Beej's Guide to Network Programming

Using Internet Sockets

Brian "Beej Jorgensen" Hall
beej@beej.us

Version 3.0.14
September 8, 2009

Copyright ? 2009 Brian "Beej Jorgensen" Hall

Contents

1. Intro

1.1. Audience

1.2. Platform and Compiler

1.3. Official Homepage and Books For Sale

1.4. Note for Solaris/SunOS Programmers

1.5. Note for Windows Programmers

1.6. Email Policy

1.7. Mirroring

1.8. Note for Translators

1.9. Copyright and Distribution

2. What is a socket?

2.1. Two Types of Internet Sockets

2.2. Low level Nonsense and Network Theory

3. IP Addresses, structs, and Data Munging

3.1. IP Addresses, versions 4 and 6

3.2. Byte Order

3.3. structs

3.4. IP Addresses, Part Deux

4. Jumping from IPv4 to IPv6

5. System Calls or Bust

5.1. getaddrinfo()—Prepare to launch!

5.2. socket()—Get the File Descriptor!

5.3. bind()—What port am I on?

5.4. connect()—Hey, you!

5.5. listen()—Will somebody please callme?

5.6. accept()—"Thank you for calling port3490."

5.7. send() and recv()—Talk to me,baby!

5.8. sendto() andrecvfrom()—Talk to me, DGRAM-style

5.9. close() andshutdown()—Get outta my face!

5.10. getpeername()—Who are you?

5.11. gethostname()—Who am I?

6. Client-Server Background

6.1. A Simple Stream Server

6.2. A Simple Stream Client

6.3. Datagram Sockets

7. Slightly Advanced Techniques

7.1. Blocking

7.2. select()—Synchronous I/O Multiplexing

7.3. Handling Partial send()s

7.4. Serialization—How to Pack Data

7.5. Son of Data Encapsulation

7.6. Broadcast Packets—Hello, World!

8. Common Questions

9. Man Pages

9.1. accept()

9.2. bind()

9.3. connect()

9.4. close()

9.5. getaddrinfo(), freeaddrinfo(),gai_strerror()

9.6. gethostname()

9.7. gethostbyname(), gethostbyaddr()

9.8. getnameinfo()

9.9. getpeername()

9.10. errno

9.11. fcntl()

9.12. htons(), htonl(),ntohs(), ntohl()

9.13. inet_ntoa(), inet_aton(),inet_addr

9.14. inet_ntop(), inet_pton()

9.15. listen()

9.16. perror(), strerror()

9.17. poll()

9.18. recv(), recvfrom()

9.19. select()

9.20. setsockopt(), getsockopt()

9.21. send(), sendto()

9.22. shutdown()

9.23. socket()

9.24. struct sockaddr and pals

10. More References

10.1. Books

10.2. Web References

10.3. RFCs

Index

1. Intro

Hey! Socket programming got you down? Is this stuff just a littletoo difficult to figure out from the man pages? You want todo cool Internet programming, but you don't have time to wade through agob of structs trying to figure out if you have to callbind() before you connect(), etc., etc.

Well, guess what! I've already done this nasty business, and I'mdying to share the information with everyone! You've come to the rightplace. This document should give the average competent C programmer theedge s/he needs to get a grip on this networking noise.

And check it out: I've finally caught up with the future (just in thenick of time, too!) and have updated the Guide for IPv6! Enjoy!

1.1. Audience

This document has been written as a tutorial, not a completereference. It is probably at its best when read by individuals who arejust starting out with socket programming and are looking for afoothold. It is certainly not the complete and total guideto sockets programming, by any means.

Hopefully, though, it'll be just enough for those man pages to startmaking sense... :-)

1.2. Platform and Compiler

The code contained within this document was compiled on a Linux PCusing Gnu's gcc compiler. Itshould, however, build on just about any platform that usesgcc. Naturally, this doesn't apply if you're programming forWindows—see the section on Windowsprogramming, below.

1.3. Official Homepage and Books For Sale

This official location of this document is http://beej.us/guide/bgnet/. There you willalso find example code and translations of the guide into variouslanguages.

To buy nicely bound print copies (some call them "books"), visithttp://beej.us/guide/url/bgbuy. I'll appreciate the purchasebecause it helps sustain my document-writing lifestyle!

1.4. Note for Solaris/SunOS Programmers

When compiling for Solaris or SunOS, you need to specify some extra command-line switchesfor linking in the proper libraries. In order to do this, simply add"-lnsl -lsocket -lresolv" to the end of the compile command,like so:

$ cc -o server server.c -lnsl -lsocket -lresolv

If you still get errors, you could try further adding a"-lxnet" to the end of that command line. I don't know whatthat does, exactly, but some people seem to need it.

Another place that you might find problems is in the call tosetsockopt(). The prototype differs from that on my Linuxbox, so instead of:

int yes=1;

enter this:

char yes='1';

As I don't have a Sun box, I haven't tested any of the aboveinformation—it's just what people have told me through email.

1.5. Note for Windows Programmers

At this point in the guide, historically, I've done a bit of baggingon Windows, simply due to the fact that I don't likeit very much. But I should really be fair and tell you that Windows hasa huge install base and is obviously a perfectly fine operatingsystem.

They say absence makes the heart grow fonder, and in this case, Ibelieve it to be true. (Or maybe it's age.) But what I can say is thatafter a decade-plus of not using Microsoft OSes for my personal work,I'm much happier! As such, I can sit back and safely say, "Sure, feelfree to use Windows!" ...Ok yes, it does make me grit my teeth to saythat.

So I still encourage you to try Linux, BSD, orsome flavor of Unix, instead.

But people like what they like, and you Windows folk will be pleasedto know that this information is generally applicable to you guys, witha few minor changes, if any.

One cool thing you can do is install Cygwin, which is a collection of Unix toolsfor Windows. I've heard on the grapevine that doing so allows all theseprograms to compile unmodified.

But some of you might want to do things the Pure Windows Way. That'svery gutsy of you, and this is what you have to do: run out and get Uniximmediately! No, no—I'm kidding. I'm supposed to beWindows-friendly(er) these days...

This is what you'll have to do (unless you install Cygwin!): first, ignore prettymuch all of the system header files I mention in here. All you need toinclude is:

#include 

Wait! You also have to make a call to WSAStartup() before doing anything elsewith the sockets library. The code to do that looks something likethis:

#include {    WSADATA wsaData;   // if this doesn't work    //WSAData wsaData; // then try this instead    // MAKEWORD(1,1) for Winsock 1.1, MAKEWORD(2,0) for Winsock 2.0:    if (WSAStartup(MAKEWORD(1,1), &wsaData) != 0) {        fprintf(stderr, "WSAStartup failed.\n");        exit(1);    }

You also have to tell your compiler to link in the Winsock library,usually called wsock32.lib or winsock32.lib,or ws2_32.lib for Winsock 2.0. Under VC++, this can bedone through the Project menu, under Settings....Click the Link tab, and look for the box titled "Object/librarymodules". Add "wsock32.lib" (or whichever lib is your preference) tothat list.

Or so I hear.

Finally, you need to call WSACleanup() when you're all throughwith the sockets library. See your online help for details.

Once you do that, the rest of the examples in this tutorial shouldgenerally apply, with a few exceptions. For one thing, you can't useclose() to close a socket—you need to use closesocket(), instead. Also, select() only works with socketdescriptors, not file descriptors (like 0 forstdin).

There is also a socket class that you can use, CSocket. Check your compilers help pagesfor more information.

To get more information about Winsock, read the Winsock FAQ and go fromthere.

Finally, I hear that Windows has no fork() system call which is, unfortunately,used in some of my examples. Maybe you have to link in a POSIX libraryor something to get it to work, or you can use CreateProcess() instead.fork() takes no arguments, and CreateProcess()takes about 48 billion arguments. If you're not up to that, the CreateThread() is a little easier todigest...unfortunately a discussion about multithreading is beyond thescope of this document. I can only talk about so much, you know!

1.6. Email Policy

I'm generally available to help out with email questions so feel free to write in, but I can't guarantee aresponse. I lead a pretty busy life and there are times when I justcan't answer a question you have. When that's the case, I usually justdelete the message. It's nothing personal; I just won't ever have thetime to give the detailed answer you require.

As a rule, the more complex the question, the less likely I am torespond. If you can narrow down your question before mailing it and besure to include any pertinent information (like platform, compiler,error messages you're getting, and anything else you think might help metroubleshoot), you're much more likely to get a response. For morepointers, read ESR's document, How To AskQuestions The Smart Way.

If you don't get a response, hack on it some more, try to find theanswer, and if it's still elusive, then write me again with theinformation you've found and hopefully it will be enough for me to helpout.

Now that I've badgered you about how to write and not write me, I'djust like to let you know that I fully appreciate all thepraise the guide has received over the years. It's a real morale boost,and it gladdens me to hear that it is being used for good! :-)Thank you!

1.7. Mirroring

You are more than welcome to mirror this site,whether publicly or privately. If you publicly mirror the site and wantme to link to it from the main page, drop me a line atbeej@beej.us.

1.8. Note for Translators

If you want to translate the guide intoanother language, write me at beej@beej.us and I'll link toyour translation from the main page. Feel free to add your name andcontact info to the translation.

Please note the license restrictions in the Copyright andDistribution section, below.

If you want me to host the translation, just ask. I'll also link toit if you want to host it; either way is fine.

1.9. Copyright and Distribution

Beej's Guide to Network Programming is Copyright ? 2009Brian "Beej Jorgensen" Hall.

With specific exceptions for source code and translations, below,this work is licensed under the Creative Commons Attribution-Noncommercial- No Derivative Works 3.0 License. To view a copy of thislicense, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to CreativeCommons, 171 Second Street, Suite 300, San Francisco, California, 94105,USA.

One specific exception to the "No Derivative Works" portion of thelicense is as follows: this guide may be freely translated into anylanguage, provided the translation is accurate, and the guide isreprinted in its entirety. The same license restrictions apply to thetranslation as to the original guide. The translation may also includethe name and contact information for the translator.

The C source code presented in this document is hereby granted to thepublic domain, and is completely free of any license restriction.

Educators are freely encouraged to recommend or supply copies of thisguide to their students.

Contact beej@beej.us for more information.

2. What is a socket?

You hear talk of "sockets" all the time, andperhaps you are wondering just what they are exactly. Well, they'rethis: a way to speak to other programs using standard Unix file descriptors.

What?

Ok—you may have heard some Unix hacker state, "Jeez,everything in Unix is a file!" What that person may havebeen talking about is the fact that when Unix programs do any sort ofI/O, they do it by reading or writing to a file descriptor. A filedescriptor is simply an integer associated with an open file. But (andhere's the catch), that file can be a network connection, a FIFO, apipe, a terminal, a real on-the-disk file, or just about anything else.Everything in Unix is a file! So when you want tocommunicate with another program over the Internet you're gonna do itthrough a file descriptor, you'd better believe it.

"Where do I get this file descriptor for network communication, Mr.Smarty-Pants?" is probably the last question on your mind right now, butI'm going to answer it anyway: You make a call to the socket() system routine. It returns thesocket descriptor, and you communicatethrough it using the specialized send()and recv() (mansend, man recv)socket calls.

"But, hey!" you might be exclaiming right about now. "If it's a filedescriptor, why in the name of Neptune can't I just use the normal read() and write() calls to communicate through thesocket?" The short answer is, "You can!" The longer answer is, "Youcan, but send() and recv() offer much greater control over yourdata transmission."

What next? How about this: there are all kinds of sockets. Thereare DARPA Internet addresses (InternetSockets), path names on a local node (Unix Sockets), CCITT X.25addresses (X.25 Sockets that you can safely ignore), and probably manyothers depending on which Unix flavor you run. This document deals onlywith the first: Internet Sockets.

2.1. Two Types of Internet Sockets

What's this? There are two types of Internetsockets? Yes. Well, no. I'm lying. There are more, but I didn't wantto scare you. I'm only going to talk about two types here. Except forthis sentence, where I'm going to tell you that "Raw Sockets" are also very powerful andyou should look them up.

All right, already. What are the two types? One is "Stream Sockets"; the other is"Datagram Sockets", whichmay hereafter be referred to as "SOCK_STREAM" and"SOCK_DGRAM", respectively. Datagram sockets aresometimes called "connectionless sockets". (Though they can be connect()'d if you really want. Seeconnect(), below.)

Stream sockets are reliable two-way connected communication streams.If you output two items into the socket in the order "1, 2", they willarrive in the order "1, 2" at the opposite end. They will also beerror-free. I'm so certain, in fact, they will be error-free, that I'mjust going to put my fingers in my ears and chant la la la laif anyone tries to claim otherwise.

What uses stream sockets? Well, you mayhave heard of the telnet application, yes?It uses stream sockets. All the characters you type need to arrive inthe same order you type them, right? Also, web browsers use the HTTP protocol which uses stream sockets to getpages. Indeed, if you telnet to a web site on port 80, and type"GET / HTTP/1.0" and hit RETURN twice, it'll dump the HTML backat you!

How do stream sockets achieve this high level of data transmissionquality? They use a protocol called "The Transmission ControlProtocol", otherwise known as "TCP" (see RFC 793 for extremely detailed infoon TCP.) TCP makes sure your data arrives sequentially and error-free.You may have heard "TCP" before as the better half of "TCP/IP" where "IP" stands for "Internet Protocol" (see RFC 791.) IP deals primarily withInternet routing and is not generally responsible for dataintegrity.

Cool. What about Datagram sockets? Whyare they called connectionless? What is the deal, here, anyway? Whyare they unreliable? Well, here are some facts: if you send a datagram,it may arrive. It may arrive out of order. If it arrives, the datawithin the packet will be error-free.

Datagram sockets also use IP for routing, but they don't use TCP;they use the "User Datagram Protocol", or "UDP" (seeRFC 768.)

Why are they connectionless? Well, basically, it's because you don'thave to maintain an open connection as you do with stream sockets. Youjust build a packet, slap an IP header on it with destinationinformation, and send it out. No connection needed. They are generallyused either when a TCP stack is unavailable or when a few droppedpackets here and there don't mean the end of the Universe. Sampleapplications: tftp (trivial file transfer protocol, a littlebrother to FTP), dhcpcd (a DHCP client), multiplayer games,streaming audio, video conferencing, etc.

"Wait a minute! tftp and dhcpcd are used totransfer binary applications from one host to another! Data can't belost if you expect the application to work when it arrives! What kindof dark magic is this?"

Well, my human friend, tftp and similar programs havetheir own protocol on top of UDP. For example, the tftp protocol saysthat for each packet that gets sent, the recipient has to send back apacket that says, "I got it!" (an "ACK" packet.) If the sender of theoriginal packet gets no reply in, say, five seconds, he'll re-transmitthe packet until he finally gets an ACK. This acknowledgment procedureis very important when implementing reliable SOCK_DGRAMapplications.

For unreliable applications like games, audio, or video, you justignore the dropped packets, or perhaps try to cleverly compensate forthem. (Quake players will know the manifestation this effect by thetechnical term: accursed lag. The word "accursed", in thiscase, represents any extremely profane utterance.)

Why would you use an unreliable underlying protocol? Two reasons:speed and speed. It's way faster to fire-and-forget than it is to keeptrack of what has arrived safely and make sure it's in order and allthat. If you're sending chat messages, TCP is great; if you're sending40 positional updates per second of the players in the world, maybe itdoesn't matter so much if one or two get dropped, and UDP is a goodchoice.

2.2. Low level Nonsense and Network Theory

Since I just mentioned layering of protocols, it's time to talkabout how networks really work, and to show some examples of how SOCK_DGRAM packets are built.Practically, you can probably skip this section. It's good background,however.

Data Encapsulation.

Hey, kids, it's time to learn about DataEncapsulation! This is very very important. It's soimportant that you might just learn about it if you take the networkscourse here at Chico State ;-). Basically, it says this: a packetis born, the packet is wrapped ("encapsulated") in a header (and rarely a footer) by thefirst protocol (say, the TFTP protocol), then the wholething (TFTP header included) is encapsulated again by the next protocol(say, UDP), then again by the next (IP),then again by the final protocol on the hardware (physical) layer (say,Ethernet).

When another computer receives the packet, the hardware strips theEthernet header, the kernel strips the IP and UDP headers, the TFTPprogram strips the TFTP header, and it finally has the data.

Now I can finally talk about the infamous Layered Network Model (aka "ISO/OSI"). This NetworkModel describes a system of network functionality that has manyadvantages over other models. For instance, you can write socketsprograms that are exactly the same without caring how the data isphysically transmitted (serial, thin Ethernet, AUI, whatever) becauseprograms on lower levels deal with it for you. The actual networkhardware and topology is transparent to the socket programmer.

Without any further ado, I'll present the layers of the full-blownmodel. Remember this for network class exams:

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

The Physical Layer is the hardware (serial, Ethernet, etc.). TheApplication Layer is just about as far from the physical layer as youcan imagine—it's the place where users interact with thenetwork.

Now, this model is so general you could probably use it as anautomobile repair guide if you really wanted to. A layered model moreconsistent with Unix might be:

• Application Layer (telnet, ftp, etc.)
• Host-to-Host Transport Layer (TCP, UDP)
• Internet Layer (IP and routing)
• Network Access Layer (Ethernet, wi-fi, or whatever)

At this point in time, you can probably see how these layerscorrespond to the encapsulation of the original data.

See how much work there is in building a simple packet? Jeez!And you have to type in the packet headers yourself using"cat"! Just kidding. All you have to do for stream socketsis send() the data out. All you have todo for datagram sockets is encapsulate the packet in the method of yourchoosing and sendto() it out. Thekernel builds the Transport Layer and Internet Layer on for you and thehardware does the Network Access Layer. Ah, modern technology.

So ends our brief foray into network theory. Oh yes, I forgot totell you everything I wanted to say about routing: nothing! That'sright, I'm not going to talk about it at all. The router strips thepacket to the IP header, consults its routing table, blah blah blah. Check out the IP RFC if you really really care. Ifyou never learn about it, well, you'll live.

3. IP Addresses, structs, and Data Munging

Here's the part of the game where we get to talk code for achange.

But first, let's discuss more non-code! Yay! First I want to talkabout IP addresses and ports for just a tad so we havethat sorted out. Then we'll talk about how the sockets API stores andmanipulates IP addresses and other data.

3.1. IP Addresses, versions 4 and 6

In the good old days back when Ben Kenobi was still called Obi WanKenobi, there was a wonderful network routing system called The InternetProtocol Version 4, also called IPv4. It had addressesmade up of four bytes (A.K.A. four "octets"), and was commonly writtenin "dots and numbers" form, like so: 192.0.2.111.

You've probably seen it around.

In fact, as of this writing, virtually every site on the Internetuses IPv4.

Everyone, including Obi Wan, was happy. Things were great, untilsome naysayer by the name of Vint Cerf warned everyone that we wereabout to run out of IPv4 addresses!

(Besides warning everyone of the Coming IPv4 Apocalypse Of Doom AndGloom, Vint Cerf isalso well-known for being The Father Of The Internet. So I really am inno position to second-guess his judgment.)

Run out of addresses? How could this be? I mean, there are likebillions of IP addresses in a 32-bit IPv4 address. Do we really havebillions of computers out there?

Yes.

Also, in the beginning, when there were only a few computers andeveryone thought a billion was an impossibly large number, some bigorganizations were generously allocated millions of IP addresses fortheir own use. (Such as Xerox, MIT, Ford, HP, IBM, GE, AT&T, andsome little company called Apple, to name a few.)

In fact, if it weren't for several stopgap measures, we would haverun out a long time ago.

But now we're living in an era where we're talking about every humanhaving an IP address, every computer, every calculator, every phone,every parking meter, and (why not) every puppy dog, as well.

And so, IPv6 was born. Since Vint Cerf is probablyimmortal (even if his physical form should pass on, heaven forbid, he isprobably already existing as some kind of hyper-intelligent ELIZA program out in the depths of theInternet2), no one wants to have to hear him say again "I told you so"if we don't have enough addresses in the next version of the InternetProtocol.

What does this suggest to you?

That we need a lot more addresses. That we need not justtwice as many addresses, not a billion times as many, not a thousandtrillion times as many, but 79 MILLION BILLION TRILLION times asmany possible addresses! That'll show 'em!

You're saying, "Beej, is that true? I have every reason todisbelieve large numbers." Well, the difference between 32 bits and 128bits might not sound like a lot; it's only 96 more bits, right? Butremember, we're talking powers here: 32 bits represents some 4 billionnumbers (2³²), while 128 bits represents about 340trillion trillion trillion numbers (for real, 2¹²⁸).That's like a million IPv4 Internets for every single star in theUniverse.

Forget this dots-and-numbers look of IPv4, too; now we've got ahexadecimal representation, with each two-byte chunk separated by acolon, like this: 2001:0db8:c9d2:aee5:73e3:934a:a5ae:9551.

That's not all! Lots of times, you'll have an IP address with lotsof zeros in it, and you can compress them between two colons. And youcan leave off leading zeros for each byte pair. For instance, each ofthese pairs of addresses are equivalent:

2001:0db8:c9d2:0012:0000:0000:0000:00512001:db8:c9d2:12::512001:0db8:ab00:0000:0000:0000:0000:00002001:db8:ab00::0000:0000:0000:0000:0000:0000:0000:0001::1

The address ::1 is the loopback address. Italways means "this machine I'm running on now". In IPv4, the loopbackaddress is 127.0.0.1.

