is create a socket. The socket function does this.
Here is a code sample :

Function socket() creates a socket and returns a descriptor which can be used in other functions. The above code will create a socket with following properties

Next we shall try to connect to some server using this socket.
We can connect to www.google.com

Apart from SOCK_STREAM type of sockets there is another type called SOCK_DGRAM which indicates the UDP protocol. This type of socket is non-connection socket. In this tutorial we shall stick to SOCK_STREAM or TCP sockets.

2. Connect socket to a server

We connect to a remote server on a certain port number. So we need 2 things, ip address and port number to connect to.

To connect to a remote server we need to do a couple of things. First is to create a sockaddr_in structure with proper values.

Have a look at the structure

The sockaddr_in has a member called sin_addr of type in_addr which has a s_addr which is nothing but a long. It contains the IP address in long format.

Function inet_addr is a very handy function to convert an IP address to a long format. This is how you do it :

So you need to know the IP address of the remote server you are connecting to. Here we used the ip address of google.com as a sample. A little later on we shall see how to find out the ip address of a given domain name.

The last thing needed is the connect function. It needs a socket and a sockaddr structure to connect to. Here is a code sample.

It cannot be any simpler. It creates a socket and then connects. If you run the program it should show Connected.
Try connecting to a port different from port 80 and you should not be able to connect which indicates that the port is not open for connection.

OK , so we are now connected. Lets do the next thing , sending some data to the remote server.

Connections are present only in tcp sockets

The concept of «connections» apply to SOCK_STREAM/TCP type of sockets. Connection means a reliable «stream» of data such that there can be multiple such streams each having communication of its own. Think of this as a pipe which is not interfered by other data.

Other sockets like UDP , ICMP , ARP dont have a concept of «connection». These are non-connection based communication. Which means you keep sending or receiving packets from anybody and everybody.

3. Send data over socket

Function send will simply send data. It needs the socket descriptor , the data to send and its size.
Here is a very simple example of sending some data to google.com ip :

In the above example , we first connect to an ip address and then send the string message «GET / HTTP/1.1\r\n\r\n» to it.
The message is actually a http command to fetch the mainpage of a website.

Now that we have send some data , its time to receive a reply from the server. So lets do it.

When sending data to a socket you are basically writing data to that socket. This is similar to writing data to a file. Hence you can also use the write function to send data to a socket. Later in this tutorial we shall use write function to send data.

4. Receive data on socket

Function recv is used to receive data on a socket. In the following example we shall send the same message as the last example and receive a reply from the server.

Here is the output of the above code :

We can see what reply was send by the server. It looks something like Html, well IT IS html. Google.com replied with the content of the page we requested. Quite simple!

When receiving data on a socket , we are basically reading the data on the socket. This is similar to reading data from a file. So we can also use the read function to read data on a socket. For example :

Now that we have received our reply, its time to close the socket.

5. Close socket

Function close is used to close the socket. Need to include the unistd.h header file for this.

6. Summary

So in the above example we learned how to

Your web browser also does the same thing when you open www.google.com
This kind of socket activity represents a socket client. A client is an application that connects to a remote system to fetch or retrieve data.

The other kind of socket application is called a socket server. A server is a system that uses sockets to receive incoming connections and provide them with data. It is just the opposite of Client. So www.google.com is a server and your web browser is a client. Or more technically www.google.com is a HTTP Server and your web browser is an HTTP client.

Now its time to do some server tasks using sockets. But before we move ahead there are a few side topics that should be covered just incase you need them.

Get ip address of hostname

When connecting to a remote host , it is necessary to have its IP address. Function gethostbyname is used for this purpose. It takes the domain name as the parameter and returns a structure of type hostent. This structure has the ip information. It is present in netdb.h . Lets have a look at this structure

The h_addr_list has the IP addresses. So now lets have some code to use them.

Output of the code would look like :

So the above code can be used to find the ip address of any domain name. Then the ip address can be used to make a connection using a socket.

Function inet_ntoa will convert an IP address in long format to dotted format. This is just the opposite of inet_addr .

So far we have see some important structures that are used. Lets revise them :

1. sockaddr_in — Connection information. Used by connect , send , recv etc.
2. in_addr — Ip address in long format
3. sockaddr
4. hostent — The ip addresses of a hostname. Used by gethostbyname

In the next part we shall look into creating servers using socket. Servers are the opposite of clients, that instead of connecting out to others, they wait for incoming connections.

Socket server

OK now onto server things. Socket servers operate in the following manner

We have already learnt how to open a socket. So the next thing would be to bind it.

1. Bind socket to a port

The bind function can be used to bind a socket to a particular «address and port» combination. It needs a sockaddr_in structure similar to connect function.

Now that bind is done, its time to make the socket listen to connections. We bind a socket to a particular IP address and a certain port number. By doing this we ensure that all incoming data which is directed towards this port number is received by this application.

This makes it obvious that you cannot have 2 sockets bound to the same port.

