A file descriptor is simply an integer associated with an open file. right now , but I’m going to answer it anyway: You make a call to the socket() system routine. System Calls or Bust¬†– What is a socket?¬†– Struct sockaddr and pals. Praise for Socket Programming Guide. When I first wrote Beej’s Guide to.

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I encourage others to provide similar information whenever possible.

Since datagram sockets aren’t connected to societ remote host, guess which piece of information we need to give before we send a packet? You can take your socket descriptor made with the socket system call and tell it to listen for incoming connections.

In our case, though, the peer’s “name” is it’s IP address and port. If you’ll recall, things like socket and listen return -1 on error, and they set the exact value of errno to let you know specifically which error occurred. This structure is a more recent invention, and is used to prep the socket address structures for subsequent use.

The last array element is NULL.

If you output two items into the socket in the soc,et “1, 2”, they will arrive in the order “1, 2” at the opposite end. When you actually do recv from it, recv will return 0.

One cool thing you can do is install Cygwinwhich is a collection of Unix tools for Windows. But have a look, then read the description that follows it. Lastly, for input parameters, we have hints. You call accept and you tell it to get the pending connection. The original one is still listening on your port and the newly created one is finally ready to send and recv.

It’ll fire off as much of the data as it can, and trust you to send the rest later. As much as Bill Gates would like to think otherwise. So they made a new one. The function will do the magic conversion if it has to, and this way your code is portable to machines of differing endianness. Also notice that it returns a pointer to a char. In the Ancient Times, there were “classes” of subnets, where the first one, two, or three bytes of the address was the network part.


The maximum lengths are: If you’re only getting one single connection ever, you can close the listening sockfd in order to prevent more incoming connections on the same port, if you so desire. So I get it, and then go through the master list and send that data to all the rest of the connected clients.

Beej’s Guide to Network Programming

Usually this information comes along as the result of a call to getaddrinfobut you can fill out your own struct sockaddr if you want to. See the section on socket for details.

Also, select only works with socket descriptors, not file descriptors like 0 for stdin. So you get wise, and you prevent the client from using any except for a couple utilities that you know are safe, like the foobar utility:.

Beej’s Guide to Network Programming

Finally, when you’re done with the linked list, you need to call freeaddrinfo to free up the memory or it will be leaked, and Some People will get upset. The code contained within this document was compiled on a Linux PC using Gnu’s gcc compiler. See your online help for details. But seriously, though, there is a danger to using broadcast packets, aocket that is: My external IP address soxket really So all you need to do to use my code is to find the place between where the data is read and the data is sent using send over the network, and stick some code in there that does the encryption.

You call accept and you tell it to get the pending connection. My favorite solution to this involves a goto statement. Now that I’ve badgered you about how to write and not write me, I’d just like to let you know that I fully appreciate all the praise the guide has received over the years. What if you’re blocking on an accept call?


When you first create the socket descriptor with socketthe kernel sets it to blocking. But others do not.

Finally, there’s an IPv4-compatibility mode for IPv6 addresses that you might come across. Let’s say that talker calls connect and specifies the listener ‘s address. You also have to tell your compiler to link in the Winsock library, usually called wsock If I log into a remote computer, it tells me I’m logged in from That is, you have a work buffer with one complete packet, and an incomplete part of the next packet!

The original one is still listening for more new connections, and the newly created one is finally ready to send and recv. With the exception of the client socoet allowed to send broadcast packets in this case.

Beej’s Guide to Network Programming Using Internet Sockets

For one thing, you can still often connect through the firewall if it’s doing some kind of masquerading or NAT or something like that. Once you have either accept ed a remote connection, or connect ed to a server, you now have what is known as a peer. But it’s a bit unwieldy to use a big string of numbers like But assuming you want to plug in or implement your own compressor or encryption sockst, it’s just a matter of thinking of your data as running through a sequence of steps between both ends.

You can also see in the above example that the last couple decimal places are not correctly preserved.