Finally, there's an IPv4-compatibility mode for IPv6 addresses thatyou might come across. If you want, for example, to represent the IPv4address 192.0.2.33 as an IPv6 address, you use the following notation:"::ffff:192.0.2.33".

We're talking serious fun.

In fact, it's such serious fun, that the Creators of IPv6 have quitecavalierly lopped off trillions and trillions of addresses for reserveduse, but we have so many, frankly, who's even counting anymore? Thereare plenty left over for every man, woman, child, puppy, and parkingmeter on every planet in the galaxy. And believe me, every planet inthe galaxy has parking meters. You know it's true.

3.1.1. Subnets

For organizational reasons, it's sometimes convenient to declare that"this first part of this IP address up through this bit is thenetwork portion of the IP address, and the remainder is thehost portion.

For instance, with IPv4, you might have 192.0.2.12, and we could saythat the first three bytes are the network and the last byte was thehost. Or, put another way, we're talking about host 12 onnetwork 192.0.2.0 (see how we zero out the byte that was thehost.)

And now for more outdated information! Ready? In the Ancient Times,there were "classes" of subnets, where the first one, two, or threebytes of the address was the network part. If you were lucky enough tohave one byte for the network and three for the host, you could have24 bits-worth of hosts on your network (24 million or so). That was a"Class A" network. On the opposite end was a "Class C", with threebytes of network, and one byte of host (256 hosts, minus a couple thatwere reserved.)

So as you can see, there were just a few Class As, a huge pile ofClass Cs, and some Class Bs in the middle.

The network portion of the IP address is described by somethingcalled the netmask, which you bitwise-AND with the IP addressto get the network number out of it. The netmask usually lookssomething like 255.255.255.0. (E.g. with that netmask, if yourIP is 192.0.2.12, then your network is 192.0.2.12 AND255.255.255.0 which gives 192.0.2.0.)

Unfortunately, it turned out that this wasn't fine-grained enough forthe eventual needs of the Internet; we were running out of Class Cnetworks quite quickly, and we were most definitely out of Class As, sodon't even bother to ask. To remedy this, The Powers That Be allowedfor the netmask to be an arbitrary number of bits, not just 8, 16, or24. So you might have a netmask of, say 255.255.255.252, whichis 30 bits of network, and 2 bits of host allowing for four hosts on thenetwork. (Note that the netmask is ALWAYS a bunch of 1-bitsfollowed by a bunch of 0-bits.)

But it's a bit unwieldy to use a big string of numbers like255.192.0.0 as a netmask. First of all, people don't have anintuitive idea of how many bits that is, and secondly, it's really notcompact. So the New Style came along, and it's much nicer. You justput a slash after the IP address, and then follow that by the number ofnetwork bits in decimal. Like this: 192.0.2.12/30.

Or, for IPv6, something like this: 2001:db8::/32 or2001:db8:5413:4028::9db9/64.

3.1.2. Port Numbers

If you'll kindly remember, I presented you earlier with the Layered Network Model which had the InternetLayer (IP) split off from the Host-to-Host Transport Layer (TCP andUDP). Get up to speed on that before the next paragraph.

Turns out that besides an IP address (used by the IP layer), thereis another address that is used by TCP (stream sockets) and,coincidentally, by UDP (datagram sockets). It is the portnumber. It's a 16-bit number that's like the local address forthe connection.

Think of the IP address as the street address of a hotel, and theport number as the room number. That's a decent analogy; maybe laterI'll come up with one involving the automobile industry.

Say you want to have a computer that handles incoming mail AND webservices—how do you differentiate between the two on a computerwith a single IP address?

Well, different services on the Internet have different well-knownport numbers. You can see them all in the BigIANA Port List or, if you're on a Unix box, in your/etc/services file. HTTP (the web) is port 80, telnet isport 23, SMTP is port 25, the game DOOMused port 666, etc. and so on. Ports under 1024 are often consideredspecial, and usually require special OS privileges to use.

And that's about it!

3.2. Byte Order

By Order of the Realm! There shall be twobyte orderings, hereafter to be known as Lame and Magnificent!

I joke, but one really is better than the other. :-)

There really is no easy way to say this, so I'll just blurt it out:your computer might have been storing bytes in reverse order behind yourback. I know! No one wanted to have to tell you.

The thing is, everyone in the Internet world has generally agreedthat if you want to represent the two-byte hex number, sayb34f, you'll store it in two sequential bytes b3followed by 4f. Makes sense, and, as Wilford Brimley would tell you, it's the RightThing To Do. This number, stored with the big end first, is calledBig-Endian.

Unfortunately, a few computers scattered here and there throughoutthe world, namely anything with an Intel or Intel-compatible processor,store the bytes reversed, so b34f would be stored in memory asthe sequential bytes 4f followed by b3. This storagemethod is called Little-Endian.

But wait, I'm not done with terminology yet! The more-saneBig-Endian is also called Network Byte Orderbecause that's the order us network types like.

Your computer stores numbers in Host Byte Order. If it'san Intel 80x86, Host Byte Order is Little-Endian. If it's a Motorola68k, Host Byte Order is Big-Endian. If it's a PowerPC, Host Byte Orderis... well, it depends!

A lot of times when you're building packets or filling out datastructures you'll need to make sure your two- and four-byte numbers arein Network Byte Order. But how can you do this if you don't know thenative Host Byte Order?

Good news! You just get to assume the Host Byte Order isn't right,and you always run the value through a function to set it to NetworkByte Order. The function will do the magic conversion if it has to, andthis way your code is portable to machines of differing endianness.

All righty. There are two types of numbers that you can convert:short (two bytes) and long (four bytes).These functions work for the unsigned variations as well.Say you want to convert a short from Host Byte Order toNetwork Byte Order. Start with "h" for "host", follow it with "to",then "n" for "network", and "s" for "short": h-to-n-s, orhtons() (read: "Host to Network Short").

It's almost too easy...

You can use every combination of "n", "h", "s", and "l" you want,not counting the really stupid ones. For example, there is NOT astolh() ("Short to Long Host") function—not at thisparty, anyway. But there are:

Basically, you'll want to convert the numbers to Network Byte Orderbefore they go out on the wire, and convert them to Host Byte Order asthey come in off the wire.

I don't know of a 64-bit variant, sorry. And if you want to dofloating point, check out the section on Serialization, far below.

Assume the numbers in this document are in Host Byte Order unless Isay otherwise.

3.3. structs

Well, we're finally here. It's time to talk about programming.In this section, I'll cover various data types used by the socketsinterface, since some of them are a real bear to figure out.

First the easy one: a socket descriptor.A socket descriptor is the following type:

int

Just a regular int.

Things get weird from here, so just read through and bear withme.

My First Struct^TM—structaddrinfo. This structure is a more recent invention, and is usedto prep the socket address structures for subsequent use. It's alsoused in host name lookups, and service name lookups. That'll make moresense later when we get to actual usage, but just know for now that it'sone of the first things you'll call when making a connection.

struct addrinfo {    int              ai_flags;     // AI_PASSIVE, AI_CANONNAME, etc.    int              ai_family;    // AF_INET, AF_INET6, AF_UNSPEC    int              ai_socktype;  // SOCK_STREAM, SOCK_DGRAM    int              ai_protocol;  // use 0 for "any"    size_t           ai_addrlen;   // size of ai_addr in bytes    struct sockaddr *ai_addr;      // struct sockaddr_in or _in6    char            *ai_canonname; // full canonical hostname    struct addrinfo *ai_next;      // linked list, next node};

You'll load this struct up a bit, and then call getaddrinfo(). It'll return a pointerto a new linked list of these structures filled out with all the goodiesyou need.

You can force it to use IPv4 or IPv6 in the ai_familyfield, or leave it as AF_UNSPEC to use whatever. This iscool because your code can be IP version-agnostic.

Note that this is a linked list: ai_next points at thenext element—there could be several results for you to choosefrom. I'd use the first result that worked, but you might havedifferent business needs; I don't know everything, man!

You'll see that the ai_addr field in the structaddrinfo is a pointer to a struct sockaddr. This is where we start gettinginto the nitty-gritty details of what's inside an IP addressstructure.

You might not usually need to write to these structures; oftentimes,a call to getaddrinfo() to fill out your structaddrinfo for you is all you'll need. You will,however, have to peer inside these structs to get thevalues out, so I'm presenting them here.

(Also, all the code written before struct addrinfo wasinvented packed all this stuff by hand, so you'll see a lot of IPv4 codeout in the wild that does exactly that. You know, in old versions ofthis guide and so on.)

Some structs are IPv4, some are IPv6, and some are both.I'll make notes of which are what.

Anyway, the struct sockaddr holds socket addressinformation for many types of sockets.

struct sockaddr {    unsigned short    sa_family;    // address family, AF_xxx    char              sa_data[14];  // 14 bytes of protocol address}; 

sa_family can be a variety of things, but it'll be AF_INET (IPv4) or AF_INET6 (IPv6) for everything we do inthis document. sa_data contains a destination addressand port number for the socket. This is rather unwieldy since you don'twant to tediously pack the address in the sa_data byhand.

To deal with struct sockaddr, programmers created aparallel structure: structsockaddr_in ("in" for "Internet") to be used with IPv4.

And this is the important bit: a pointer to a structsockaddr_in can be cast to a pointer to a structsockaddr and vice-versa. So even though connect()wants a struct sockaddr*, you can still use a structsockaddr_in and cast it at the last minute!

// (IPv4 only--see struct sockaddr_in6 for IPv6)struct sockaddr_in {    short int          sin_family;  // Address family, AF_INET    unsigned short int sin_port;    // Port number    struct in_addr     sin_addr;    // Internet address    unsigned char      sin_zero[8]; // Same size as struct sockaddr};

This structure makes it easy to reference elements of the socketaddress. Note that sin_zero (which is included to padthe structure to the length of a struct sockaddr) should beset to all zeros with the function memset(). Also, noticethat sin_family corresponds to sa_familyin a struct sockaddr and should be set to"AF_INET". Finally, the sin_port must bein Network Byte Order (by using htons()!)

Let's dig deeper! You see the sin_addr field is astruct in_addr. What is that thing? Well, not to beoverly dramatic, but it's one of the scariest unions of all time:

// (IPv4 only--see struct in6_addr for IPv6)// Internet address (a structure for historical reasons)struct in_addr {    uint32_t s_addr; // that's a 32-bit int (4 bytes)};

Whoa! Well, it used to be a union, but now those daysseem to be gone. Good riddance. So if you have declaredina to be of type struct sockaddr_in, thenina.sin_addr.s_addr references the 4-byte IP address (inNetwork Byte Order). Note that even if your system still uses theGod-awful union for struct in_addr, you can still referencethe 4-byte IP address in exactly the same way as I did above (this dueto #defines.)

What about IPv6? Similar structs existfor it, as well:

// (IPv6 only--see struct sockaddr_in and struct in_addr for IPv4)struct sockaddr_in6 {    u_int16_t       sin6_family;   // address family, AF_INET6    u_int16_t       sin6_port;     // port number, Network Byte Order    u_int32_t       sin6_flowinfo; // IPv6 flow information    struct in6_addr sin6_addr;     // IPv6 address    u_int32_t       sin6_scope_id; // Scope ID};struct in6_addr {    unsigned char   s6_addr[16];   // IPv6 address};

Note that IPv6 has an IPv6 address and a port number, just like IPv4has an IPv4 address and a port number.

Also note that I'm not going to talk about the IPv6 flow informationor Scope ID fields for the moment... this is just a starter guide.:-)

Last but not least, here is another simple structure, structsockaddr_storage that is designedto be large enough to hold both IPv4 and IPv6 structures. (See, forsome calls, sometimes you don't know in advance if it's going to fillout your struct sockaddr with an IPv4 or IPv6 address. Soyou pass in this parallel structure, very similar to structsockaddr except larger, and then cast it to the type youneed:

struct sockaddr_storage {    sa_family_t  ss_family;     // address family    // all this is padding, implementation specific, ignore it:    char      __ss_pad1[_SS_PAD1SIZE];    int64_t   __ss_align;    char      __ss_pad2[_SS_PAD2SIZE];};

What's important is that you can see the address family in thess_family field—check this to see if it'sAF_INET or AF_INET6 (for IPv4 orIPv6). Then you can cast it to a struct sockaddr_in orstruct sockaddr_in6 if you wanna.

3.4. IP Addresses, Part Deux

Fortunately for you, there are a bunch of functions that allow you tomanipulate IP addresses. No need to figure them out byhand and stuff them in a long with the<< operator.

First, let's say you have a struct sockaddr_in ina, andyou have an IP address "10.12.110.57" or"2001:db8:63b3:1::3490" that you want to store into it. Thefunction you want to use, inet_pton(), converts an IP address in numbers-and-dotsnotation into either a struct in_addr or a structin6_addr depending on whether you specify AF_INETor AF_INET6. ("pton" stands for "presentation tonetwork"—you can call it "printable to network" if that's easierto remember.) The conversion can be made as follows:

struct sockaddr_in sa; // IPv4struct sockaddr_in6 sa6; // IPv6inet_pton(AF_INET, "192.0.2.1", &(sa.sin_addr)); // IPv4inet_pton(AF_INET6, "2001:db8:63b3:1::3490", &(sa6.sin6_addr)); // IPv6

(Quick note: the old way of doing things used a function called inet_addr() or another function calledinet_aton(); these are now obsoleteand don't work with IPv6.)

Now, the above code snippet isn't very robust because there is noerror checking. See, inet_pton() returns-1 on error, or 0 if the address is messed up. So checkto make sure the result is greater than 0 before using!

All right, now you can convert string IP addresses to their binaryrepresentations. What about the other way around? What if you have astruct in_addr and you want to print it in numbers-and-dotsnotation? (Or a struct in6_addr that you want in, uh,"hex-and-colons" notation.) In this case, you'll want to use thefunction inet_ntop() ("ntop" means"network to presentation"—you can call it "network to printable"if that's easier to remember), like this:

// IPv4:char ip4[INET_ADDRSTRLEN];  // space to hold the IPv4 stringstruct sockaddr_in sa;      // pretend this is loaded with somethinginet_ntop(AF_INET, &(sa.sin_addr), ip4, INET_ADDRSTRLEN);printf("The IPv4 address is: %s\n", ip4);// IPv6:char ip6[INET6_ADDRSTRLEN]; // space to hold the IPv6 stringstruct sockaddr_in6 sa6;    // pretend this is loaded with somethinginet_ntop(AF_INET6, &(sa6.sin6_addr), ip6, INET6_ADDRSTRLEN);printf("The address is: %s\n", ip6);

When you call it, you'll pass the address type (IPv4 or IPv6), theaddress, a pointer to a string to hold the result, and the maximumlength of that string. (Two macros conveniently hold the size of thestring you'll need to hold the largest IPv4 or IPv6 address:INET_ADDRSTRLEN and INET6_ADDRSTRLEN.)

(Another quick note to mention once again the old way of doingthings: the historical function to do this conversion was called inet_ntoa(). It's also obsolete andwon't work with IPv6.)

Lastly, these functions only work with numeric IPaddresses—they won't do any nameserver DNS lookup on a hostname,like "www.example.com". You will use getaddrinfo() to dothat, as you'll see later on.

3.4.1. Private (Or Disconnected) Networks

Lots of places have a firewall that hides the network from the rest of theworld for their own protection. And often times, the firewalltranslates "internal" IP addresses to "external" (that everyone else inthe world knows) IP addresses using a process called NetworkAddress Translation, or NAT.

Are you getting nervous yet? "Where's he going with all this weirdstuff?"

Well, relax and buy yourself a non-alcoholic (or alcoholic) drink,because as a beginner, you don't even have to worry about NAT, sinceit's done for you transparently. But I wanted to talk about the networkbehind the firewall in case you started getting confused by the networknumbers you were seeing.

For instance, I have a firewall at home. I have two static IPv4addresses allocated to me by the DSL company, and yet I have sevencomputers on the network. How is this possible? Two computers can'tshare the same IP address, or else the data wouldn't know which one togo to!

The answer is: they don't share the same IP addresses. They are on aprivate network with 24 million IP addresses allocated to it. They areall just for me. Well, all for me as far as anyone else is concerned.Here's what's happening:

If I log into a remote computer, it tells me I'm logged in from192.0.2.33 which is the public IP address my ISP has provided to me.But if I ask my local computer what it's IP address is, it says10.0.0.5. Who is translating the IP address from one to the other?That's right, the firewall! It's doing NAT!

10.x.x.x is one of a few reservednetworks that are only to be used either on fully disconnected networks,or on networks that are behind firewalls. The details of which privatenetwork numbers are available for you to use are outlined in RFC 1918,but some common ones you'll see are 10.x.x.x and 192.168.x.x, where xis 0-255, generally. Less common is172.y.x.x, where y goesbetween 16 and 31.

Networks behind a NATing firewall don't need to be on oneof these reserved networks, but they commonly are.

(Fun fact! My external IP address isn't really 192.0.2.33. The192.0.2.x network is reserved for make-believe "real" IPaddresses to be used in documentation, just like this guide!Wowzers!)

IPv6 has private networks, too, in a sense. They'llstart with fdxx: (or maybe in the futurefcXX:), as per RFC 4193. NAT and IPv6 don'tgenerally mix, however (unless you're doing the IPv6 to IPv4 gatewaything which is beyond the scope of this document)—in theoryyou'll have so many addresses at your disposal that you won't need touse NAT any longer. But if you want to allocate addresses for yourselfon a network that won't route outside, this is how to do it.

4. Jumping from IPv4 to IPv6

But I just want to know what to change in my code toget it going with IPv6! Tell me now!

Ok! Ok!

Almost everything in here is something I've gone over, above, butit's the short version for the impatient. (Of course, there is morethan this, but this is what applies to the guide.)

1. First of all, try to use getaddrinfo() to get all thestruct sockaddr info, instead of packing the structures byhand. This will keep you IP version-agnostic, and will eliminate manyof the subsequent steps.
2. Any place that you find you're hard-coding anything related to theIP version, try to wrap up in a helper function.
3. Change AF_INET to AF_INET6.
4. Change PF_INET to PF_INET6.

5. Change INADDR_ANY assignments toin6addr_any assignments, which are slightlydifferent:

   struct sockaddr_in sa;struct sockaddr_in6 sa6;sa.sin_addr.s_addr = INADDR_ANY;  // use my IPv4 addresssa6.sin6_addr = in6addr_any; // use my IPv6 address

   Also, the value IN6ADDR_ANY_INIT can be used as aninitializer when the struct in6_addr is declared, likeso:

   struct in6_addr ia6 = IN6ADDR_ANY_INIT;

6. Instead of struct sockaddr_in use structsockaddr_in6, being sure to add "6" to the fields as appropriate(see structs, above). There isno sin6_zero field.
7. Instead of struct in_addr use structin6_addr, being sure to add "6" to the fields as appropriate (seestructs, above).
8. Instead of inet_aton() or inet_addr(), useinet_pton().
9. Instead of inet_ntoa(), useinet_ntop().
10. Instead of gethostbyname(), use the superiorgetaddrinfo().
11. Instead of gethostbyaddr(), use the superior getnameinfo() (althoughgethostbyaddr() can still work with IPv6).
12. INADDR_BROADCAST no longer works. Use IPv6 multicastinstead.

Et voila!

5. System Calls or Bust

This is the section where we get into the system calls (and otherlibrary calls) that allow you to access the network functionality of aUnix box, or any box that supports the sockets API for that matter (BSD,Windows, Linux, Mac, what-have-you.) When you call one of thesefunctions, the kernel takes over and does all the work for youautomagically.

The place most people get stuck around here is what order to callthese things in. In that, the man pages are no use,as you've probably discovered. Well, to help with that dreadfulsituation, I've tried to lay out the system calls in the followingsections in exactly (approximately) the same orderthat you'll need to call them in your programs.

That, coupled with a few pieces of sample code here and there,some milk and cookies (which I fear you will have to supply yourself),and some raw guts and courage, and you'll be beaming data around theInternet like the Son of Jon Postel!

(Please note that for brevity, many code snippets below do notinclude necessary error checking. And they very commonly assume thatthe result from calls to getaddrinfo() succeed and return avalid entry in the linked list. Both of these situations are properlyaddressed in the stand-alone programs, though, so use those as amodel.)

5.1. getaddrinfo()—Prepare to launch!

This is a real workhorse of a function witha lot of options, but usage is actually pretty simple. It helps set upthe structs you need later on.

A tiny bit of history: it used to be that you would use a functioncalled gethostbyname() to do DNS lookups. Then you'dload that information by hand into a struct sockaddr_in,and use that in your calls.

This is no longer necessary, thankfully. (Nor is it desirable, ifyou want to write code that works for both IPv4 and IPv6!) In thesemodern times, you now have the function getaddrinfo() thatdoes all kinds of good stuff for you, including DNS and service namelookups, and fills out the structs you need, besides!

Let's take a look!