2. Listen for incoming connections on the socket

After binding a socket to a port the next thing we need to do is listen for connections. For this we need to put the socket in listening mode. Function listen is used to put the socket in listening mode. Just add the following line after bind.

Thats all. Now comes the main part of accepting new connections.

3. Accept connection

Function accept is used for this. Here is the code

Program output

Run the program. It should show

So now this program is waiting for incoming connections on port 8888. Dont close this program , keep it running.
Now a client can connect to it on this port. We shall use the telnet client for testing this. Open a terminal and type

On the terminal you shall get

And the server output will show

So we can see that the client connected to the server. Try the above process till you get it perfect.

4. Get the ip address of the connected client

You can get the ip address of client and the port of connection by using the sockaddr_in structure passed to accept function. It is very simple :

We accepted an incoming connection but closed it immediately. This was not very productive. There are lots of things that can be done after an incoming connection is established. Afterall the connection was established for the purpose of communication. So lets reply to the client.

We can simply use the write function to write something to the socket of the incoming connection and the client should see it. Here is an example :

Run the above code in 1 terminal. And connect to this server using telnet from another terminal and you should see this :

So the client(telnet) received a reply from server.

We can see that the connection is closed immediately after that simply because the server program ends after accepting and sending reply. A server like www.google.com is always up to accept incoming connections.

It means that a server is supposed to be running all the time. Afterall its a server meant to serve. So we need to keep our server RUNNING non-stop. The simplest way to do this is to put the accept in a loop so that it can receive incoming connections all the time.

5. Live Server

So a live server will be alive for all time. Lets code this up :

We havent done a lot there. Just the accept was put in a loop.

Now run the program in 1 terminal , and open 3 other terminals. From each of the 3 terminal do a telnet to the server port.

Each of the telnet terminal would show :

And the server terminal would show

So now the server is running nonstop and the telnet terminals are also connected nonstop. Now close the server program.
All telnet terminals would show «Connection closed by foreign host.»
Good so far. But still there is not effective communication between the server and the client.

The server program accepts connections in a loop and just send them a reply, after that it does nothing with them. Also it is not able to handle more than 1 connection at a time. So now its time to handle the connections , and handle multiple connections together.

6. Handle multiple socket connections with threads

To handle every connection we need a separate handling code to run along with the main server accepting connections.
One way to achieve this is using threads. The main server program accepts a connection and creates a new thread to handle communication for the connection, and then the server goes back to accept more connections.

On Linux threading can be done with the pthread (posix threads) library. It would be good to read some small tutorial about it if you dont know anything about it. However the usage is not very complicated.

We shall now use threads to create handlers for each connection the server accepts. Lets do it pal.

Run the above server and open 3 terminals like before. Now the server will create a thread for each client connecting to it.

The telnet terminals would show :

This one looks good , but the communication handler is also quite dumb. After the greeting it terminates. It should stay alive and keep communicating with the client.

One way to do this is by making the connection handler wait for some message from a client as long as the client is connected. If the client disconnects , the connection handler ends.

So the connection handler can be rewritten like this :

The above connection handler takes some input from the client and replies back with the same. Simple! Here is how the telnet output might look

So now we have a server thats communicative. Thats useful now.

Linking the pthread library

When compiling programs that use the pthread library you need to link the library. This is done like this :

Conclusion

By now you must have learned the basics of socket programming in C. You can try out some experiments like writing a chat client or something similar.

If you think that the tutorial needs some addons or improvements or any of the code snippets above dont work then feel free to make a comment below so that it gets fixed.

A Tech Enthusiast, Blogger, Linux Fan and a Software Developer. Writes about Computer hardware, Linux and Open Source software and coding in Python, Php and Javascript. He can be reached at [email protected] .

150 thoughts on “ Socket programming in C on Linux – The Ultimate Guide for Beginners ”

thank you soo much , helpful and perfect explanation

You never mention CLOSING the socket. Let’s suppose we have clients contacting a server to receive a file of arbitrary length. So the client now has to read the response from the server until the server reaches the end-of-file EOF. Can the server simply close the socket when it has finished ? Or will the client receive a “Socket reset by peer” ?

Thank you! Exactly what I was looking for in a post about the basics of sockets in C.

Brilliantly written and explained.

The tutorial is very well written and it teaches the main concept of the Linux sockets and the examples are useful, thanks for your work.

Your http read code usually gets a ‘bad request’ response.
This is because http 1.1 requires a host = in the header. Something like
char * message = “GET / HTTP/1.1\r\nHost: example.com\r\n\r\n”;
if( send(socketID , message , strlen(message) , 0)

Thanks for this tutorial.
Is the second like below supposed to be commented out?

//Now join the thread , so that we dont terminate before the thread
//pthread_join( sniffer_thread , NULL);
puts(“Handler assigned”);

Explanation is good. Concepts are made very clear with detailed explanation and example.

Very nice and helpful tutorial , thank you a lot for sharing
I would like to make a question in you example. I would like to send two variables from client to server (a) & (b) and to make the sumution of those to server and after that the server to reply the result to client.
which change can i make to your example , can you help me?

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