Bind udp socket linux
In our discussion of socketswe covered an example of programming with connection-oriented sockets: As with TCP sockets, this tutorial will focus on the basics. There are tutorials on the web that delve into far greater detail. On-line manual pages will provide you with the latest information on bind udp socket linux parameters and functions.
Since there's only one form of datagram service, there are no variations of the protocol, so the last argument, protocolbind udp socket linux zero. Our code for creating a UDP socket looks like this:. When we talk about naming a socket, we are talking about assigning a transport address to the socket a port number in IP networking.
In sockets, this operation is called binding an address and the bind system call is used to do this. The analogy is that of assigning a phone number to the line that you requested from the phone company in step 1 or that of assigning an address to a mailbox.
The transport address is defined in a socket address structure. Because sockets were designed to work with various different types of communication interfaces, the interface is very general. Instead of accepting, say, a port number as a parameter, it takes a sockaddr structure bind udp socket linux actual format is determined on the address family type of network you're using. For example, if you're using UNIX domain bind udp socket linux, bind actually creates a file in the file system.
The first parameter, socketis the socket that was created with the socket system call. For the second parameter, the structure sockaddr is a generic container that just allows the OS to be able to read the first couple of bytes that identify the address family.
The address family determines what variant of the sockaddr struct to use that contains elements that make sense for that specific communication type.
Before calling bindwe need to fill out this structure. The three key parts we need to set are: You can explicitly assign a transport address port or allow the operating system to assign one. If you're a client and don't need a well-known port that others can use to locate you since they will only respond to your messagesyou can just let the operating system pick any available port number by specifying port 0.
If you're a server, you'll generally pick a specific number since clients will need to know a port number to which to address messages. This is just your machine's IP address. With IP, your machine will have one IP address for each network interface. For example, if your machine has both Wi-Fi and ethernet connections, that machine will have two addresses, one for each interface. Most of the time, we don't care to specify a specific interface and can let the operating system use whatever it wants.
The special address for this is 0. Since the address structure may differ based on the type of transport used, the third parameter specifies the length of that structure. This example asks the operating system to pick any available port number by setting the port to 0. If you want to use a specific port number, change the line. You might have noticed the htonl and htons references in the previous code block.
These convert bind udp socket linux and two-byte numbers into network representations. Integers are stored in memory and sent across the network as sequences of bytes. There are two common ways of storing these bytes: Little endian representation stores the least-significant bytes in low memory. Big bind udp socket linux representation stores the least-significant bytes in high memory. The Intel x86 family uses the little endian format.
Old Motorola processors and the PowerPC used by Macs before their switch to the Intel architecture use the big endian format. Internet headers standardized on using the big endian format. If you're on an Intel processor and set the value of a port to 1, hex equivalent 04d2it will be be stored in memory as d For processors that use the big endian bind udp socket linux, these macros do absolutely nothing.
For those that use the little endian format most processors, these daysthe macros flip the sequence of either four or two bytes. Using the macros, however, ensures that your code remains portable regardless of the architecture to which you compile. With TCP sockets, we had to establish a connection before we could communicate. With UDP, our sockets are connectionless. Bind udp socket linux, we can send messages immediately. Since we do not have a connection, the messages have to be addressed to their destination.
Instead of using a bind udp socket linux system call, we will use sendtowhich allows us to specify the destination. The address is identified through the sockaddr structure, the same way as it is in bind and as we did when using connect for TCP sockets. The first parameter, socketis the socket that was created with the socket system call and named via bind. The second parameter, bufferprovides the starting address of the message we want to send.
The flags parameter is 0 and not useful for UDP sockets. As with bindthe final parameter is simply the length of the address structure: The server's address will contain the IP address of the server machine as well as the port number that corresponds to a socket listening on that port on that machine. The IP address is a four-byte 32 bit value in network byte order see htonl above.
In most cases, you'll know the name of the machine but not its IP address. An easy way of getting the IP address is with the gethostbyname library libc function. Gethostbyname accepts a host bind udp socket linux as a parameter and returns a hostent structure:. There may be more than one IP addresses for a host.
In practice, you should be able to use any of the addresses or you may want to pick one that matches a particular subnet. We'll be bind udp socket linux here and just use the first address in the list. For example, suppose you want to find the addresses for google. The code will look like this:. Here's the code for sending a message to the address of a machine in host.
The variable fd is the socket which was created with the socket system call. With TCP sockets, a server would set up a socket for listening via a listen system bind udp socket linux and then call accept to wait for a connection.
A server can immediately listen for messages once it has a socket. We use the recvfrom system call to wait for an incoming datagram on a specific transport address IP address and port number. The first parameter, socket is a socket that we created ahead of time and used bind.
The port number assigned to that socket via the bind call tells us on what port recvfrom will wait bind udp socket linux data. The incoming data will be placed into the memory at buffer and no more than length bytes will be transferred that's the size of your buffer. We will ignore flags here. You can look at the man page for recvfrom for details on this. This parameter allows us to process out-of-band data, peek at an incoming message without removing it from the queue, or block until the request is fully satisfied.
We can safely ignore these and use 0. If you do not care bind udp socket linux identify the sender, you can set both of these to zero but you will then have no way to reply to the sender.
Let's examine a simple server. We'll create a socket, bind it to all available IP addresses on the machine but to a specific port number. Then we will loop, receiving messages and printing their contents. We now have a client sending a message to a server. What if the server wants to send a message back to that client?
There is no connection so the server cannot just write the response back. Fortunately, the recvfrom call gave us the address of the server. It was placed in remaddr:. The server can use that address in sendto and send a message back to the recipient's address. Close the socket With TCP sockets, we saw that we can use the bind udp socket linux system call to close a socket or to terminate communication in a single direction.
Since there is no concept of a connection in UDP, there is no need to call shutdown. However, the socket still uses up a file descriptor in the kernel, so we can free that up with the close system call just as we do with files. For questions or bind udp socket linux about this site, contact Paul Krzyzanowski, gro.
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Programming with UDP sockets.