#include #include #include int getaddrinfo(const char *node,     // e.g. "www.example.com" or IP                const char *service,  // e.g. "http" or port number                const struct addrinfo *hints,                struct addrinfo **res);

You give this function three input parameters, and it gives you apointer to a linked-list, res, of results.

The node parameter is the host name to connect to, oran IP address.

Next is the parameter service, which can be a portnumber, like "80", or the name of a particular service (found in The IANA Port List or the/etc/services file on your Unix machine) like "http" or"ftp" or "telnet" or "smtp" or whatever.

Finally, the hints parameter points to a structaddrinfo that you've already filled out with relevantinformation.

Here's a sample call if you're a server who wants to listen on yourhost's IP address, port 3490. Note that this doesn't actually do anylistening or network setup; it merely sets up structures we'll uselater:

int status;struct addrinfo hints;struct addrinfo *servinfo;  // will point to the resultsmemset(&hints, 0, sizeof hints); // make sure the struct is emptyhints.ai_family = AF_UNSPEC;     // don't care IPv4 or IPv6hints.ai_socktype = SOCK_STREAM; // TCP stream socketshints.ai_flags = AI_PASSIVE;     // fill in my IP for meif ((status = getaddrinfo(NULL, "3490", &hints, &servinfo)) != 0) {    fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));    exit(1);}// servinfo now points to a linked list of 1 or more struct addrinfos// ... do everything until you don't need servinfo anymore ....freeaddrinfo(servinfo); // free the linked-list

Notice that I set the ai_family toAF_UNSPEC, thereby saying that I don't care if we useIPv4 or IPv6. You can set it to AF_INET orAF_INET6 if you want one or the other specifically.

Also, you'll see the AI_PASSIVE flag in there; thistells getaddrinfo() to assign the address of my local hostto the socket structures. This is nice because then you don't have tohardcode it. (Or you can put a specific address in as the firstparameter to getaddrinfo() where I currently haveNULL, up there.)

Then we make the call. If there's an error(getaddrinfo() returns non-zero), we can print it out usingthe function gai_strerror(), as you see. If everythingworks properly, though, servinfo will point to a linked listof struct addrinfos, each of which contains a structsockaddr of some kind that we can use later! Nifty!

Finally, when we're eventually all done with the linked list thatgetaddrinfo() so graciously allocated for us, we can (andshould) free it all up with a call to freeaddrinfo().

Here's a sample call if you're a client who wants to connect to aparticular server, say "www.example.net" port 3490. Again, this doesn'tactually connect, but it sets up the structures we'll use later:

int status;struct addrinfo hints;struct addrinfo *servinfo;  // will point to the resultsmemset(&hints, 0, sizeof hints); // make sure the struct is emptyhints.ai_family = AF_UNSPEC;     // don't care IPv4 or IPv6hints.ai_socktype = SOCK_STREAM; // TCP stream sockets// get ready to connectstatus = getaddrinfo("www.example.net", "3490", &hints, &servinfo);// servinfo now points to a linked list of 1 or more struct addrinfos// etc.

I keep saying that servinfo is a linked list with allkinds of address information. Let's write a quick demo program to showoff this information. This shortprogram will print the IP addresses for whatever host youspecify on the command line:

/*** showip.c -- show IP addresses for a host given on the command line*/#include #include #include #include #include #include int main(int argc, char *argv[]){    struct addrinfo hints, *res, *p;    int status;    char ipstr[INET6_ADDRSTRLEN];    if (argc != 2) {        fprintf(stderr,"usage: showip hostname\n");        return 1;    }    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version    hints.ai_socktype = SOCK_STREAM;    if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));        return 2;    }    printf("IP addresses for %s:\n\n", argv[1]);    for(p = res;p != NULL; p = p->ai_next) {        void *addr;        char *ipver;        // get the pointer to the address itself,        // different fields in IPv4 and IPv6:        if (p->ai_family == AF_INET) { // IPv4            struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;            addr = &(ipv4->sin_addr);            ipver = "IPv4";        } else { // IPv6            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;            addr = &(ipv6->sin6_addr);            ipver = "IPv6";        }        // convert the IP to a string and print it:        inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);        printf("  %s: %s\n", ipver, ipstr);    }    freeaddrinfo(res); // free the linked list    return 0;}

As you see, the code calls getaddrinfo() on whatever youpass on the command line, that fills out the linked list pointed to byres, and then we can iterate over the list and print stuffout or do whatever.

(There's a little bit of ugliness there where we have to dig into thedifferent types of struct sockaddrs depending on the IPversion. Sorry about that! I'm not sure of a better way aroundit.)

Sample run! Everyone loves screenshots:

$ showip www.example.netIP addresses for www.example.net:  IPv4: 192.0.2.88$ showip ipv6.example.comIP addresses for ipv6.example.com:  IPv4: 192.0.2.101  IPv6: 2001:db8:8c00:22::171

Now that we have that under control, we'll use the results we getfrom getaddrinfo() to pass to other socket functions and,at long last, get our network connection established! Keep reading!

5.2. socket()—Get the File Descriptor!

I guess I can put it off no longer—I have to talk about thesocket() system call. Here's thebreakdown:

#include #include int socket(int domain, int type, int protocol); 

But what are these arguments? They allow you to say what kind ofsocket you want (IPv4 or IPv6, stream or datagram, and TCP or UDP).

It used to be people would hardcode these values, and you canabsolutely still do that. (domain isPF_INET or PF_INET6, typeis SOCK_STREAM or SOCK_DGRAM, andprotocol can be set to 0 to choose theproper protocol for the given type. Or you can callgetprotobyname() to look up the protocol you want, "tcp" or"udp".)

(This PF_INET thing is a close relative of the AF_INET that you can use when initializingthe sin_family field in your struct sockaddr_in.In fact, they're so closely related that they actually have the samevalue, and many programmers will call socket() and passAF_INET as the first argument instead ofPF_INET. Now, get some milk and cookies, because it'stimes for a story. Once upon a time, a long time ago, it was thoughtthat maybe a address family (what the "AF" in "AF_INET"stands for) might support several protocols that were referred to bytheir protocol family (what the "PF" in "PF_INET" standsfor). That didn't happen. And they all lived happily ever after, TheEnd. So the most correct thing to do is to use AF_INETin your struct sockaddr_in and PF_INET inyour call to socket().)

Anyway, enough of that. What you really want to do is use the valuesfrom the results of the call to getaddrinfo(), and feedthem into socket() directly like this:

int s;struct addrinfo hints, *res;// do the lookup// [pretend we already filled out the "hints" struct]getaddrinfo("www.example.com", "http", &hints, &res);// [again, you should do error-checking on getaddrinfo(), and walk// the "res" linked list looking for valid entries instead of just// assuming the first one is good (like many of these examples do.)// See the section on client/server for real examples.]s = socket(res->ai_family, res->ai_socktype, res->ai_protocol);

socket() simply returns to you a socketdescriptor that you can use in later system calls, or-1 on error. The global variable errno is setto the error's value (see the errno man page for more details, and aquick note on using errno in multithreaded programs.)

Fine, fine, fine, but what good is this socket? The answer isthat it's really no good by itself, and you need to read on and makemore system calls for it to make any sense.

5.3. bind()—What port am I on?

Once you have a socket, you might have to associatethat socket with a port on your local machine. (Thisis commonly done if you're going to listen() for incoming connections on aspecific port—multiplayer network games do this when they tellyou to "connect to 192.168.5.10 port 3490".) The port number is used bythe kernel to match an incoming packet to a certain process's socketdescriptor. If you're going to only be doing a connect() (because you're the client, notthe server), this is probably be unnecessary. Read it anyway, just forkicks.

Here is the synopsis for the bind() systemcall:

#include #include int bind(int sockfd, struct sockaddr *my_addr, int addrlen);

sockfd is the socket file descriptor returned bysocket(). my_addr is a pointer to astruct sockaddr that contains information about youraddress, namely, port and IP address.addrlen is the length in bytes of that address.

Whew. That's a bit to absorb in one chunk. Let's have anexample that binds the socket to the host the program is running on,port 3490:

struct addrinfo hints, *res;int sockfd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_STREAM;hints.ai_flags = AI_PASSIVE;     // fill in my IP for megetaddrinfo(NULL, "3490", &hints, &res);// make a socket:sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);// bind it to the port we passed in to getaddrinfo():bind(sockfd, res->ai_addr, res->ai_addrlen);

By using the AI_PASSIVE flag, I'm telling the programto bind to the IP of the host it's running on. If you want to bind to aspecific local IP address, drop the AI_PASSIVE and put anIP address in for the first argument to getaddrinfo().

bind() also returns -1on error and sets errno to the error'svalue.

Lots of old code manually packs the struct sockaddr_inbefore calling bind(). Obviously this is IPv4-specific,but there's really nothing stopping you from doing the same thing withIPv6, except that using getaddrinfo() is going to beeasier, generally. Anyway, the old code looks something like this:

// !!! THIS IS THE OLD WAY !!!int sockfd;struct sockaddr_in my_addr;sockfd = socket(PF_INET, SOCK_STREAM, 0);my_addr.sin_family = AF_INET;my_addr.sin_port = htons(MYPORT);     // short, network byte ordermy_addr.sin_addr.s_addr = inet_addr("10.12.110.57");memset(my_addr.sin_zero, '\0', sizeof my_addr.sin_zero);bind(sockfd, (struct sockaddr *)&my_addr, sizeof my_addr);

In the above code, you could also assign INADDR_ANY tothe s_addr field if you wanted to bind to your local IPaddress (like the AI_PASSIVE flag, above.) The IPv6version of INADDR_ANY is a global variablein6addr_any that is assigned into the sin6_addrfield of your struct sockaddr_in6. (There is also a macroIN6ADDR_ANY_INIT that you can use in a variableinitializer.)

Another thing to watch out for when calling bind():don't go underboard with your port numbers. All portsbelow 1024 are RESERVED (unless you're the superuser)! You can have anyport number above that, right up to 65535 (provided they aren't alreadybeing used by another program.)

Sometimes, you might notice, you try to rerun a server andbind() fails, claiming "Addressalready in use." What does that mean? Well, a little bit of a socketthat was connected is still hanging around in the kernel, and it'shogging the port. You can either wait for it to clear (a minute or so),or add code to your program allowing it to reuse the port, likethis:

int yes=1;//char yes='1'; // Solaris people use this// lose the pesky "Address already in use" error messageif (setsockopt(listener,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) {    perror("setsockopt");    exit(1);} 

One small extra final note aboutbind(): there are times when you won't absolutely have tocall it. If you are connect()ing to aremote machine and you don't care what your local port is (as is thecase with telnet where you only care about the remote port),you can simply call connect(), it'll check to see if thesocket is unbound, and will bind() it to an unused localport if necessary.

5.4. connect()—Hey, you!

Let's just pretend for a few minutes that you'rea telnet application. Your user commands you (just like in the movieTRON) to get a socket file descriptor. Youcomply and call socket(). Next, the user tells you toconnect to "10.12.110.57" on port "23" (the standardtelnet port.) Yow! What do you do now?

Lucky for you, program, you're now perusing the section onconnect()—how to connect to a remote host. So readfuriously onward! No time to lose!

The connect() call is as follows:

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

sockfd is our friendly neighborhood socketfile descriptor, as returned by the socket() call,serv_addr is a struct sockaddrcontaining the destination port and IP address, andaddrlen is the length in bytes of the server addressstructure.

All of this information can be gleaned from the results of thegetaddrinfo() call, which rocks.

Is this starting to make more sense? I can't hear you from here, soI'll just have to hope that it is. Let's have an example where we makea socket connection to "www.example.com", port 3490:

struct addrinfo hints, *res;int sockfd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;hints.ai_socktype = SOCK_STREAM;getaddrinfo("www.example.com", "3490", &hints, &res);// make a socket:sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);// connect!connect(sockfd, res->ai_addr, res->ai_addrlen);

Again, old-school programs filled out their own structsockaddr_ins to pass to connect(). You can do thatif you want to. See the similar note in the bind() section, above.

Be sure to check the return value fromconnect()—it'll return -1 on errorand set the variable errno.

Also, notice that we didn't callbind(). Basically, we don't care about our local portnumber; we only care where we're going (the remote port). The kernelwill choose a local port for us, and the site we connect to willautomatically get this information from us. No worries.

5.5. listen()—Will somebody please callme?

Ok, time for a change of pace. What if you don'twant to connect to a remote host. Say, just for kicks, that you want towait for incoming connections and handle them in some way. The processis two step: first you listen(), then youaccept() (see below.)

The listen call is fairly simple, but requires a bit ofexplanation:

int listen(int sockfd, int backlog); 

sockfd is the usual socket file descriptorfrom the socket() system call.backlog is the number ofconnections allowed on the incoming queue. What does that mean? Well,incoming connections are going to wait in this queue until youaccept() them (see below) and this is the limit on how manycan queue up. Most systems silently limit this number to about 20; youcan probably get away with setting it to 5 or10.

Again, as per usual, listen() returns-1 and sets errno onerror.

Well, as you can probably imagine, we need to callbind() before we call listen() so that theserver is running on a specific port. (You have to be able to tell yourbuddies which port to connect to!) So if you're going to be listeningfor incoming connections, the sequence of system calls you'll makeis:

getaddrinfo();socket();bind();listen();/* accept() goes here */ 

I'll just leave that in the place of sample code, since it'sfairly self-explanatory. (The code in the accept()section, below, is more complete.) The really tricky part of this wholesha-bang is the call to accept().

5.6. accept()—"Thank you for calling port3490."

Get ready—the accept() callis kinda weird! What's going to happen is this: someone far far awaywill try to connect() to your machine on a port that youare listen()ing on. Their connection will be queued upwaiting to be accept()ed. You call accept()and you tell it to get the pending connection. It'll return to you abrand new socket file descriptor to use for this singleconnection! That's right, suddenly you have two socket filedescriptors for the price of one! The original one is stilllistening for more new connections, and the newly created one is finallyready to send() and recv(). We're there!

The call is as follows:

#include #include int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen); 

sockfd is thelisten()ing socket descriptor. Easy enough.addr will usually be a pointer to a localstruct sockaddr_storage. This is where the informationabout the incoming connection will go (and with it you can determinewhich host is calling you from which port). addrlen is alocal integer variable that should be set to sizeof(structsockaddr_storage) before its address is passed toaccept(). accept() will not put more thanthat many bytes into addr. If it puts fewer in, it'llchange the value of addrlen to reflect that.

Guess what? accept() returns -1 and setserrno if an error occurs. Betcha didn't figure that.

Like before, this is a bunch to absorb in one chunk, so here's asample code fragment for your perusal:

#include #include #include #include #define MYPORT "3490"  // the port users will be connecting to#define BACKLOG 10     // how many pending connections queue will holdint main(void){    struct sockaddr_storage their_addr;    socklen_t addr_size;    struct addrinfo hints, *res;    int sockfd, new_fd;    // !! don't forget your error checking for these calls !!    // first, load up address structs with getaddrinfo():    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whichever    hints.ai_socktype = SOCK_STREAM;    hints.ai_flags = AI_PASSIVE;     // fill in my IP for me    getaddrinfo(NULL, MYPORT, &hints, &res);    // make a socket, bind it, and listen on it:    sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);    bind(sockfd, res->ai_addr, res->ai_addrlen);    listen(sockfd, BACKLOG);    // now accept an incoming connection:    addr_size = sizeof their_addr;    new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size);    // ready to communicate on socket descriptor new_fd!    .    .    .

Again, note that we will use the socket descriptornew_fd for all send() andrecv() calls. If you're only getting one single connectionever, you can close() the listening sockfdin order to prevent more incoming connections on the same port, if youso desire.

5.7. send() and recv()—Talk to me,baby!

These two functions are for communicating over stream sockets orconnected datagram sockets. If you want to use regular unconnecteddatagram sockets, you'll need to see the section on sendto() andrecvfrom(), below.

The send() call:

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

sockfd is the socket descriptor you want tosend data to (whether it's the one returned bysocket() or the one you got withaccept().) msg is a pointerto the data you want to send, and len is thelength of that data in bytes. Just set flags to0. (See the send() man pagefor more information concerning flags.)

Some sample code might be:

char *msg = "Beej was here!";int len, bytes_sent;...len = strlen(msg);bytes_sent = send(sockfd, msg, len, 0);... 

send() returns the number of bytes actuallysent out—this might be less than the number you told it tosend! See, sometimes you tell it to send a whole gob of data andit just can't handle it. It'll fire off as much of the data as it can,and trust you to send the rest later. Remember, if the value returnedby send() doesn't match the value in len,it's up to you to send the rest of the string. The good news is this:if the packet is small (less than 1K or so) it will probablymanage to send the whole thing all in one go. Again, -1is returned on error, and errno is set to the errornumber.

The recv() call is similar in manyrespects:

int recv(int sockfd, void *buf, int len, int flags);

sockfd is the socket descriptor to readfrom, buf is the buffer to read the informationinto, len is the maximum length of the buffer,and flags can again be set to0. (See the recv() man pagefor flag information.)

recv() returns the number of bytes actuallyread into the buffer, or -1 on error (witherrno set, accordingly.)

Wait! recv() can return0. This can mean only one thing: the remote sidehas closed the connection on you! A return value of0 is recv()'s way of lettingyou know this has occurred.

There, that was easy, wasn't it? You can now pass data back andforth on stream sockets! Whee! You're a Unix NetworkProgrammer!

5.8. sendto() andrecvfrom()—Talk to me, DGRAM-style

"This is all fine and dandy," I hear yousaying, "but where does this leave me with unconnected datagramsockets?" No problemo, amigo. We have just the thing.

Since datagram sockets aren't connected to a remote host, guess whichpiece of information we need to give before we send a packet? That'sright! The destination address! Here's the scoop:

int sendto(int sockfd, const void *msg, int len, unsigned int flags,           const struct sockaddr *to, socklen_t tolen); 

As you can see, this call is basically the same as the call tosend() with the addition of two other pieces ofinformation. to is a pointer to a structsockaddr (which will probably be another structsockaddr_in or struct sockaddr_in6 or structsockaddr_storage that you cast at the last minute) which containsthe destination IP address and port.tolen, an int deep-down, can simply be setto sizeof *to or sizeof(struct sockaddr_storage).

To get your hands on the destination address structure, you'llprobably either get it from getaddrinfo(), or fromrecvfrom(), below, or you'll fill it out by hand.

Just like with send(),sendto() returns the number of bytes actually sent(which, again, might be less than the number of bytes you told it tosend!), or -1 on error.

Equally similar are recv() andrecvfrom(). The synopsis ofrecvfrom() is:

int recvfrom(int sockfd, void *buf, int len, unsigned int flags,             struct sockaddr *from, int *fromlen); 

Again, this is just like recv() with theaddition of a couple fields. from is a pointer to alocal struct sockaddr_storagethat will be filled with the IP address and port of the originatingmachine. fromlen is a pointer to a localint that should be initialized to sizeof *from orsizeof(struct sockaddr_storage). When the function returns,fromlen will contain the length of the address actuallystored in from.

recvfrom() returns the number of bytesreceived, or -1 on error (witherrno set accordingly.)

So, here's a question: why do we use structsockaddr_storage as the socket type? Why not structsockaddr_in? Because, you see, we want to not tie ourselves downto IPv4 or IPv6. So we use the generic structsockaddr_storage which we know will be big enough for either.

(So... here's another question: why isn't structsockaddr itself big enough for any address? We even cast thegeneral-purpose struct sockaddr_storage to thegeneral-purpose struct sockaddr! Seems extraneous andredundant, huh. The answer is, it just isn't big enough, and I'd guessthat changing it at this point would be Problematic. So they made a newone.)

Remember, if you connect() a datagramsocket, you can then simply use send() andrecv() for all your transactions. The socket itself isstill a datagram socket and the packets still use UDP, but the socketinterface will automatically add the destination and source informationfor you.

5.9. close() andshutdown()—Get outta my face!

Whew! You've been send()ing andrecv()ing data all day long, and you've had it.You're ready to close the connection on your socket descriptor. This iseasy. You can just use the regular Unix file descriptorclose() function:

close(sockfd); 

This will prevent any more reads and writes to the socket. Anyoneattempting to read or write the socket on the remote end will receive anerror.

Just in case you want a little more control over how the socketcloses, you can use the shutdown()function. It allows you to cut off communication in a certaindirection, or both ways (just like close() does.)Synopsis:

int shutdown(int sockfd, int how); 

sockfd is the socket file descriptor youwant to shutdown, and how is one of thefollowing:

shutdown() returns 0 onsuccess, and -1 on error (witherrno set accordingly.)

If you deign to use shutdown() on unconnecteddatagram sockets, it will simply make the socket unavailable for furthersend() and recv() calls(remember that you can use these if you connect()your datagram socket.)

It's important to note that shutdown()doesn't actually close the file descriptor—it just changes itsusability. To free a socket descriptor, you need to useclose().

Nothing to it.

(Except to remember that if you're using Windowsand Winsock that you should call closesocket() instead ofclose().)

5.10. getpeername()—Who are you?

This function is so easy.

It's so easy, I almost didn't give it its own section. But hereit is anyway.

The function getpeername() will tell you who is at the other endof a connected stream socket. The synopsis:

#include int getpeername(int sockfd, struct sockaddr *addr, int *addrlen); 

sockfd is the descriptor of the connectedstream socket, addr is a pointer to astruct sockaddr (or a struct sockaddr_in) thatwill hold the information about the other side of the connection, andaddrlen is a pointer to an int, thatshould be initialized to sizeof *addr or sizeof(structsockaddr).

The function returns -1 on error and setserrno accordingly.

Once you have their address, you can use inet_ntop(), getnameinfo(), or gethostbyaddr() to print or get moreinformation. No, you can't get their login name. (Ok, ok. If theother computer is running an ident daemon, this is possible. This,however, is beyond the scope of this document. Check out RFC 1413 for more info.)

5.11. gethostname()—Who am I?

Even easier than getpeername()is the function gethostname(). It returns the name of thecomputer that your program is running on. The name can then be used bygethostbyname(), below, todetermine the IP address of your local machine.

What could be more fun? I could think of a few things, but theydon't pertain to socket programming. Anyway, here's thebreakdown:

#include int gethostname(char *hostname, size_t size); 

The arguments are simple: hostname is apointer to an array of chars that will contain the hostname upon thefunction's return, and size is the length inbytes of the hostname array.

The function returns 0 on successfulcompletion, and -1 on error, settingerrno as usual.

6. Client-Server Background

It's a client-server world, baby. Justabout everything on the network deals with client processes talking toserver processes and vice-versa. Take telnet, for instance.When you connect to a remote host on port 23 with telnet (the client), aprogram on that host (called telnetd, the server) springs tolife. It handles the incoming telnet connection, sets you up with alogin prompt, etc.

Client-Server Interaction.

The exchange of information between client and server issummarized in the above diagram.

Note that the client-server pair can speakSOCK_STREAM, SOCK_DGRAM, oranything else (as long as they're speaking the same thing.) Some goodexamples of client-server pairs aretelnet/telnetd,ftp/ftpd, orFirefox/Apache. Every time you useftp, there's a remote program,ftpd, that serves you.

Often, there will only be one server on a machine, and that serverwill handle multiple clients using fork(). The basic routine is: server willwait for a connection, accept() it, and fork()a child process to handle it. This is what our sample server does inthe next section.

6.1. A Simple Stream Server

All this server does is send the string"Hello, World!\n" out over a stream connection. All you needto do to test this server is run it in one window, and telnet to it fromanother with:

$ telnet remotehostname 3490

where remotehostname is the name of the machine you'rerunning it on.

The server code:

/*** server.c -- a stream socket server demo*/#include #include #include #include #include #include #include #include #include #include #include #include #define PORT "3490"  // the port users will be connecting to#define BACKLOG 10     // how many pending connections queue will holdvoid sigchld_handler(int s){    while(waitpid(-1, NULL, WNOHANG) > 0);}// get sockaddr, IPv4 or IPv6:void *get_in_addr(struct sockaddr *sa){    if (sa->sa_family == AF_INET) {        return &(((struct sockaddr_in*)sa)->sin_addr);    }    return &(((struct sockaddr_in6*)sa)->sin6_addr);}int main(void){    int sockfd, new_fd;  // listen on sock_fd, new connection on new_fd    struct addrinfo hints, *servinfo, *p;    struct sockaddr_storage their_addr; // connector's address information    socklen_t sin_size;    struct sigaction sa;    int yes=1;    char s[INET6_ADDRSTRLEN];    int rv;    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC;    hints.ai_socktype = SOCK_STREAM;    hints.ai_flags = AI_PASSIVE; // use my IP    if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));        return 1;    }    // loop through all the results and bind to the first we can    for(p = servinfo; p != NULL; p = p->ai_next) {        if ((sockfd = socket(p->ai_family, p->ai_socktype,                p->ai_protocol)) == -1) {            perror("server: socket");            continue;        }        if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,                sizeof(int)) == -1) {            perror("setsockopt");            exit(1);        }        if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {            close(sockfd);            perror("server: bind");            continue;        }        break;    }    if (p == NULL)  {        fprintf(stderr, "server: failed to bind\n");        return 2;    }    freeaddrinfo(servinfo); // all done with this structure    if (listen(sockfd, BACKLOG) == -1) {        perror("listen");        exit(1);    }    sa.sa_handler = sigchld_handler; // reap all dead processes    sigemptyset(&sa.sa_mask);    sa.sa_flags = SA_RESTART;    if (sigaction(SIGCHLD, &sa, NULL) == -1) {        perror("sigaction");        exit(1);    }    printf("server: waiting for connections...\n");    while(1) {  // main accept() loop        sin_size = sizeof their_addr;        new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);        if (new_fd == -1) {            perror("accept");            continue;        }        inet_ntop(their_addr.ss_family,            get_in_addr((struct sockaddr *)&their_addr),            s, sizeof s);        printf("server: got connection from %s\n", s);        if (!fork()) { // this is the child process            close(sockfd); // child doesn't need the listener            if (send(new_fd, "Hello, world!", 13, 0) == -1)                perror("send");            close(new_fd);            exit(0);        }        close(new_fd);  // parent doesn't need this    }    return 0;}

In case you're curious, I have the code in one bigmain() function for (I feel) syntactic clarity.Feel free to split it into smaller functions if it makes you feelbetter.

(Also, this whole sigaction()thing might be new to you—that's ok. The code that's there isresponsible for reaping zombie processes thatappear as the fork()ed child processes exit. If you makelots of zombies and don't reap them, your system administrator willbecome agitated.)

You can get the data from this server by using the clientlisted in the next section.

6.2. A Simple Stream Client

This guy's even easier than the server. Allthis client does is connect to the host you specify on the command line,port 3490. It gets the string that the server sends.

The clientsource:

/*** client.c -- a stream socket client demo*/#include #include #include #include #include #include #include #include #include #include #define PORT "3490" // the port client will be connecting to #define MAXDATASIZE 100 // max number of bytes we can get at once // get sockaddr, IPv4 or IPv6:void *get_in_addr(struct sockaddr *sa){    if (sa->sa_family == AF_INET) {        return &(((struct sockaddr_in*)sa)->sin_addr);    }    return &(((struct sockaddr_in6*)sa)->sin6_addr);}int main(int argc, char *argv[]){    int sockfd, numbytes;      char buf[MAXDATASIZE];    struct addrinfo hints, *servinfo, *p;    int rv;    char s[INET6_ADDRSTRLEN];    if (argc != 2) {        fprintf(stderr,"usage: client hostname\n");        exit(1);    }    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC;    hints.ai_socktype = SOCK_STREAM;    if ((rv = getaddrinfo(argv[1], PORT, &hints, &servinfo)) != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));        return 1;    }    // loop through all the results and connect to the first we can    for(p = servinfo; p != NULL; p = p->ai_next) {        if ((sockfd = socket(p->ai_family, p->ai_socktype,                p->ai_protocol)) == -1) {            perror("client: socket");            continue;        }        if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {            close(sockfd);            perror("client: connect");            continue;        }        break;    }    if (p == NULL) {        fprintf(stderr, "client: failed to connect\n");        return 2;    }    inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),            s, sizeof s);    printf("client: connecting to %s\n", s);    freeaddrinfo(servinfo); // all done with this structure    if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {        perror("recv");        exit(1);    }    buf[numbytes] = '\0';    printf("client: received '%s'\n",buf);    close(sockfd);    return 0;}

Notice that if you don't run the server before you run the client,connect() returns"Connection refused". Veryuseful.

6.3. Datagram Sockets

We've already covered the basics of UDP datagram sockets with ourdiscussion of sendto() and recvfrom(), above,so I'll just present a couple of sample programs: talker.cand listener.c.

listener sits on a machinewaiting for an incoming packet on port 4950. talker sends apacket to that port, on the specified machine, that contains whateverthe user enters on the command line.

Here is the source forlistener.c:

/*** listener.c -- a datagram sockets "server" demo*/#include #include #include #include #include #include #include #include #include #include #define MYPORT "4950"    // the port users will be connecting to#define MAXBUFLEN 100// get sockaddr, IPv4 or IPv6:void *get_in_addr(struct sockaddr *sa){    if (sa->sa_family == AF_INET) {        return &(((struct sockaddr_in*)sa)->sin_addr);    }    return &(((struct sockaddr_in6*)sa)->sin6_addr);}int main(void){    int sockfd;    struct addrinfo hints, *servinfo, *p;    int rv;    int numbytes;    struct sockaddr_storage their_addr;    char buf[MAXBUFLEN];    socklen_t addr_len;    char s[INET6_ADDRSTRLEN];    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4    hints.ai_socktype = SOCK_DGRAM;    hints.ai_flags = AI_PASSIVE; // use my IP    if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));        return 1;    }    // loop through all the results and bind to the first we can    for(p = servinfo; p != NULL; p = p->ai_next) {        if ((sockfd = socket(p->ai_family, p->ai_socktype,                p->ai_protocol)) == -1) {            perror("listener: socket");            continue;        }        if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {            close(sockfd);            perror("listener: bind");            continue;        }        break;    }    if (p == NULL) {        fprintf(stderr, "listener: failed to bind socket\n");        return 2;    }    freeaddrinfo(servinfo);    printf("listener: waiting to recvfrom...\n");    addr_len = sizeof their_addr;    if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0,        (struct sockaddr *)&their_addr, &addr_len)) == -1) {        perror("recvfrom");        exit(1);    }    printf("listener: got packet from %s\n",        inet_ntop(their_addr.ss_family,            get_in_addr((struct sockaddr *)&their_addr),            s, sizeof s));    printf("listener: packet is %d bytes long\n", numbytes);    buf[numbytes] = '\0';    printf("listener: packet contains \"%s\"\n", buf);    close(sockfd);    return 0;}

Notice that in our call to getaddrinfo() we're finallyusing SOCK_DGRAM. Also, note that there's no need tolisten() or accept(). This is one of theperks of using unconnected datagram sockets!

Next comes the source for talker.c:

/*** talker.c -- a datagram "client" demo*/#include #include #include #include #include #include #include #include #include #include #define SERVERPORT "4950"    // the port users will be connecting toint main(int argc, char *argv[]){    int sockfd;    struct addrinfo hints, *servinfo, *p;    int rv;    int numbytes;    if (argc != 3) {        fprintf(stderr,"usage: talker hostname message\n");        exit(1);    }    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC;    hints.ai_socktype = SOCK_DGRAM;    if ((rv = getaddrinfo(argv[1], SERVERPORT, &hints, &servinfo)) != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));        return 1;    }    // loop through all the results and make a socket    for(p = servinfo; p != NULL; p = p->ai_next) {        if ((sockfd = socket(p->ai_family, p->ai_socktype,                p->ai_protocol)) == -1) {            perror("talker: socket");            continue;        }        break;    }    if (p == NULL) {        fprintf(stderr, "talker: failed to bind socket\n");        return 2;    }    if ((numbytes = sendto(sockfd, argv[2], strlen(argv[2]), 0,             p->ai_addr, p->ai_addrlen)) == -1) {        perror("talker: sendto");        exit(1);    }    freeaddrinfo(servinfo);    printf("talker: sent %d bytes to %s\n", numbytes, argv[1]);    close(sockfd);    return 0;}

And that's all there is to it! Run listener on somemachine, then run talker on another. Watch them communicate!Fun G-rated excitement for the entire nuclear family!

You don't even have to run the server this time! You can runtalker by itself, and it just happily fires packets off intothe ether where they disappear if no one is ready with arecvfrom() on the other side. Remember: data sent usingUDP datagram sockets isn't guaranteed to arrive!

Except for one more tiny detail that I've mentioned many times in thepast: connected datagramsockets. I need to talk about this here, since we're in the datagramsection of the document. Let's say that talker callsconnect() and specifies the listener's address.From that point on, talker may only sent to and receive fromthe address specified by connect(). For this reason, youdon't have to use sendto() and recvfrom(); youcan simply use send() and recv().

7. Slightly Advanced Techniques

These aren't really advanced, but they'regetting out of the more basic levels we've already covered. In fact, ifyou've gotten this far, you should consider yourself fairly accomplishedin the basics of Unix network programming! Congratulations!

So here we go into the brave new world of some of the moreesoteric things you might want to learn about sockets. Have atit!

7.1. Blocking

Blocking. You've heard about it—now whatthe heck is it? In a nutshell, "block" is techie jargon for "sleep".You probably noticed that when you run listener, above, itjust sits there until a packet arrives. What happened is that it calledrecvfrom(), there was no data, and sorecvfrom() is said to "block" (that is, sleep there) untilsome data arrives.

Lots of functions block. accept() blocks.All the recv() functions block. The reason theycan do this is because they're allowed to. When you first create thesocket descriptor with socket(), the kernel sets itto blocking. If you don't want a socket to be blocking, you have tomake a call to fcntl():

#include #include ...sockfd = socket(PF_INET, SOCK_STREAM, 0);fcntl(sockfd, F_SETFL, O_NONBLOCK);... 

By setting a socket to non-blocking, you can effectively "poll"the socket for information. If you try to read from a non-blockingsocket and there's no data there, it's not allowed to block—itwill return -1 and errno will be set toEWOULDBLOCK.

Generally speaking, however, this type of polling is a bad idea.If you put your program in a busy-wait looking for data on the socket,you'll suck up CPU time like it was going out of style. A more elegantsolution for checking to see if there's data waiting to be read comes inthe following section on select().

7.2. select()—Synchronous I/O Multiplexing

This function is somewhat strange, but it's veryuseful. Take the following situation: you are a server and you want tolisten for incoming connections as well as keep reading from theconnections you already have.

No problem, you say, just an accept() and acouple of recv()s. Not so fast, buster! What ifyou're blocking on an accept() call? How are yougoing to recv() data at the same time? "Usenon-blocking sockets!" No way! You don't want to be a CPU hog. What,then?

select() gives you the power to monitorseveral sockets at the same time. It'll tell you which ones are readyfor reading, which are ready for writing, and which sockets have raisedexceptions, if you really want to know that.

This being said, in modern times select(), though veryportable, is one of the slowest methods for monitoring sockets. Onepossible alternative is libevent, orsomething similar, that encapsulates all the system-dependent stuffinvolved with getting socket notifications.

Without any further ado, I'll offer the synopsis ofselect():

#include #include #include int select(int numfds, fd_set *readfds, fd_set *writefds,           fd_set *exceptfds, struct timeval *timeout); 

The function monitors "sets" of file descriptors; in particularreadfds, writefds, andexceptfds. If you want to see if you can readfrom standard input and some socket descriptor,sockfd, just add the file descriptors0 and sockfd to the setreadfds. The parameternumfds should be set to the values of the highestfile descriptor plus one. In this example, it should be set tosockfd+1, since it is assuredly higher thanstandard input (0).

When select() returns,readfds will be modified to reflect which of thefile descriptors you selected which is ready for reading. You can testthem with the macro FD_ISSET(), below.

Before progressing much further, I'll talk about how to manipulatethese sets. Each set is of the type fd_set. The followingmacros operate on this type:

Finally, what is this weirded out struct timeval?Well, sometimes you don't want to wait forever for someone to send yousome data. Maybe every 96 seconds you want to print "Still Going..." tothe terminal even though nothing has happened. This time structureallows you to specify a timeout period. If the time is exceeded andselect() still hasn't found any ready filedescriptors, it'll return so you can continue processing.

The struct timeval has the follow fields:

struct timeval {    int tv_sec;     // seconds    int tv_usec;    // microseconds}; 

Just set tv_sec to the number of seconds towait, and set tv_usec to the number ofmicroseconds to wait. Yes, that's microseconds,not milliseconds. There are 1,000 microseconds in a millisecond, and1,000 milliseconds in a second. Thus, there are 1,000,000 microsecondsin a second. Why is it "usec"? The "u" is supposed to look like theGreek letter μ (Mu) that we use for "micro". Also, when the functionreturns, timeout might beupdated to show the time still remaining. This depends on what flavorof Unix you're running.

Yay! We have a microsecond resolution timer! Well, don't count onit. You'll probably have to wait some part of your standard Unixtimeslice no matter how small you set your structtimeval.

Other things of interest: If you set the fields in yourstruct timeval to 0,select() will timeout immediately, effectivelypolling all the file descriptors in your sets. If you set theparameter timeout to NULL, it will never timeout,and will wait until the first file descriptor is ready. Finally, if youdon't care about waiting for a certain set, you can just set it to NULLin the call to select().

The following code snippet waits 2.5 seconds forsomething to appear on standard input:

/*** select.c -- a select() demo*/#include #include #include #include #define STDIN 0  // file descriptor for standard inputint main(void){    struct timeval tv;    fd_set readfds;    tv.tv_sec = 2;    tv.tv_usec = 500000;    FD_ZERO(&readfds);    FD_SET(STDIN, &readfds);    // don't care about writefds and exceptfds:    select(STDIN+1, &readfds, NULL, NULL, &tv);    if (FD_ISSET(STDIN, &readfds))        printf("A key was pressed!\n");    else        printf("Timed out.\n");    return 0;} 

If you're on a line buffered terminal, the key you hit should beRETURN or it will time out anyway.

Now, some of you might think this is a great way to wait for dataon a datagram socket—and you are right: it might be.Some Unices can use select in this manner, and some can't. You shouldsee what your local man page says on the matter if you want to attemptit.

Some Unices update the time in your struct timeval toreflect the amount of time still remaining before a timeout. But othersdo not. Don't rely on that occurring if you want to be portable. (Usegettimeofday() if you need totrack time elapsed. It's a bummer, I know, but that's the way itis.)

What happens if a socket in the read set closes the connection?Well, in that case, select() returns with thatsocket descriptor set as "ready to read". When you actually dorecv() from it, recv() willreturn 0. That's how you know the client hasclosed the connection.

One more note of interest about select(): if you have asocket that is listen()ing, you cancheck to see if there is a new connection by putting that socket's filedescriptor in the readfds set.

And that, my friends, is a quick overview of the almightyselect() function.

But, by popular demand, here is an in-depth example.Unfortunately, the difference between the dirt-simple example, above, andthis one here is significant. But have a look, then read thedescription that follows it.

This program actslike a simple multi-user chat server. Start it running in one window,then telnet to it ("telnet hostname9034") from multiple other windows. When you type somethingin one telnet session, it should appear in all theothers.

/*** selectserver.c -- a cheezy multiperson chat server*/#include #include #include #include #include #include #include #include #include #define PORT "9034"   // port we're listening on// get sockaddr, IPv4 or IPv6:void *get_in_addr(struct sockaddr *sa){    if (sa->sa_family == AF_INET) {        return &(((struct sockaddr_in*)sa)->sin_addr);    }    return &(((struct sockaddr_in6*)sa)->sin6_addr);}int main(void){    fd_set master;    // master file descriptor list    fd_set read_fds;  // temp file descriptor list for select()    int fdmax;        // maximum file descriptor number    int listener;     // listening socket descriptor    int newfd;        // newly accept()ed socket descriptor    struct sockaddr_storage remoteaddr; // client address    socklen_t addrlen;    char buf[256];    // buffer for client data    int nbytes;    char remoteIP[INET6_ADDRSTRLEN];    int yes=1;        // for setsockopt() SO_REUSEADDR, below    int i, j, rv;    struct addrinfo hints, *ai, *p;    FD_ZERO(&master);    // clear the master and temp sets    FD_ZERO(&read_fds);    // get us a socket and bind it    memset(&hints, 0, sizeof hints);    hints.ai_family = AF_UNSPEC;    hints.ai_socktype = SOCK_STREAM;    hints.ai_flags = AI_PASSIVE;    if ((rv = getaddrinfo(NULL, PORT, &hints, &ai)) != 0) {        fprintf(stderr, "selectserver: %s\n", gai_strerror(rv));        exit(1);    }        for(p = ai; p != NULL; p = p->ai_next) {        listener = socket(p->ai_family, p->ai_socktype, p->ai_protocol);        if (listener < 0) {             continue;        }                // lose the pesky "address already in use" error message        setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int));        if (bind(listener, p->ai_addr, p->ai_addrlen) < 0) {            close(listener);            continue;        }        break;    }    // if we got here, it means we didn't get bound    if (p == NULL) {        fprintf(stderr, "selectserver: failed to bind\n");        exit(2);    }    freeaddrinfo(ai); // all done with this    // listen    if (listen(listener, 10) == -1) {        perror("listen");        exit(3);    }    // add the listener to the master set    FD_SET(listener, &master);    // keep track of the biggest file descriptor    fdmax = listener; // so far, it's this one    // main loop    for(;;) {        read_fds = master; // copy it        if (select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1) {            perror("select");            exit(4);        }        // run through the existing connections looking for data to read        for(i = 0; i <= fdmax; i++) {            if (FD_ISSET(i, &read_fds)) { // we got one!!                if (i == listener) {                    // handle new connections                    addrlen = sizeof remoteaddr;                    newfd = accept(listener,                        (struct sockaddr *)&remoteaddr,                        &addrlen);                    if (newfd == -1) {                        perror("accept");                    } else {                        FD_SET(newfd, &master); // add to master set                        if (newfd > fdmax) {    // keep track of the max                            fdmax = newfd;                        }                        printf("selectserver: new connection from %s on "                            "socket %d\n",                            inet_ntop(remoteaddr.ss_family,                                get_in_addr((struct sockaddr*)&remoteaddr),                                remoteIP, INET6_ADDRSTRLEN),                            newfd);                    }                } else {                    // handle data from a client                    if ((nbytes = recv(i, buf, sizeof buf, 0)) <= 0) {                        // got error or connection closed by client                        if (nbytes == 0) {                            // connection closed                            printf("selectserver: socket %d hung up\n", i);                        } else {                            perror("recv");                        }                        close(i); // bye!                        FD_CLR(i, &master); // remove from master set                    } else {                        // we got some data from a client                        for(j = 0; j <= fdmax; j++) {                            // send to everyone!                            if (FD_ISSET(j, &master)) {                                // except the listener and ourselves                                if (j != listener && j != i) {                                    if (send(j, buf, nbytes, 0) == -1) {                                        perror("send");                                    }                                }                            }                        }                    }                } // END handle data from client            } // END got new incoming connection        } // END looping through file descriptors    } // END for(;;)--and you thought it would never end!        return 0;}

Notice I have two file descriptor sets in the code:master and read_fds. Thefirst, master, holds all the socket descriptorsthat are currently connected, as well as the socket descriptor that islistening for new connections.

The reason I have the master set is thatselect() actually changes theset you pass into it to reflect which sockets are ready to read. SinceI have to keep track of the connections from one call ofselect() to the next, I must store these safelyaway somewhere. At the last minute, I copy themaster into the read_fds,and then call select().

But doesn't this mean that every time I get a new connection, Ihave to add it to the master set? Yup! Andevery time a connection closes, I have to remove it from themaster set? Yes, it does.

Notice I check to see when the listenersocket is ready to read. When it is, it means I have a new connectionpending, and I accept() it and add it to themaster set. Similarly, when a client connectionis ready to read, and recv() returns0, I know the client has closed the connection, andI must remove it from the master set.

If the client recv() returns non-zero,though, I know some data has been received. So I get it, and then gothrough the master list and send that data to allthe rest of the connected clients.

And that, my friends, is a less-than-simple overview of thealmighty select() function.

In addition, here is a bonus afterthought: there is another functioncalled poll() which behaves much the same wayselect() does, but with a different system for managing thefile descriptor sets. Check it out!

7.3. Handling Partial send()s

Remember back in the section aboutsend(), above, when I said thatsend() might not send all the bytes you asked itto? That is, you want it to send 512 bytes, but it returns 412. Whathappened to the remaining 100 bytes?

Well, they're still in your little buffer waiting to be sent out.Due to circumstances beyond your control, the kernel decided not to sendall the data out in one chunk, and now, my friend, it's up to you to getthe data out there.

You could write a function like this to do it,too:

#include #include int sendall(int s, char *buf, int *len){    int total = 0;        // how many bytes we've sent    int bytesleft = *len; // how many we have left to send    int n;    while(total < *len) {        n = send(s, buf+total, bytesleft, 0);        if (n == -1) { break; }        total += n;        bytesleft -= n;    }    *len = total; // return number actually sent here    return n==-1?-1:0; // return -1 on failure, 0 on success} 

In this example, s is the socket you wantto send the data to, buf is the buffer containingthe data, and len is a pointer to anint containing the number of bytes in the buffer.

The function returns -1 on error (anderrno is still set from the call tosend().) Also, the number of bytes actually sentis returned in len. This will be the same numberof bytes you asked it to send, unless there was an error.sendall() will do it's best, huffing and puffing,to send the data out, but if there's an error, it gets back to you rightaway.

For completeness, here's a sample call to the function:

char buf[10] = "Beej!";int len;len = strlen(buf);if (sendall(s, buf, &len) == -1) {    perror("sendall");    printf("We only sent %d bytes because of the error!\n", len);} 

What happens on the receiver's end when part of a packet arrives?If the packets are variable length, how does the receiver know when onepacket ends and another begins? Yes, real-world scenarios are a royalpain in the donkeys. You probably have to encapsulate (remember that from thedata encapsulation section way back thereat the beginning?) Read on for details!

7.4. Serialization—How to Pack Data

It's easy enough to send text data acrossthe network, you're finding, but what happens if you want to send some"binary" data like ints or floats? It turnsout you have a few options.

1. Convert the number into text with a function likesprintf(), then send the text. The receiver will parse thetext back into a number using a function likestrtol().
2. Just send the data raw, passing a pointer to the data tosend().
3. Encode the number into a portable binary form. The receiver willdecode it.

Sneak preview! Tonight only!

[Curtain raises]

Beej says, "I prefer Method Three, above!"

[THE END]

(Before I begin this section in earnest, I should tell you that thereare libraries out there for doing this, and rolling your own andremaining portable and error-free is quite a challenge. So hunt aroundand do your homework before deciding to implement this stuff yourself.I include the information here for those curious about how things likethis work.)

Actually all the methods, above, have their drawbacks and advantages,but, like I said, in general, I prefer the third method. First, though,let's talk about some of the drawbacks and advantages to the other two.

The first method, encoding the numbers as text before sending, hasthe advantage that you can easily print and read the data that's comingover the wire. Sometimes a human-readable protocol is excellent to usein a non-bandwidth-intensive situation, such as with Internet Relay Chat (IRC).However, it has the disadvantage that it is slow to convert, and theresults almost always take up more space than the original number!

Method two: passing the raw data. This one is quite easy (butdangerous!): just take a pointer to the data to send, and call send withit.

double d = 3490.15926535;send(s, &d, sizeof d, 0);  /* DANGER--non-portable! */

The receiver gets it like this:

double d;recv(s, &d, sizeof d, 0);  /* DANGER--non-portable! */

Fast, simple—what's not to like? Well, it turns out that notall architectures represent a double (or intfor that matter) with the same bit representation or even the same byteordering! The code is decidedly non-portable. (Hey—maybe youdon't need portability, in which case this is nice and fast.)

When packing integer types, we've already seen how the htons()-class of functions can help keepthings portable by transforming the numbers into Network Byte Order, and how that's the Right Thing to do.Unfortunately, there are no similar functions for floattypes. Is all hope lost?

Fear not! (Were you afraid there for a second? No? Not even alittle bit?) There is something we can do: we can pack (or "marshal",or "serialize", or one of a thousand million other names) the data intoa known binary format that the receiver can unpack on the remoteside.

What do I mean by "known binary format"? Well, we've already seenthe htons() example, right? It changes (or "encodes", ifyou want to think of it that way) a number from whatever the host formatis into Network Byte Order. To reverse (unencode) the number, thereceiver calls ntohs().

But didn't I just get finished saying there wasn't any such functionfor other non-integer types? Yes. I did. And since there's nostandard way in C to do this, it's a bit of a pickle (that a gratuitouspun there for you Python fans).

The thing to do is to pack the data into a known format and send thatover the wire for decoding. For example, to pack floats,here's something quick and dirty withplenty of room for improvement:

#include uint32_t htonf(float f){    uint32_t p;    uint32_t sign;    if (f < 0) { sign = 1; f = -f; }    else { sign = 0; }            p = ((((uint32_t)f)&0x7fff)<<16) | (sign<<31); // whole part and sign    p |= (uint32_t)(((f - (int)f) * 65536.0f))&0xffff; // fraction    return p;}float ntohf(uint32_t p){    float f = ((p>>16)&0x7fff); // whole part    f += (p&0xffff) / 65536.0f; // fraction    if (((p>>31)&0x1) == 0x1) { f = -f; } // sign bit set    return f;}

The above code is sort of a naive implementation that stores afloat in a 32-bit number. The high bit (31) is used tostore the sign of the number ("1" means negative), and the next sevenbits (30-16) are used to store the whole number portion of thefloat. Finally, the remaining bits (15-0) are used tostore the fractional portion of the number.

Usage is fairly straightforward:

#include int main(void){    float f = 3.1415926, f2;    uint32_t netf;    netf = htonf(f);  // convert to "network" form    f2 = ntohf(netf); // convert back to test    printf("Original: %f\n", f);        // 3.141593    printf(" Network: 0x%08X\n", netf); // 0x0003243F    printf("Unpacked: %f\n", f2);       // 3.141586    return 0;}

On the plus side, it's small, simple, and fast. On the minus side,it's not an efficient use of space and the range is severelyrestricted—try storing a number greater-than 32767 in there andit won't be very happy! You can also see in the above example that thelast couple decimal places are not correctly preserved.

What can we do instead? Well, The Standard for storingfloating point numbers is known as IEEE-754. Most computers use this formatinternally for doing floating point math, so in those cases, strictlyspeaking, conversion wouldn't need to be done. But if you want yoursource code to be portable, that's an assumption you can't necessarilymake. (On the other hand, if you want things to be fast, you shouldoptimize this out on platforms that don't need to do it! That's whathtons() and its ilk do.)

Here's some code that encodesfloats and doubles into IEEE-754 format. (Mostly—itdoesn't encode NaN or Infinity, but it could be modified to dothat.)

#define pack754_32(f) (pack754((f), 32, 8))#define pack754_64(f) (pack754((f), 64, 11))#define unpack754_32(i) (unpack754((i), 32, 8))#define unpack754_64(i) (unpack754((i), 64, 11))uint64_t pack754(long double f, unsigned bits, unsigned expbits){    long double fnorm;    int shift;    long long sign, exp, significand;    unsigned significandbits = bits - expbits - 1; // -1 for sign bit    if (f == 0.0) return 0; // get this special case out of the way    // check sign and begin normalization    if (f < 0) { sign = 1; fnorm = -f; }    else { sign = 0; fnorm = f; }    // get the normalized form of f and track the exponent    shift = 0;    while(fnorm >= 2.0) { fnorm /= 2.0; shift++; }    while(fnorm < 1.0) { fnorm *= 2.0; shift--; }    fnorm = fnorm - 1.0;    // calculate the binary form (non-float) of the significand data    significand = fnorm * ((1LL<>significandbits)&((1LL< 0) { result *= 2.0; shift--; }    while(shift < 0) { result /= 2.0; shift++; }    // sign it    result *= (i>>(bits-1))&1? -1.0: 1.0;    return result;}

I put some handy macros up there at the top for packing and unpacking32-bit (probably a float) and 64-bit (probably adouble) numbers, but the pack754() functioncould be called directly and told to encode bits-worth ofdata (expbits of which are reserved for the normalizednumber's exponent.)

Here's sample usage:

#include #include  // defines uintN_t types#include  // defines PRIx macrosint main(void){    float f = 3.1415926, f2;    double d = 3.14159265358979323, d2;    uint32_t fi;    uint64_t di;    fi = pack754_32(f);    f2 = unpack754_32(fi);    di = pack754_64(d);    d2 = unpack754_64(di);    printf("float before : %.7f\n", f);    printf("float encoded: 0x%08" PRIx32 "\n", fi);    printf("float after  : %.7f\n\n", f2);    printf("double before : %.20lf\n", d);    printf("double encoded: 0x%016" PRIx64 "\n", di);    printf("double after  : %.20lf\n", d2);    return 0;}

The above code produces this output:

float before : 3.1415925float encoded: 0x40490FDAfloat after  : 3.1415925double before : 3.14159265358979311600double encoded: 0x400921FB54442D18double after  : 3.14159265358979311600

Another question you might have is how do you packstructs? Unfortunately for you, the compiler is free toput padding all over the place in a struct, and that meansyou can't portably send the whole thing over the wire in one chunk.(Aren't you getting sick of hearing "can't do this", "can't do that"?Sorry! To quote a friend, "Whenever anything goes wrong, I always blameMicrosoft." This one might not be Microsoft's fault, admittedly, but myfriend's statement is completely true.)

Back to it: the best way to send the struct over thewire is to pack each field independently and then unpack them into thestruct when they arrive on the other side.

That's a lot of work, is what you're thinking. Yes, it is. Onething you can do is write a helper function to help pack the data foryou. It'll be fun! Really!

In the book "The Practice ofProgramming" by Kernighan and Pike, they implementprintf()-like functions called pack() andunpack() that do exactly this. I'd link to them, butapparently those functions aren't online with the rest of thesource from the book.

(The Practice of Programming is an excellent read. Zeus saves akitten every time I recommend it.)

At this point, I'm going to drop a pointer to the BSD-licensed Typed Parameter Language C API which I've neverused, but looks completely respectable. Python and Perl programmerswill want to check out their language's pack() andunpack() functions for accomplishing the same thing. AndJava has a big-ol' Serializable interface that can be used in a similarway.

But if you want to write your own packing utility in C, K&P'strick is to use variable argument lists to makeprintf()-like functions to build the packets. Here's a version I cooked up on my ownbased on that which hopefully will be enough to give you an idea of howsuch a thing can work.

(This code references the pack754() functions, above.The packi*() functions operate like the familiarhtons() family, except they pack into a chararray instead of another integer.)

#include #include #include #include #include // various bits for floating point types--// varies for different architecturestypedef float float32_t;typedef double float64_t;/*** packi16() -- store a 16-bit int into a char buffer (like htons())*/ void packi16(unsigned char *buf, unsigned int i){    *buf++ = i>>8; *buf++ = i;}/*** packi32() -- store a 32-bit int into a char buffer (like htonl())*/ void packi32(unsigned char *buf, unsigned long i){    *buf++ = i>>24; *buf++ = i>>16;    *buf++ = i>>8;  *buf++ = i;}/*** unpacki16() -- unpack a 16-bit int from a char buffer (like ntohs())*/ unsigned int unpacki16(unsigned char *buf){    return (buf[0]<<8) | buf[1];}/*** unpacki32() -- unpack a 32-bit int from a char buffer (like ntohl())*/ unsigned long unpacki32(unsigned char *buf){    return (buf[0]<<24) | (buf[1]<<16) | (buf[2]<<8) | buf[3];}/*** pack() -- store data dictated by the format string in the buffer****  h - 16-bit              l - 32-bit**  c - 8-bit char          f - float, 32-bit**  s - string (16-bit length is automatically prepended)*/ int32_t pack(unsigned char *buf, char *format, ...){    va_list ap;    int16_t h;    int32_t l;    int8_t c;    float32_t f;    char *s;    int32_t size = 0, len;    va_start(ap, format);    for(; *format != '\0'; format++) {        switch(*format) {        case 'h': // 16-bit            size += 2;            h = (int16_t)va_arg(ap, int); // promoted            packi16(buf, h);            buf += 2;            break;        case 'l': // 32-bit            size += 4;            l = va_arg(ap, int32_t);            packi32(buf, l);            buf += 4;            break;        case 'c': // 8-bit            size += 1;            c = (int8_t)va_arg(ap, int); // promoted            *buf++ = (c>>0)&0xff;            break;        case 'f': // float            size += 4;            f = (float32_t)va_arg(ap, double); // promoted            l = pack754_32(f); // convert to IEEE 754            packi32(buf, l);            buf += 4;            break;        case 's': // string            s = va_arg(ap, char*);            len = strlen(s);            size += len + 2;            packi16(buf, len);            buf += 2;            memcpy(buf, s, len);            buf += len;            break;        }    }    va_end(ap);    return size;}/*** unpack() -- unpack data dictated by the format string into the buffer*/void unpack(unsigned char *buf, char *format, ...){    va_list ap;    int16_t *h;    int32_t *l;    int32_t pf;    int8_t *c;    float32_t *f;    char *s;    int32_t len, count, maxstrlen=0;    va_start(ap, format);    for(; *format != '\0'; format++) {        switch(*format) {        case 'h': // 16-bit            h = va_arg(ap, int16_t*);            *h = unpacki16(buf);            buf += 2;            break;        case 'l': // 32-bit            l = va_arg(ap, int32_t*);            *l = unpacki32(buf);            buf += 4;            break;        case 'c': // 8-bit            c = va_arg(ap, int8_t*);            *c = *buf++;            break;        case 'f': // float            f = va_arg(ap, float32_t*);            pf = unpacki32(buf);            buf += 4;            *f = unpack754_32(pf);            break;        case 's': // string            s = va_arg(ap, char*);            len = unpacki16(buf);            buf += 2;            if (maxstrlen > 0 && len > maxstrlen) count = maxstrlen - 1;            else count = len;            memcpy(s, buf, count);            s[count] = '\0';            buf += len;            break;        default:            if (isdigit(*format)) { // track max str len                maxstrlen = maxstrlen * 10 + (*format-'0');            }        }        if (!isdigit(*format)) maxstrlen = 0;    }    va_end(ap);}

And here is a demonstrationprogram of the above code that packs some data intobuf and then unpacks it into variables. Note that whencalling unpack() with a string argument (format specifier"s"), it's wise to put a maximum length count in front ofit to prevent a buffer overrun, e.g. "96s". Be wary whenunpacking data you get over the network—a malicious user mightsend badly-constructed packets in an effort to attack your system!

#include // various bits for floating point types--// varies for different architecturestypedef float float32_t;typedef double float64_t;int main(void){    unsigned char buf[1024];    int8_t magic;    int16_t monkeycount;    int32_t altitude;    float32_t absurdityfactor;    char *s = "Great unmitigated Zot!  You've found the Runestaff!";    char s2[96];    int16_t packetsize, ps2;    packetsize = pack(buf, "chhlsf", (int8_t)'B', (int16_t)0, (int16_t)37,             (int32_t)-5, s, (float32_t)-3490.6677);    packi16(buf+1, packetsize); // store packet size in packet for kicks    printf("packet is %" PRId32 " bytes\n", packetsize);    unpack(buf, "chhl96sf", &magic, &ps2, &monkeycount, &altitude, s2,        &absurdityfactor);    printf("'%c' %" PRId32" %" PRId16 " %" PRId32            " \"%s\" %f\n", magic, ps2, monkeycount,            altitude, s2, absurdityfactor);    return 0;}

Whether you roll your own code or use someone else's, it's a goodidea to have a general set of data packing routines for the sake ofkeeping bugs in check, rather than packing each bit by hand eachtime.

When packing the data, what's a good format to use? Excellentquestion. Fortunately, RFC 4506, the External DataRepresentation Standard, already defines binary formats for a bunch ofdifferent types, like floating point types, integer types, arrays, rawdata, etc. I suggest conforming to that if you're going to roll thedata yourself. But you're not obligated to. The Packet Police are notright outside your door. At least, I don't think they are.

In any case, encoding the data somehow or another before you send itis the right way of doing things!

7.5. Son of Data Encapsulation

What does it really mean to encapsulate data, anyway? In thesimplest case, it means you'll stick a header on there with either someidentifying information or a packet length, or both.

What should your header look like? Well, it's just some binarydata that represents whatever you feel is necessary to complete yourproject.

Wow. That's vague.

Okay. For instance, let's say you have a multi-user chat programthat uses SOCK_STREAMs. When a user types ("says")something, two pieces of information need to be transmitted to theserver: what was said and who said it.

So far so good? "What's the problem?" you're asking.

The problem is that the messages can be of varying lengths. Oneperson named "tom" might say, "Hi", and another person named"Benjamin" might say, "Hey guys what is up?"

So you send() all this stuff to the clientsas it comes in. Your outgoing data stream looks like this:

t o m H i B e n j a m i n H e y g u y s w h a t i s u p ?

And so on. How does the client know when one message starts andanother stops? You could, if you wanted, make all messages the samelength and just call the sendall() weimplemented, above. But that wastesbandwidth! We don't want to send() 1024 bytes just so"tom" can say "Hi".

So we encapsulate the data in a tiny headerand packet structure. Both the client and server know how to pack andunpack (sometimes referred to as "marshal" and "unmarshal") this data.Don't look now, but we're starting to define aprotocol that describes how a client and servercommunicate!

In this case, let's assume the user name is a fixed length of 8characters, padded with '\0'. And then let'sassume the data is variable length, up to a maximum of 128characters. Let's have a look a sample packet structure that we mightuse in this situation:

1. len (1 byte, unsigned)—The total length of thepacket, counting the 8-byte user name and chat data.
2. name (8 bytes)—The user's name, NUL-padded ifnecessary.
3. chatdata(n-bytes)—The data itself, no more than 128 bytes.The length of the packet should be calculated as the length of this dataplus 8 (the length of the name field, above).

Why did I choose the 8-byte and 128-byte limits for the fields? Ipulled them out of the air, assuming they'd be long enough. Maybe,though, 8 bytes is too restrictive for your needs, and you can have a30-byte name field, or whatever. The choice is up to you.

Using the above packet definition, the first packet would consistof the following information (in hex and ASCII):

   0A     74 6F 6D 00 00 00 00 00      48 69(length)  T  o  m    (padding)         H  i

And the second is similar:

   18     42 65 6E 6A 61 6D 69 6E      48 65 79 20 67 75 79 73 20 77 ...(length)  B  e  n  j  a  m  i  n       H  e  y     g  u  y  s     w  ...

(The length is stored in Network Byte Order, of course. In thiscase, it's only one byte so it doesn't matter, but generally speakingyou'll want all your binary integers to be stored in Network Byte Orderin your packets.)

When you're sending this data, you should be safe and use acommand similar to sendall(), above, so youknow all the data is sent, even if it takes multiple calls tosend() to get it all out.

Likewise, when you're receiving this data, you need to do a bit of extrawork. To be safe, you should assume that you might receive a partialpacket (like maybe we receive "18 42 656E 6A" from Benjamin, above, but that's all we get in thiscall to recv()). We need to callrecv() over and over again until the packet iscompletely received.

But how? Well, we know the number of bytes we need to receive intotal for the packet to be complete, since that number is tacked on thefront of the packet. We also know the maximum packet size is 1+8+128,or 137 bytes (because that's how we defined the packet.)

There are actually a couple things you can do here. Since you knowevery packet starts off with a length, you can call recv()just to get the packet length. Then once you have that, you can call itagain specifying exactly the remaining length of the packet (possiblyrepeatedly to get all the data) until you have the complete packet.The advantage of this method is that you only need a buffer largeenough for one packet, while the disadvantage is that you need to callrecv() at least twice to get all the data.

Another option is just to call recv() and say the amountyou're willing to receive is the maximum number of bytes in a packet.Then whatever you get, stick it onto the back of a buffer, and finallycheck to see if the packet is complete. Of course, you might get someof the next packet, so you'll need to have room for that.

What you can do is declare an array big enough for two packets.This is your work array where you will reconstruct packets as theyarrive.

Every time you recv() data, you'll append itinto the work buffer and check to see if the packet is complete. Thatis, the number of bytes in the buffer is greater than or equal to thelength specified in the header (+1, because the length in the headerdoesn't include the byte for the length itself.) If the number of bytesin the buffer is less than 1, the packet is not complete, obviously.You have to make a special case for this, though, since the first byteis garbage and you can't rely on it for the correct packetlength.

Once the packet is complete, you can do with it what youwill. Use it, and remove it from your work buffer.

Whew! Are you juggling that in your head yet? Well, here's thesecond of the one-two punch: you might have read past the end of onepacket and onto the next in a single recv() call.That is, you have a work buffer with one complete packet, and anincomplete part of the next packet! Bloody heck. (But this is why youmade your work buffer large enough to hold twopackets—in case this happened!)

Since you know the length of the first packet from the header, andyou've been keeping track of the number of bytes in the work buffer, youcan subtract and calculate how many of the bytes in the work bufferbelong to the second (incomplete) packet. When you've handled the firstone, you can clear it out of the work buffer and move the partial secondpacket down the to front of the buffer so it's all ready to go for thenext recv().

(Some of you readers will note that actually moving the partialsecond packet to the beginning of the work buffer takes time, and theprogram can be coded to not require this by using a circular buffer.Unfortunately for the rest of you, a discussion on circular buffers isbeyond the scope of this article. If you're still curious, grab a datastructures book and go from there.)

I never said it was easy. Ok, I did say it was easy. And it is;you just need practice and pretty soon it'll come to you naturally. ByExcalibur I swear it!

7.6. Broadcast Packets—Hello, World!

So far, this guide has talked about sending data from one host to oneother host. But it is possible, I insist, that you can, with the properauthority, send data to multiple hosts at the same time!

With UDP (only UDP, not TCP) and standard IPv4, thisis done through a mechanism called broadcasting. With IPv6, broadcasting isn'tsupported, and you have to resort to the often superior technique ofmulticasting, which, sadly I won't be discussing at thistime. But enough of the starry-eyed future—we're stuck in the32-bit present.

But wait! You can't just run off and start broadcasting willy-nilly;You have to set the socket option SO_BROADCAST before you can send abroadcast packet out on the network. It's like a one of those littleplastic covers they put over the missile launch switch! That's just howmuch power you hold in your hands!

But seriously, though, there is a danger to using broadcast packets,and that is: every system that receives a broadcast packet must undo allthe onion-skin layers of data encapsulation until it finds out what portthe data is destined to. And then it hands the data over or discardsit. In either case, it's a lot of work for each machine that receivesthe broadcast packet, and since it is all of them on the local network,that could be a lot of machines doing a lot of unnecessary work. Whenthe game Doom first came out, this was a complaint about its networkcode.

Now, there is more than one way to skin a cat... wait a minute. Isthere really more than one way to skin a cat? What kind of is that? Uh, and likewise, there is more than one way to send abroadcast packet. So, to get to the meat and potatoes of the wholething: how do you specify the destination address for a broadcastmessage? There are two common ways:

1. Send the data to a specific subnet's broadcast address. This is thesubnet's network number with all one-bits set for the host portion ofthe address. For instance, at home my network is 192.168.1.0, mynetmask is 255.255.255.0, so the last byte of the address is my hostnumber (because the first three bytes, according to the netmask, are thenetwork number). So my broadcast address is 192.168.1.255. Under Unix,the ifconfig command will actually give you all this data.(If you're curious, the bitwise logic to get your broadcast address isnetwork_number OR (NOT netmask).) You can sendthis type of broadcast packet to remote networks as well as your localnetwork, but you run the risk of the packet being dropped by thedestination's router. (If they didn't drop it, then some random smurfcould start flooding their LAN with broadcast traffic.)
2. Send the data to the "global" broadcast address. This is 255.255.255.255, akaINADDR_BROADCAST. Manymachines will automatically bitwise AND this with your network number toconvert it to a network broadcast address, but some won't. It varies.Routers do not forward this type of broadcast packet off your localnetwork, ironically enough.

So what happens if you try to send data on the broadcast addresswithout first setting the SO_BROADCAST socket option?Well, let's fire up good old talker andlistener and see what happens.

$ talker 192.168.1.2 foosent 3 bytes to 192.168.1.2$ talker 192.168.1.255 foosendto: Permission denied$ talker 255.255.255.255 foosendto: Permission denied

Yes, it's not happy at all...because we didn't set theSO_BROADCAST socket option. Do that, and now you cansendto() anywhere you want!

In fact, that's the only difference between a UDPapplication that can broadcast and one that can't. So let's take theold talker application and add one section that sets theSO_BROADCAST socket option. We'll call this programbroadcaster.c:

/*** broadcaster.c -- a datagram "client" like talker.c, except**                  this one can broadcast*/#include #include #include #include #include #include #include #include #include #include #define SERVERPORT 4950    // the port users will be connecting toint main(int argc, char *argv[]){    int sockfd;    struct sockaddr_in their_addr; // connector's address information    struct hostent *he;    int numbytes;    int broadcast = 1;    //char broadcast = '1'; // if that doesn't work, try this    if (argc != 3) {        fprintf(stderr,"usage: broadcaster hostname message\n");        exit(1);    }    if ((he=gethostbyname(argv[1])) == NULL) {  // get the host info        perror("gethostbyname");        exit(1);    }    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {        perror("socket");        exit(1);    }    // this call is what allows broadcast packets to be sent:    if (setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &broadcast,        sizeof broadcast) == -1) {        perror("setsockopt (SO_BROADCAST)");        exit(1);    }    their_addr.sin_family = AF_INET;     // host byte order    their_addr.sin_port = htons(SERVERPORT); // short, network byte order    their_addr.sin_addr = *((struct in_addr *)he->h_addr);    memset(their_addr.sin_zero, '\0', sizeof their_addr.sin_zero);    if ((numbytes=sendto(sockfd, argv[2], strlen(argv[2]), 0,             (struct sockaddr *)&their_addr, sizeof their_addr)) == -1) {        perror("sendto");        exit(1);    }    printf("sent %d bytes to %s\n", numbytes,        inet_ntoa(their_addr.sin_addr));    close(sockfd);    return 0;}

What's different between this and a "normal" UDP client/serversituation? Nothing! (With the exception of the client being allowed tosend broadcast packets in this case.) As such, go ahead and run the oldUDP listener program in onewindow, and broadcaster in another. You should be now beable to do all those sends that failed, above.

$ broadcaster 192.168.1.2 foosent 3 bytes to 192.168.1.2$ broadcaster 192.168.1.255 foosent 3 bytes to 192.168.1.255$ broadcaster 255.255.255.255 foosent 3 bytes to 255.255.255.255

And you should see listener responding that it got thepackets. (If listener doesn't respond, it could be becauseit's bound to an IPv6 address. Try changing theAF_UNSPEC in listener.c toAF_INET to force IPv4.)

Well, that's kind of exciting. But now fire up listener onanother machine next to you on the same network so that you have twocopies going, one on each machine, and run broadcaster againwith your broadcast address... Hey! Both listeners get thepacket even though you only called sendto() once!Cool!

If the listener gets data you send directly to it, but notdata on the broadcast address, it could be that you have a firewall on your local machine that is blocking thepackets. (Yes, Pat and Bapper, thankyou for realizing before I did that this is why my sample code wasn'tworking. I told you I'd mention you in the guide, and here you are. Sonyah.)

Again, be careful with broadcast packets. Since every machine on theLAN will be forced to deal with the packet whether itrecvfrom()s it or not, it can present quite a load to theentire computing network. They are definitely to be used sparingly andappropriately.

8. Common Questions

Where can I get those header files?

If you don't have them on your systemalready, you probably don't need them. Check the manual for yourparticular platform. If you're building for Windows,you only need to #include .

What do I do when bind() reports"Address already in use"?

You have to use setsockopt()with the SO_REUSEADDR option onthe listening socket. Check out the section on bind() and the section on select() for an example.

How do I get a list of open sockets on thesystem?

Use the netstat. Check theman page for full details, but you should get some goodoutput just typing:

$ netstat

The only trick is determining which socket is associated withwhich program. :-)

How can I view the routing table?

Run the route command (in/sbin on most Linuxes) or the commandnetstat -r.

How can I run the client and server programs if I onlyhave one computer? Don't I need a network to write networkprograms?

Fortunately for you, virtually all machines implement a loopback network "device" that sits in the kerneland pretends to be a network card. (This is the interface listed as"lo" in the routing table.)

Pretend you're logged into a machine named"goat". Run the client in one windowand the server in another. Or start the server in the background("server &") and run the client in the samewindow. The upshot of the loopback device is that you can eitherclient goat or client localhost(since "localhost" is likely defined inyour /etc/hosts file) and you'll have the clienttalking to the server without a network!

In short, no changes are necessary to any of the code to make itrun on a single non-networked machine! Huzzah!

How can I tell if the remote side has closedconnection?

You can tell because recv() willreturn 0.

How do I implement a "ping" utility? What is ICMP? Where can I find out more about raw sockets and SOCK_RAW?

All your raw sockets questions will be answered in W. Richard Stevens' UNIX Network Programming books.Also, look in the ping/ subdirectory in Stevens' UNIXNetwork Programming source code, availableonline.

How do I change or shorten the timeout on a call toconnect()?

Instead of giving you exactly the same answer that W. RichardStevens would give you, I'll just refer you to lib/connect_nonb.c in the UNIX NetworkProgramming source code.

The gist of it is that you make a socket descriptor withsocket(), set it tonon-blocking, call connect(), and if all goes wellconnect() will return -1 immediately anderrno will be set to EINPROGRESS. Then youcall select() with whatevertimeout you want, passing the socket descriptor in both the read andwrite sets. If it doesn't timeout, it means the connect()call completed. At this point, you'll have to usegetsockopt() with the SO_ERROR option to getthe return value from the connect() call, which should bezero if there was no error.

Finally, you'll probably want to set the socket back to be blockingagain before you start transferring data over it.

Notice that this has the added benefit of allowing your program to dosomething else while it's connecting, too. You could, for example, setthe timeout to something low, like 500 ms, and update an indicatoronscreen each timeout, then call select() again. Whenyou've called select() and timed-out, say, 20 times, you'llknow it's time to give up on the connection.

Like I said, check out Stevens' source for a perfectly excellentexample.

How do I build for Windows?

First, delete Windows and install Linux or BSD.};-). No, actually, just see the section on building forWindows in the introduction.

How do I build for Solaris/SunOS? I keep getting linkererrors when I try to compile!

The linker errors happen because Sun boxes don'tautomatically compile in the socket libraries. See the section on building for Solaris/SunOS in theintroduction for an example of how to do this.

Why does select() keep falling outon a signal?

Signals tend to cause blocked system calls to return-1 with errno set to EINTR.When you set up a signal handler with sigaction(), you can set the flag SA_RESTART, which is supposed torestart the system call after it was interrupted.

Naturally, this doesn't always work.

My favorite solution to this involves agoto statement. You know thisirritates your professors to no end, so go for it!

select_restart:if ((err = select(fdmax+1, &readfds, NULL, NULL, NULL)) == -1) {    if (errno == EINTR) {        // some signal just interrupted us, so restart        goto select_restart;    }    // handle the real error here:    perror("select");} 

Sure, you don't need to usegoto in this case; you can use otherstructures to control it. But I think thegoto statement is actuallycleaner.

How can I implement a timeout on a call torecv()?

Use select()! It allows you to specify atimeout parameter for socket descriptors that you're looking to readfrom. Or, you could wrap the entire functionality in a single function,like this:

#include #include #include #include int recvtimeout(int s, char *buf, int len, int timeout){    fd_set fds;    int n;    struct timeval tv;    // set up the file descriptor set    FD_ZERO(&fds);    FD_SET(s, &fds);    // set up the struct timeval for the timeout    tv.tv_sec = timeout;    tv.tv_usec = 0;    // wait until timeout or data received    n = select(s+1, &fds, NULL, NULL, &tv);    if (n == 0) return -2; // timeout!    if (n == -1) return -1; // error    // data must be here, so do a normal recv()    return recv(s, buf, len, 0);}...// Sample call to recvtimeout():n = recvtimeout(s, buf, sizeof buf, 10); // 10 second timeoutif (n == -1) {    // error occurred    perror("recvtimeout");}else if (n == -2) {    // timeout occurred} else {    // got some data in buf}... 

Notice that recvtimeout()returns -2 in case of a timeout. Why not return0? Well, if you recall, a return value of0 on a call to recv() means that the remoteside closed the connection. So that return value is already spoken for,and -1 means "error", so I chose -2 as mytimeout indicator.

How do I encrypt or compress the data beforesending it through the socket?

One easy way to do encryption is to use SSL (securesockets layer), but that's beyond the scope of this guide. (Check out the OpenSSLproject for more info.)

But assuming you want to plug in or implement your own compressoror encryption system, it's just a matter of thinking of your data asrunning through a sequence of steps between both ends. Each stepchanges the data in some way.

1. server reads data from file (or wherever)
2. server encrypts/compresses data (you add this part)
3. server send()s encrypted data

Now the other way around:

1. client recv()s encrypted data
2. client decrypts/decompresses data (you add this part)
3. client writes data to file (or wherever)

If you're going to compress and encrypt, just remember to compressfirst. :-)

Just as long as the client properly undoes what the server does,the data will be fine in the end no matter how many intermediate stepsyou add.

So all you need to do to use my code is to find the place betweenwhere the data is read and the data is sent (usingsend()) over the network, and stick some code inthere that does the encryption.

What is this"PF_INET" I keep seeing? Is it related toAF_INET?

Yes, yes it is. See the section onsocket() for details.

How can I write a server that accepts shell commandsfrom a client and executes them?

For simplicity, lets say the clientconnect()s, send()s, andclose()s the connection (that is, there are nosubsequent system calls without the client connecting again.)

The process the client follows is this:

1. connect() to server
2. send("/sbin/ls > /tmp/client.out")
3. close() the connection

Meanwhile, the server is handling the data and executingit:

1. accept() the connection from the client
2. recv(str) the command string
3. close() the connection
4. system(str) to run the command

Beware! Having the server executewhat the client says is like giving remote shell access and people cando things to your account when they connect to the server. Forinstance, in the above example, what if the client sends "rm -rf~"? It deletes everything in your account, that's what!

So you get wise, and you prevent the client from using any exceptfor a couple utilities that you know are safe, like thefoobar utility:

if (!strncmp(str, "foobar", 6)) {    sprintf(sysstr, "%s > /tmp/server.out", str);    system(sysstr);} 

But you're still unsafe, unfortunately: what if the client enters"foobar; rm -rf ~"? The safest thing to do is towrite a little routine that puts an escape ("\")character in front of all non-alphanumeric characters (including spaces,if appropriate) in the arguments for the command.

As you can see, security is a pretty big issue when the serverstarts executing things the client sends.

I'm sending a slew of data, but when Irecv(), it only receives 536 bytes or 1460 bytes ata time. But if I run it on my local machine, it receives all the dataat the same time. What's going on?

You're hitting the MTU—the maximum size thephysical medium can handle. On the local machine, you're using theloopback device which can handle 8K or more no problem. But onEthernet, which can only handle 1500 bytes with a header, you hit thatlimit. Over a modem, with 576 MTU (again, with header), you hit theeven lower limit.

You have to make sure all the data is being sent, first of all.(See the sendall()function implementation for details.) Once you're sure of that, then youneed to call recv() in a loop until all your datais read.

Read the section Son of DataEncapsulation for details on receiving complete packets of datausing multiple calls to recv().

I'm on a Windows box and I don't have thefork() system call or any kind of structsigaction. What to do?

If they're anywhere, they'll be in POSIX librariesthat may have shipped with your compiler. Since I don't have a Windowsbox, I really can't tell you the answer, but I seem to remember thatMicrosoft has a POSIX compatibility layer and that's wherefork() would be. (And maybe evensigaction.)

Search the help that came with VC++ for "fork" or "POSIX" and see if itgives you any clues.

If that doesn't work at all, ditch thefork()/sigaction stuff and replace it with theWin32 equivalent: CreateProcess(). I don't know howto use CreateProcess()—it takes a bazillionarguments, but it should be covered in the docs that came with VC++.

I'm behind a firewall—how do I let peopleoutside the firewall know my IP address so they can connect to mymachine?

Unfortunately, the purpose of a firewall is to preventpeople outside the firewall from connecting to machines inside thefirewall, so allowing them to do so is basically considered a breach ofsecurity.

This isn't to say that all is lost. For one thing, you can stilloften connect() through the firewall if it's doingsome kind of masquerading or NAT or something like that. Just designyour programs so that you're always the one initiating the connection,and you'll be fine.

If that's not satisfactory, youcan ask your sysadmins to poke a hole in the firewall so that people canconnect to you. The firewall can forward to you either through it's NATsoftware, or through a proxy or something like that.

Be aware that a hole in the firewall is nothing to be takenlightly. You have to make sure you don't give bad people access to theinternal network; if you're a beginner, it's a lot harder to makesoftware secure than you might imagine.

Don't make your sysadmin mad at me.;-)

How do I writea packet sniffer? How do I put my Ethernet interface into promiscuousmode?

For those not in the know, when a network card is in "promiscuousmode", it will forward ALL packets to the operating system, not justthose that were addressed to this particular machine. (We're talkingEthernet-layer addresses here, not IP addresses--but since ethernet islower-layer than IP, all IP addresses are effectively forwarded aswell. See the section Low Level Nonsense andNetwork Theory for more info.)

This is the basis for how a packet sniffer works. It puts theinterface into promiscuous mode, then the OS gets every single packetthat goes by on the wire. You'll have a socket of some type that youcan read this data from.

Unfortunately, the answer to the question varies depending on theplatform, but if you Google for, for instance, "windows promiscuous ioctl" you'll probably get somewhere. There's what lookslike a decent writeup in Linux Journal,as well.

How can I set a custom timeout value fora TCP or UDP socket?

It depends on your system. You might search the net for SO_RCVTIMEO and SO_SNDTIMEO (for use with setsockopt()) to see if your systemsupports such functionality.

The Linux man page suggests using alarm() orsetitimer() as a substitute.

How can I tell which ports are available to use? Is there a list of"official" port numbers?

Usually this isn't an issue. If you're writing, say, a webserver, then it's a good idea to use the well-known port 80 for yoursoftware. If you're writing just your own specialized server, thenchoose a port at random (but greater than 1023) and give it a try.

If the port is already in use, you'll get an "Address already in use"error when you try to bind(). Choose another port. (It'sa good idea to allow the user of your software to specify an alternateport either with a config file or a command line switch.)

There is a list of official portnumbers maintained by the Internet Assigned Numbers Authority(IANA). Just because something (over 1023) is in that list doesn't meanyou can't use the port. For instance, Id Software's DOOM uses the sameport as "mdqs", whatever that is. All that matters is that no one elseon the same machine is using that port when you want to useit.

9. Man Pages

In the Unix world, there are a lot of manuals.They have little sections that describe individual functions that youhave at your disposal.

Of course, manual would be too much of a thing to type. Imean, no one in the Unix world, including myself, likes to type thatmuch. Indeed I could go on and on at great length about how much Iprefer to be terse but instead I shall be brief and not bore you withlong-winded diatribes about how utterly amazingly brief I prefer to bein virtually all circumstances in their entirety.

[Applause]

Thank you. What I am getting at is that these pages are called "manpages" in the Unix world, and I have included my own personal truncatedvariant here for your reading enjoyment. The thing is, many of thesefunctions are way more general purpose than I'm letting on, but I'm onlygoing to present the parts that are relevant for Internet SocketsProgramming.

But wait! That's not all that's wrong with my man pages:

• They are incomplete and only show the basics from the guide.
• There are many more man pages than this in the real world.
• They are different than the ones on your system.
• The header files might be different for certain functions on yoursystem.
• The function parameters might be different for certain functions on yoursystem.

If you want the real information, check your local Unix man pages bytyping man whatever, where "whatever" is something thatyou're incredibly interested in, such as "accept". (I'm sureMicrosoft Visual Studio has something similar in their help section.But "man" is better because it is one byte more concise than "help".Unix wins again!)

So, if these are so flawed, why even include them at all in theGuide? Well, there are a few reasons, but the best are that (a) theseversions are geared specifically toward network programming and areeasier to digest than the real ones, and (b) these versions containexamples!

Oh! And speaking of the examples, I don't tend to put in all theerror checking because it really increases the length of the code. Butyou should absolutely do error checking pretty much any time you makeany of the system calls unless you're totally 100% sure it's not goingto fail, and you should probably do it even then!

9.1. accept()

Accept an incoming connection on a listening socket

Prototypes

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

Description

Once you've gone through the trouble of getting aSOCK_STREAM socket and setting itup for incoming connections with listen(), then you callaccept() to actually get yourself a new socket descriptorto use for subsequent communication with the newly connected client.

The old socket that you are using for listening is still there, andwill be used for further accept() calls as they comein.

accept() will normally block, and you can useselect() to peek on the listening socket descriptor aheadof time to see if it's "ready to read". If so, then there's a newconnection waiting to be accept()ed! Yay! Alternatively,you could set the O_NONBLOCK flagon the listening socket using fcntl(),and then it will never block, choosing instead to return-1 with errno set to EWOULDBLOCK.

The socket descriptor returned by accept() is a bonafide socket descriptor, open and connected to the remote host. You haveto close() it when you're done with it.

Return Value

accept() returns the newly connected socket descriptor,or -1 on error, with errno setappropriately.

Example

struct sockaddr_storage their_addr;socklen_t addr_size;struct addrinfo hints, *res;int sockfd, new_fd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_STREAM;hints.ai_flags = AI_PASSIVE;     // fill in my IP for megetaddrinfo(NULL, MYPORT, &hints, &res);// make a socket, bind it, and listen on it:sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);bind(sockfd, res->ai_addr, res->ai_addrlen);listen(sockfd, BACKLOG);// now accept an incoming connection:addr_size = sizeof their_addr;new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size);// ready to communicate on socket descriptor new_fd!

See Also

socket(),getaddrinfo(),listen(),struct sockaddr_in

9.2. bind()

Associate a socket with an IP address and port number

Prototypes

#include #include int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);

Description

When a remote machine wants to connect to yourserver program, it needs two pieces of information: theIP address and the port number.The bind() call allows you to do just that.

First, you call getaddrinfo() to load up a structsockaddr with the destination address and port information. Thenyou call socket() to get a socket descriptor, and then youpass the socket and address into bind(), and the IP addressand port are magically (using actual magic) bound to the socket!

If you don't know your IP address, or you know you only have one IPaddress on the machine, or you don't care which of the machine's IPaddresses is used, you can simply pass the AI_PASSIVEflag in the hints parameter togetaddrinfo(). What this does is fill in the IP addresspart of the struct sockaddr with a special value that tellsbind() that it should automatically fill in this host's IPaddress.

What what? What special value is loaded into the structsockaddr's IP address to cause it to auto-fill the address withthe current host? I'll tell you, but keep in mind this is only ifyou're filling out the struct sockaddr by hand; if not, usethe results from getaddrinfo(), as per above. In IPv4, thesin_addr.s_addr field of the structsockaddr_in structure is set to INADDR_ANY. InIPv6, the sin6_addr field of the structsockaddr_in6 structure is assigned into from the global variablein6addr_any. Or, if you're declaring a new structin6_addr, you can initialize it toIN6ADDR_ANY_INIT.

Lastly, the addrlen parameter should be set tosizeof my_addr.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

// modern way of doing things with getaddrinfo()struct addrinfo hints, *res;int sockfd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_STREAM;hints.ai_flags = AI_PASSIVE;     // fill in my IP for megetaddrinfo(NULL, "3490", &hints, &res);// make a socket:// (you should actually walk the "res" linked list and error-check!)sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);// bind it to the port we passed in to getaddrinfo():bind(sockfd, res->ai_addr, res->ai_addrlen);

// example of packing a struct by hand, IPv4struct sockaddr_in myaddr;int s;myaddr.sin_family = AF_INET;myaddr.sin_port = htons(3490);// you can specify an IP address:inet_pton(AF_INET, "63.161.169.137", &myaddr.sin_addr.s_addr);// or you can let it automatically select one:myaddr.sin_addr.s_addr = INADDR_ANY;s = socket(PF_INET, SOCK_STREAM, 0);bind(s, (struct sockaddr*)&myaddr, sizeof myaddr);

See Also

getaddrinfo(),socket(),struct sockaddr_in,struct in_addr

9.3. connect()

Connect a socket to a server

Prototypes

#include #include int connect(int sockfd, const struct sockaddr *serv_addr,            socklen_t addrlen);

Description

Once you've built a socket descriptor with thesocket() call, you can connect() that socketto a remote server using the well-named connect() systemcall. All you need to do is pass it the socket descriptor and theaddress of the server you're interested in getting to know better. (Oh,and the length of the address, which is commonly passed to functionslike this.)

Usually this information comes along as the result of a call togetaddrinfo(), but you can fill out your own structsockaddr if you want to.

If you haven't yet called bind() on the socketdescriptor, it is automatically bound to your IP address and a randomlocal port. This is usually just fine with you if you're not a server,since you really don't care what your local port is; you only care whatthe remote port is so you can put it in the serv_addrparameter. You can call bind() if you reallywant your client socket to be on a specific IP address and port, butthis is pretty rare.

Once the socket is connect()ed, you're free tosend() and recv() data on it to your heart'scontent.

Special note: if youconnect() a SOCK_DGRAM UDP socket to aremote host, you can use send() and recv() aswell as sendto() and recvfrom(). If youwant.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

// connect to www.example.com port 80 (http)struct addrinfo hints, *res;int sockfd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_STREAM;// we could put "80" instead on "http" on the next line:getaddrinfo("www.example.com", "http", &hints, &res);// make a socket:sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);// connect it to the address and port we passed in to getaddrinfo():connect(sockfd, res->ai_addr, res->ai_addrlen);

See Also

socket(),bind()

9.4. close()

Close a socket descriptor

Prototypes

#include int close(int s);

Description

After you've finished using the socket forwhatever demented scheme you have concocted and you don't want tosend() or recv() or, indeed, do anythingelse at all with the socket, you can close() it, andit'll be freed up, never to be used again.

The remote side can tell if this happens one of two ways. One: if theremote side calls recv(), it will return 0.Two: if the remote side calls send(), it'll receive asignal SIGPIPE and send() will return-1 and errno will be set to EPIPE.

Windows users: the function you need to useis called closesocket(), notclose(). If you try to use close() on asocket descriptor, it's possible Windows will get angry... And youwouldn't like it when it's angry.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

s = socket(PF_INET, SOCK_DGRAM, 0);...// a whole lotta stuff...*BRRRONNNN!*...close(s);  // not much to it, really.

See Also

socket(),shutdown()

9.5. getaddrinfo(), freeaddrinfo(),gai_strerror()

Get information about a host name and/or service and load up astruct sockaddr with the result.

Prototypes

#include #include #include int getaddrinfo(const char *nodename, const char *servname,                const struct addrinfo *hints, struct addrinfo **res);void freeaddrinfo(struct addrinfo *ai);const char *gai_strerror(int ecode);struct addrinfo {  int     ai_flags;          // AI_PASSIVE, AI_CANONNAME, ...  int     ai_family;         // AF_xxx  int     ai_socktype;       // SOCK_xxx  int     ai_protocol;       // 0 (auto) or IPPROTO_TCP, IPPROTO_UDP   socklen_t  ai_addrlen;     // length of ai_addr  char   *ai_canonname;      // canonical name for nodename  struct sockaddr  *ai_addr; // binary address  struct addrinfo  *ai_next; // next structure in linked list};

Description

getaddrinfo() is an excellent function that will returninformation on a particular host name (such as its IP address) and loadup a struct sockaddr for you, taking care of the grittydetails (like if it's IPv4 or IPv6.) It replaces the old functionsgethostbyname() and getservbyname().Thedescription, below, contains a lot of information that might be a littledaunting, but actual usage is pretty simple. It might be worth it tocheck out the examples first.

The host name that you're interested in goes in thenodename parameter. The address can be either a hostname, like "www.example.com", or an IPv4 or IPv6 address (passed as astring). This parameter can also be NULL if you're usingthe AI_PASSIVE flag (see below.)

The servname parameter is basically the port number.It can be a port number (passed as a string, like "80"), or it can be aservice name, like "http" or "tftp" or "smtp" or "pop", etc. Well-knownservice names can be found in the IANA PortList or in your /etc/services file.

Lastly, for input parameters, we have hints. This isreally where you get to define what the getaddinfo()function is going to do. Zero the whole structure before use withmemset(). Let's take a look at the fields you need to setup before use.

The ai_flags can be set to a variety of things, buthere are a couple important ones. (Multiple flags can be specified bybitwise-ORing them together with the | operator.)Check your man page for the complete list of flags.

AI_CANONNAME causes the ai_canonnameof the result to the filled out with the host's canonical (real) name.AI_PASSIVE causes the result's IP address tobe filled out with INADDR_ANY (IPv4)orin6addr_any (IPv6); this causes a subsequent call tobind() to auto-fill the IP address of the structsockaddr with the address of the current host. That's excellentfor setting up a server when you don't want to hardcode the address.

If you do use the AI_PASSIVE, flag, then you can passNULL in the nodename (sincebind() will fill it in for you later.)

Continuing on with the input paramters, you'll likely want to setai_family to AF_UNSPEC which tellsgetaddrinfo() to look for both IPv4 and IPv6 addresses.You can also restrict yourself to one or the other withAF_INET or AF_INET6.

Next, the socktype field should be set toSOCK_STREAM or SOCK_DGRAM, depending onwhich type of socket you want.

Finally, just leave ai_protocol at 0 toautomatically choose your protocol type.

Now, after you get all that stuff in there, you canfinally make the call to getaddrinfo()!

Of course, this is where the fun begins. The res willnow point to a linked list of struct addrinfos, and you cango through this list to get all the addresses that match what you passedin with the hints.

Now, it's possible to get some addresses that don't work for onereason or another, so what the Linux man page does is loops through thelist doing a call to socket() and connect()(or bind() if you're setting up a server with theAI_PASSIVE flag) until it succeeds.

Finally, when you're done with the linked list, you need to callfreeaddrinfo() to free up the memory (or it will be leaked,and Some People will get upset.)

Return Value

Returns zero on success, or nonzero on error. If it returns nonzero,you can use the function gai_strerror() to get a printableversion of the error code in the return value.

Example

// code for a client connecting to a server// namely a stream socket to www.example.com on port 80 (http)// either IPv4 or IPv6int sockfd;  struct addrinfo hints, *servinfo, *p;int rv;memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC; // use AF_INET6 to force IPv6hints.ai_socktype = SOCK_STREAM;if ((rv = getaddrinfo("www.example.com", "http", &hints, &servinfo)) != 0) {    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));    exit(1);}// loop through all the results and connect to the first we canfor(p = servinfo; p != NULL; p = p->ai_next) {    if ((sockfd = socket(p->ai_family, p->ai_socktype,            p->ai_protocol)) == -1) {        perror("socket");        continue;    }    if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {        close(sockfd);        perror("connect");        continue;    }    break; // if we get here, we must have connected successfully}if (p == NULL) {    // looped off the end of the list with no connection    fprintf(stderr, "failed to connect\n");    exit(2);}freeaddrinfo(servinfo); // all done with this structure

// code for a server waiting for connections// namely a stream socket on port 3490, on this host's IP// either IPv4 or IPv6.int sockfd;  struct addrinfo hints, *servinfo, *p;int rv;memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC; // use AF_INET6 to force IPv6hints.ai_socktype = SOCK_STREAM;hints.ai_flags = AI_PASSIVE; // use my IP addressif ((rv = getaddrinfo(NULL, "3490", &hints, &servinfo)) != 0) {    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));    exit(1);}// loop through all the results and bind to the first we canfor(p = servinfo; p != NULL; p = p->ai_next) {    if ((sockfd = socket(p->ai_family, p->ai_socktype,            p->ai_protocol)) == -1) {        perror("socket");        continue;    }    if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {        close(sockfd);        perror("bind");        continue;    }    break; // if we get here, we must have connected successfully}if (p == NULL) {    // looped off the end of the list with no successful bind    fprintf(stderr, "failed to bind socket\n");    exit(2);}freeaddrinfo(servinfo); // all done with this structure

See Also

gethostbyname(),getnameinfo()

9.6. gethostname()

Returns the name of the system

Prototypes

#include int gethostname(char *name, size_t len);

Description

Your system has a name. They all do. Thisis a slightly more Unixy thing than the rest of the networky stuff we'vebeen talking about, but it still has its uses.

For instance, you can get your host name, and then call gethostbyname() to find out yourIP address.

The parameter name should point to a buffer that will holdthe host name, and len is the size of that buffer in bytes.gethostname() won't overwrite the end of the buffer (itmight return an error, or it might just stop writing), and it willNUL-terminate the string if there's room for it in thebuffer.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

char hostname[128];gethostname(hostname, sizeof hostname);printf("My hostname: %s\n", hostname);

See Also

gethostbyname()

9.7. gethostbyname(), gethostbyaddr()

Get an IP address for a hostname, or vice-versa

Prototypes

#include #include struct hostent *gethostbyname(const char *name); // DEPRECATED!struct hostent *gethostbyaddr(const char *addr, int len, int type);

Description

PLEASENOTE: these two functions are superseded by getaddrinfo()and getnameinfo()! In particular,gethostbyname() doesn't work well with IPv6.

These functions map back and forth between host names and IPaddresses. For instance, if you have "www.example.com", you can usegethostbyname() to get its IP address and store it in astruct in_addr.

Conversely, if you have a struct in_addr or astruct in6_addr, you can use gethostbyaddr()to get the hostname back. gethostbyaddr() isIPv6 compatible, but you should use the newer shiniergetnameinfo() instead.

(If you have a string containing an IP address in dots-and-numbersformat that you want to look up the hostname of, you'd be better offusing getaddrinfo() with the AI_CANONNAMEflag.)

gethostbyname() takes a string like "www.yahoo.com", andreturns a struct hostent which contains tons ofinformation, including the IP address. (Otherinformation is the official host name, a list of aliases, the addresstype, the length of the addresses, and the list of addresses—it'sa general-purpose structure that's pretty easy to use for our specificpurposes once you see how.)

gethostbyaddr() takes a struct in_addr orstruct in6_addr and brings you up a corresponding host name(if there is one), so it's sort of the reverse ofgethostbyname(). As for parameters, even thoughaddr is a char*, you actually want to pass in apointer to a struct in_addr. len should besizeof(struct in_addr), and type should beAF_INET.

So what is this struct hostentthat gets returned? It has a number of fields that contain informationabout the host in question.

Return Value

Returns a pointer to a resultant struct hostent orsuccess, or NULL on error.

Instead of the normal perror() and all that stuff you'dnormally use for error reporting, these functions have parallel resultsin the variable h_errno, which can be printed using thefunctions herror() or hstrerror(). These work just like theclassic errno, perror(), andstrerror() functions you're used to.

Example

// THIS IS A DEPRECATED METHOD OF GETTING HOST NAMES// use getaddrinfo() instead!#include #include #include #include #include #include #include int main(int argc, char *argv[]){    int i;    struct hostent *he;    struct in_addr **addr_list;    if (argc != 2) {        fprintf(stderr,"usage: ghbn hostname\n");        return 1;    }    if ((he = gethostbyname(argv[1])) == NULL) {  // get the host info        herror("gethostbyname");        return 2;    }    // print information about this host:    printf("Official name is: %s\n", he->h_name);    printf("    IP addresses: ");    addr_list = (struct in_addr **)he->h_addr_list;    for(i = 0; addr_list[i] != NULL; i++) {        printf("%s ", inet_ntoa(*addr_list[i]));    }    printf("\n");    return 0;}

// THIS HAS BEEN SUPERCEDED// use getnameinfo() instead!struct hostent *he;struct in_addr ipv4addr;struct in6_addr ipv6addr;inet_pton(AF_INET, "192.0.2.34", &ipv4addr);he = gethostbyaddr(&ipv4addr, sizeof ipv4addr, AF_INET);printf("Host name: %s\n", he->h_name);inet_pton(AF_INET6, "2001:db8:63b3:1::beef", &ipv6addr);he = gethostbyaddr(&ipv6addr, sizeof ipv6addr, AF_INET6);printf("Host name: %s\n", he->h_name);

See Also

getaddrinfo(),getnameinfo(),gethostname(),errno,perror(),strerror(),struct in_addr

9.8. getnameinfo()

Look up the host name and service name information for a givenstruct sockaddr.

Prototypes

#include #include int getnameinfo(const struct sockaddr *sa, socklen_t salen,                char *host, size_t hostlen,                char *serv, size_t servlen, int flags);

Description

This function is the opposite of getaddrinfo(), that is,this function takes an already loaded struct sockaddr anddoes a name and service name lookup on it. It replaces the oldgethostbyaddr() and getservbyport()functions.

You have to pass in a pointer to a struct sockaddr(which in actuality is probably a struct sockaddr_in orstruct sockaddr_in6 that you've cast) in thesa parameter, and the length of that structin the salen.

The resultant host name and service name will be written to the areapointed to by the host and servparameters. Of course, you have to specify the max lengths of thesebuffers in hostlen and servlen.

Finally, there are several flags you can pass, but here a a couplegood ones. NI_NOFQDN will cause the hostto only contain the host name, not the whole domain name.NI_NAMEREQD will cause the function to fail if the namecannot be found with a DNS lookup (if you don't specify this flag andthe name can't be found, getnameinfo() will put a stringversion of the IP address in host instead.)

As always, check your local man pages for the full scoop.

Return Value

Returns zero on success, or non-zero on error. If the return valueis non-zero, it can be passed to gai_strerror() to get ahuman-readable string. See getaddrinfo for moreinformation.

Example

struct sockaddr_in6 sa; // could be IPv4 if you wantchar host[1024];char service[20];// pretend sa is full of good information about the host and port...getnameinfo(&sa, sizeof sa, host, sizeof host, service, sizeof service, 0);printf("   host: %s\n", host);    // e.g. "www.example.com"printf("service: %s\n", service); // e.g. "http"

See Also

getaddrinfo(),gethostbyaddr()

9.9. getpeername()

Return address info about the remote side of the connection

Prototypes

#include int getpeername(int s, struct sockaddr *addr, socklen_t *len);

Description

Once you have either accept()eda remote connection, or connect()ed to a server, you nowhave what is known as a peer. Your peer is simply thecomputer you're connected to, identified by an IPaddress and a port.So...

getpeername() simply returns a structsockaddr_in filled with information about the machine you'reconnected to.

Why is it called a "name"? Well, there are a lot of different kindsof sockets, not just Internet Sockets like we're using in this guide,and so "name" was a nice generic term that covered all cases. In ourcase, though, the peer's "name" is it's IP address and port.

Although the function returns the size of the resultant address inlen, you must preload len with the size ofaddr.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

// assume s is a connected socketsocklen_t len;struct sockaddr_storage addr;char ipstr[INET6_ADDRSTRLEN];int port;len = sizeof addr;getpeername(s, (struct sockaddr*)&addr, &len);// deal with both IPv4 and IPv6:if (addr.ss_family == AF_INET) {    struct sockaddr_in *s = (struct sockaddr_in *)&addr;    port = ntohs(s->sin_port);    inet_ntop(AF_INET, &s->sin_addr, ipstr, sizeof ipstr);} else { // AF_INET6    struct sockaddr_in6 *s = (struct sockaddr_in6 *)&addr;    port = ntohs(s->sin6_port);    inet_ntop(AF_INET6, &s->sin6_addr, ipstr, sizeof ipstr);}printf("Peer IP address: %s\n", ipstr);printf("Peer port      : %d\n", port);

See Also

gethostname(),gethostbyname(),gethostbyaddr()

9.10. errno

Holds the error code for the last system call

Prototypes

#include int errno;

Description

This is the variable that holds error informationfor a lot of system calls. If you'll recall, things likesocket() and listen() return -1on error, and they set the exact value of errno to let youknow specifically which error occurred.

The header file errno.h lists a bunch of constantsymbolic names for errors, such as EADDRINUSE,EPIPE, ECONNREFUSED, etc. Your local manpages will tell you what codes can be returned as an error, and you canuse these at run time to handle different errors in different ways.

Or, more commonly, you can call perror() or strerror() to get a human-readableversion of the error.

One thing to note, for you multithreading enthusiasts, is that onmost systems errno is defined in a threadsafe manner. (Thatis, it's not actually a global variable, but it behaves just like aglobal variable would in a single-threaded environment.)

Return Value

The value of the variable is the latest error to have transpired, whichmight be the code for "success" if the last action succeeded.

Example

s = socket(PF_INET, SOCK_STREAM, 0);if (s == -1) {    perror("socket"); // or use strerror()}tryagain:if (select(n, &readfds, NULL, NULL) == -1) {    // an error has occurred!!    // if we were only interrupted, just restart the select() call:    if (errno == EINTR) goto tryagain;  // AAAA!  goto!!!    // otherwise it's a more serious error:    perror("select");    exit(1);}

See Also

perror(),strerror()

9.11. fcntl()

Control socket descriptors

Prototypes

#include #include int fcntl(int s, int cmd, long arg);

Description

This function is typically used to do file lockingand other file-oriented stuff, but it also has a couple socket-relatedfunctions that you might see or use from time to time.

Parameter s is the socket descriptor you wish to operateon, cmd should be set to F_SETFL, and arg can be one ofthe following commands. (Like I said, there's more tofcntl() than I'm letting on here, but I'm trying to staysocket-oriented.)

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Different uses of the fcntl() system call actually havedifferent return values, but I haven't covered them here because they'renot socket-related. See your local fcntl() man page formore information.

Example

int s = socket(PF_INET, SOCK_STREAM, 0);fcntl(s, F_SETFL, O_NONBLOCK);  // set to non-blockingfcntl(s, F_SETFL, O_ASYNC);     // set to asynchronous I/O

See Also

Blocking,send()

9.12. htons(), htonl(),ntohs(), ntohl()

Convert multi-byte integer types from host byte order tonetwork byte order

Prototypes

#include uint32_t htonl(uint32_t hostlong);uint16_t htons(uint16_t hostshort);uint32_t ntohl(uint32_t netlong);uint16_t ntohs(uint16_t netshort);

Description

Just to make you really unhappy, different computers usedifferent byte orderings internally for their multibyte integers (i.e.any integer that's larger than a char.) The upshot ofthis is that if you send() a two-byte shortint from an Intel box to a Mac (before they became Intel boxes,too, I mean), what one computer thinks is the number 1,the other will think is the number 256, andvice-versa.

The way to get around this problem is foreveryone to put aside their differences and agree that Motorola and IBMhad it right, and Intel did it the weird way, and so we all convert ourbyte orderings to "big-endian" before sending them out. Since Intel isa "little-endian" machine, it's far more politically correct to call ourpreferred byte ordering "Network Byte Order". So these functionsconvert from your native byte order to network byte order and backagain.

(This means on Intel these functions swap all the bytes around, andon PowerPC they do nothing because the bytes are already in NetworkByte Order. But you should always use them in your code anyway, sincesomeone might want to build it on an Intel machine and still have thingswork properly.)

Note that the types involved are 32-bit (4 byte, probablyint) and 16-bit (2 byte, very likely short)numbers. 64-bit machines might have a htonll() for 64-bitints, but I've not seen it. You'll just have to write yourown.

Anyway, the way these functions work is that you first decide ifyou're converting from host (your machine's) byte order orfrom network byte order. If "host", the the first letter of thefunction you're going to call is "h". Otherwise it's "n" for "network".The middle of the function name is always "to" because you're convertingfrom one "to" another, and the penultimate letter shows what you're convertingto. The last letter is the size of the data, "s" for short,or "l" for long. Thus:

Return Value

Each function returns the converted value.

Example

uint32_t some_long = 10;uint16_t some_short = 20;uint32_t network_byte_order;// convert and sendnetwork_byte_order = htonl(some_long);send(s, &network_byte_order, sizeof(uint32_t), 0);some_short == ntohs(htons(some_short)); // this  is true

9.13. inet_ntoa(), inet_aton(),inet_addr

Convert IP addresses from a dots-and-number string to astruct in_addr and back

Prototypes

#include #include #include // ALL THESE ARE DEPRECATED!  Use inet_pton()  or inet_ntop() instead!!char *inet_ntoa(struct in_addr in);int inet_aton(const char *cp, struct in_addr *inp);in_addr_t inet_addr(const char *cp);

Description

These functions are deprecated because they don't handle IPv6!Use inet_ntop() or inet_pton() instead! Theyare included here because they can still be found in the wild.

All of these functions convert from a struct in_addr (partof your struct sockaddr_in, most likely) to a string indots-and-numbers format (e.g. "192.168.5.10") and vice-versa. If youhave an IP address passed on the command line or something, this is theeasiest way to get a struct in_addr toconnect() to, or whatever. If you need more power, trysome of the DNS functions like gethostbyname() or attempt acoup d'état in your local country.

The function inet_ntoa() converts a network address in astruct in_addr to a dots-and-numbers format string. The"n" in "ntoa" stands for network, and the "a" stands for ASCII forhistorical reasons (so it's "Network To ASCII"—the "toa" suffixhas an analogous friend in the C library called atoi()which converts an ASCII string to an integer.)

The function inet_aton() is the opposite, convertingfrom a dots-and-numbers string into a in_addr_t (which isthe type of the field s_addr in your structin_addr.)

Finally, the function inet_addr() is an older functionthat does basically the same thing as inet_aton(). It'stheoretically deprecated, but you'll see it a lot and the police won'tcome get you if you use it.

Return Value

inet_aton() returns non-zero if the address is a validone, and it returns zero if the address is invalid.

inet_ntoa() returns the dots-and-numbers string in astatic buffer that is overwritten with each call to the function.

inet_addr() returns the address as anin_addr_t, or -1 if there's an error. (Thatis the same result as if you tried to convert the string "255.255.255.255", which is a valid IP address.This is why inet_aton() is better.)

Example

struct sockaddr_in antelope;char *some_addr;inet_aton("10.0.0.1", &antelope.sin_addr); // store IP in antelopesome_addr = inet_ntoa(antelope.sin_addr); // return the IPprintf("%s\n", some_addr); // prints "10.0.0.1"// and this call is the same as the inet_aton() call, above:antelope.sin_addr.s_addr = inet_addr("10.0.0.1");

See Also

inet_ntop(),inet_pton(),gethostbyname(),gethostbyaddr()

9.14. inet_ntop(), inet_pton()

Convert IP addresses to human-readable form andback.

Prototypes

#include const char *inet_ntop(int af, const void *src,                      char *dst, socklen_t size);int inet_pton(int af, const char *src, void *dst);

Description

These functions are for dealing with human-readable IP addresses andconverting them to their binary representation for use with variousfunctions and system calls. The "n" stands for "network", and "p" for"presentation". Or "text presentation". But you can think of it as"printable". "ntop" is "network to printable". See?

Sometimes you don't want to look at a pile of binary numbers whenlooking at an IP address. You want it in a nice printable form, like192.0.2.180, or 2001:db8:8714:3a90::12. In that case,inet_ntop() is for you.

inet_ntop() takes the address family in theaf parameter (either AF_INET orAF_INET6). The src parameter should be apointer to either a struct in_addr or structin6_addr containing the address you wish to convert to a string.Finally dst and size are the pointer tothe destination string and the maximum length of that string.

What should the maximum length of the dst string be?What is the maximum length for IPv4 and IPv6 addresses? Fortunatelythere are a couple of macros to help you out. The maximum lengths are:INET_ADDRSTRLEN and INET6_ADDRSTRLEN.

Other times, you might have a string containing an IP address inreadable form, and you want to pack it into a structsockaddr_in or a struct sockaddr_in6. In that case,the opposite funcion inet_pton() is what you're after.

inet_pton() also takes an address family (eitherAF_INET or AF_INET6) in theaf parameter. The src parameter is apointer to a string containing the IP address in printable form. Lastlythe dst parameter points to where the result should bestored, which is probably a struct in_addr or structin6_addr.

These functions don't do DNS lookups—you'll needgetaddinfo() for that.

Return Value

inet_ntop() returns the dst parameter onsuccess, or NULL on failure (and errno isset).

inet_pton() returns 1 on success. Itreturns -1 if there was an error (errno isset), or 0 if the input isn't a valid IP address.

Example

// IPv4 demo of inet_ntop() and inet_pton()struct sockaddr_in sa;char str[INET_ADDRSTRLEN];// store this IP address in sa:inet_pton(AF_INET, "192.0.2.33", &(sa.sin_addr));// now get it back and print itinet_ntop(AF_INET, &(sa.sin_addr), str, INET_ADDRSTRLEN);printf("%s\n", str); // prints "192.0.2.33"

// IPv6 demo of inet_ntop() and inet_pton()// (basically the same except with a bunch of 6s thrown around)struct sockaddr_in6 sa;char str[INET6_ADDRSTRLEN];// store this IP address in sa:inet_pton(AF_INET6, "2001:db8:8714:3a90::12", &(sa.sin6_addr));// now get it back and print itinet_ntop(AF_INET6, &(sa.sin6_addr), str, INET6_ADDRSTRLEN);printf("%s\n", str); // prints "2001:db8:8714:3a90::12"

// Helper function you can use://Convert a struct sockaddr address to a string, IPv4 and IPv6:char *get_ip_str(const struct sockaddr *sa, char *s, size_t maxlen){    switch(sa->sa_family) {        case AF_INET:            inet_ntop(AF_INET, &(((struct sockaddr_in *)sa)->sin_addr),                    s, maxlen);            break;        case AF_INET6:            inet_ntop(AF_INET6, &(((struct sockaddr_in6 *)sa)->sin6_addr),                    s, maxlen);            break;        default:            strncpy(s, "Unknown AF", maxlen);            return NULL;    }    return s;}

See Also

getaddrinfo()

9.15. listen()

Tell a socket to listen for incoming connections

Prototypes

#include int listen(int s, int backlog);

Description

You can take your socket descriptor (made withthe socket() system call) and tell it to listen forincoming connections. This is what differentiates the servers from theclients, guys.

The backlog parameter can mean a couple different thingsdepending on the system you on, but loosely it is how many pendingconnections you can have before the kernel starts rejecting new ones.So as the new connections come in, you should be quick toaccept() them so that the backlog doesn't fill. Trysetting it to 10 or so, and if your clients start getting "Connectionrefused" under heavy load, set it higher.

Before calling listen(), your server should callbind() to attach itself to a specific port number. Thatport number (on the server's IP address) will be the one that clientsconnect to.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

struct addrinfo hints, *res;int sockfd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_STREAM;hints.ai_flags = AI_PASSIVE;     // fill in my IP for megetaddrinfo(NULL, "3490", &hints, &res);// make a socket:sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);// bind it to the port we passed in to getaddrinfo():bind(sockfd, res->ai_addr, res->ai_addrlen);listen(sockfd, 10); // set s up to be a server (listening) socket// then have an accept() loop down here somewhere

See Also

accept(),bind(),socket()

9.16. perror(), strerror()

Print an error as a human-readable string

Prototypes

#include #include    // for strerror()void perror(const char *s);char *strerror(int errnum);

Description

Since so many functionsreturn -1 on error and set the value of the variable errno to be some number, it would sure be niceif you could easily print that in a form that made sense to you.

Mercifully, perror() does that. If you want moredescription to be printed before the error, you can point the parameters to it (or you can leave s as NULLand nothing additional will be printed.)

In a nutshell, this function takes errno values, likeECONNRESET, and prints them nicely, like "Connectionreset by peer."

The function strerror() is very similar toperror(), except it returns a pointer to the error messagestring for a given value (you usually pass in the variableerrno.)

Return Value

strerror() returns a pointer to the error messagestring.

Example

int s;s = socket(PF_INET, SOCK_STREAM, 0);if (s == -1) { // some error has occurred    // prints "socket error: " + the error message:    perror("socket error");}// similarly:if (listen(s, 10) == -1) {    // this prints "an error: " + the error message from errno:    printf("an error: %s\n", strerror(errno));}

See Also

errno

9.17. poll()

Test for events on multiple sockets simultaneously

Prototypes

#include int poll(struct pollfd *ufds, unsigned int nfds, int timeout);

Description

This function is very similar toselect() in that they both watch sets of file descriptorsfor events, such as incoming data ready to recv(), socketready to send() data to, out-of-band data ready torecv(), errors, etc.

The basic idea is that you pass an array of nfdsstruct pollfds in ufds, along with a timeout inmilliseconds (1000 milliseconds in a second.) The timeoutcan be negative if you want to wait forever. If no event happens on anyof the socket descriptors by the timeout, poll() willreturn.

Each element in the array of struct pollfds representsone socket descriptor, and contains the following fields:

struct pollfd {    int fd;         // the socket descriptor    short events;   // bitmap of events we're interested in    short revents;  // when poll() returns, bitmap of events that occurred};

Before calling poll(), load fd with thesocket descriptor (if you set fd to a negative number, thisstruct pollfd is ignored and its revents fieldis set to zero) and then construct the events field bybitwise-ORing the following macros:

Once the poll() call returns, the reventsfield will be constructed as a bitwise-OR of the above fields, tellingyou which descriptors actually have had that event occur. Additionally,these other fields might be present:

Return Value

Returns the number of elements in the ufds array that havehad event occur on them; this can be zero if the timeout occurred. Alsoreturns -1 on error (and errno will be setaccordingly.)

Example

int s1, s2;int rv;char buf1[256], buf2[256];struct pollfd ufds[2];s1 = socket(PF_INET, SOCK_STREAM, 0);s2 = socket(PF_INET, SOCK_STREAM, 0);// pretend we've connected both to a server at this point//connect(s1, ...)...//connect(s2, ...)...// set up the array of file descriptors.//// in this example, we want to know when there's normal or out-of-band// data ready to be recv()'d...ufds[0].fd = s1;ufds[0].events = POLLIN | POLLPRI; // check for normal or out-of-bandufds[1] = s2;ufds[1].events = POLLIN; // check for just normal data// wait for events on the sockets, 3.5 second timeoutrv = poll(ufds, 2, 3500);if (rv == -1) {    perror("poll"); // error occurred in poll()} else if (rv == 0) {    printf("Timeout occurred!  No data after 3.5 seconds.\n");} else {    // check for events on s1:    if (ufds[0].revents & POLLIN) {        recv(s1, buf1, sizeof buf1, 0); // receive normal data    }    if (ufds[0].revents & POLLPRI) {        recv(s1, buf1, sizeof buf1, MSG_OOB); // out-of-band data    }    // check for events on s2:    if (ufds[1].revents & POLLIN) {        recv(s1, buf2, sizeof buf2, 0);    }}

See Also

select()

9.18. recv(), recvfrom()

Receive data on a socket

Prototypes

#include #include ssize_t recv(int s, void *buf, size_t len, int flags);ssize_t recvfrom(int s, void *buf, size_t len, int flags,                 struct sockaddr *from, socklen_t *fromlen);

Description

Once you have a socket up andconnected, you can read incoming data from the remote side using therecv() (for TCP SOCK_STREAM sockets) andrecvfrom() (for UDP SOCK_DGRAM sockets).

Both functions take the socket descriptor s, a pointer tothe buffer buf, the size (in bytes) of the bufferlen, and a set of flags that control how thefunctions work.

Additionally, the recvfrom() takes astruct sockaddr*,from that will tell you where the data came from, and willfill in fromlen with the size of structsockaddr. (You must also initialize fromlen to be thesize of from or struct sockaddr.)

So what wondrous flags can you pass into this function? Here aresome of them, but you should check your local man pages for moreinformation and what is actually supported on your system. Youbitwise-or these together, or just set flags to0 if you want it to be a regular vanillarecv().

When you call recv(), it will block until there is somedata to read. If you want to not block, set the socket to non-blockingor check with select() or poll() to see ifthere is incoming data before calling recv() orrecvfrom().

Return Value

Returns the number of bytes actually received (which might be lessthan you requested in the len parameter), or -1on error (and errno will be set accordingly.)

If the remote side has closed the connection, recv()will return 0. This is the normal method for determiningif the remote side has closed the connection. Normality is good,rebel!

Example

// stream sockets and recv()struct addrinfo hints, *res;int sockfd;char buf[512];int byte_count;// get host info, make socket, and connect itmemset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_STREAM;getaddrinfo("www.example.com", "3490", &hints, &res);sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);connect(sockfd, res->ai_addr, res->ai_addrlen);// all right!  now that we're connected, we can receive some data!byte_count = recv(sockfd, buf, sizeof buf, 0);printf("recv()'d %d bytes of data in buf\n", byte_count);

// datagram sockets and recvfrom()struct addrinfo hints, *res;int sockfd;int byte_count;socklen_t fromlen;struct sockaddr_storage addr;char buf[512];char ipstr[INET6_ADDRSTRLEN];// get host info, make socket, bind it to port 4950memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whicheverhints.ai_socktype = SOCK_DGRAM;hints.ai_flags = AI_PASSIVE;getaddrinfo(NULL, "4950", &hints, &res);sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);bind(sockfd, res->ai_addr, res->ai_addrlen);// no need to accept(), just recvfrom():fromlen = sizeof addr;byte_count = recvfrom(sockfd, buf, sizeof buf, 0, &addr, &fromlen);printf("recv()'d %d bytes of data in buf\n", byte_count);printf("from IP address %s\n",    inet_ntop(addr.ss_family,        addr.ss_family == AF_INET?            ((struct sockadd_in *)&addr)->sin_addr:            ((struct sockadd_in6 *)&addr)->sin6_addr,        ipstr, sizeof ipstr);

See Also

send(),sendto(),select(),poll(),Blocking

9.19. select()

Check if sockets descriptors are ready to read/write

Prototypes

#include int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,           struct timeval *timeout);FD_SET(int fd, fd_set *set);FD_CLR(int fd, fd_set *set);FD_ISSET(int fd, fd_set *set);FD_ZERO(fd_set *set);

Description

The select() function gives you away to simultaneously check multiple sockets to see if they have datawaiting to be recv()d, or if you can send()data to them without blocking, or if some exception has occurred.

You populate your sets of socket descriptors using the macros, likeFD_SET(), above. Once you have the set, you pass it intothe function as one of the following parameters: readfds ifyou want to know when any of the sockets in the set is ready torecv() data, writefds if any of the sockets isready to send() data to, and/or exceptfds if youneed to know when an exception (error) occurs on any of the sockets.Any or all of these parameters can be NULL if you're notinterested in those types of events. After select()returns, the values in the sets will be changed to show which are readyfor reading or writing, and which have exceptions.

The first parameter, n is the highest-numbered socketdescriptor (they're just ints, remember?) plus one.

Lastly, the struct timeval,timeout, at the end—this lets you tellselect() how long to check these sets for. It'll returnafter the timeout, or when an event occurs, whichever is first. Thestruct timeval has two fields: tv_sec is thenumber of seconds, to which is added tv_usec, the number ofmicroseconds (1,000,000 microseconds in a second.)

The helper macros do the following:

Return Value

Returns the number of descriptors in the set on success,0 if the timeout was reached, or -1 onerror (and errno will be set accordingly.) Also, the setsare modified to show which sockets are ready.

Example

int s1, s2, n;fd_set readfds;struct timeval tv;char buf1[256], buf2[256];// pretend we've connected both to a server at this point//s1 = socket(...);//s2 = socket(...);//connect(s1, ...)...//connect(s2, ...)...// clear the set ahead of timeFD_ZERO(&readfds);// add our descriptors to the setFD_SET(s1, &readfds);FD_SET(s2, &readfds);// since we got s2 second, it's the "greater", so we use that for// the n param in select()n = s2 + 1;// wait until either socket has data ready to be recv()d (timeout 10.5 secs)tv.tv_sec = 10;tv.tv_usec = 500000;rv = select(n, &readfds, NULL, NULL, &tv);if (rv == -1) {    perror("select"); // error occurred in select()} else if (rv == 0) {    printf("Timeout occurred!  No data after 10.5 seconds.\n");} else {    // one or both of the descriptors have data    if (FD_ISSET(s1, &readfds)) {        recv(s1, buf1, sizeof buf1, 0);    }    if (FD_ISSET(s2, &readfds)) {        recv(s1, buf2, sizeof buf2, 0);    }}

See Also

poll()

9.20. setsockopt(), getsockopt()

Set various options for a socket

Prototypes

#include #include int getsockopt(int s, int level, int optname, void *optval,               socklen_t *optlen);int setsockopt(int s, int level, int optname, const void *optval,               socklen_t optlen);

Description

Sockets are fairlyconfigurable beasts. In fact, they are so configurable, I'm not evengoing to cover it all here. It's probably system-dependent anyway. ButI will talk about the basics.

Obviously, these functions get and set certain options on a socket.On a Linux box, all the socket information is in the man page for socketin section 7. (Type: "man 7 socket" to get all thesegoodies.)

As for parameters, s is the socket you're talking about,level should be set to SOL_SOCKET.Then you set the optname to the name you're interested in.Again, see your man page for all the options, but here are some of themost fun ones:

As for the parameter optval, it's usually a pointer to anint indicating the value in question. For booleans, zerois false, and non-zero is true. And that's an absolute fact, unlessit's different on your system. If there is no parameter to be passed,optval can be NULL.

The final parameter, optlen, is filled out for you bygetsockopt() and you have to specify it forsetsockopt(), where it will probably besizeof(int).

Warning: on some systems (notably Sun and Windows), the optioncan be a char instead of an int, and is setto, for example, a character value of '1' instead of anint value of 1. Again, check your own manpages for more info with "man setsockopt" and "man 7socket"!

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

int optval;int optlen;char *optval2;// set SO_REUSEADDR on a socket to true (1):optval = 1;setsockopt(s1, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval);// bind a socket to a device name (might not work on all systems):optval2 = "eth1"; // 4 bytes long, so 4, below:setsockopt(s2, SOL_SOCKET, SO_BINDTODEVICE, optval2, 4);// see if the SO_BROADCAST flag is set:getsockopt(s3, SOL_SOCKET, SO_BROADCAST, &optval, &optlen);if (optval != 0) {    print("SO_BROADCAST enabled on s3!\n");}

See Also

fcntl()

9.21. send(), sendto()

Send data out over a socket

Prototypes

#include #include ssize_t send(int s, const void *buf, size_t len, int flags);ssize_t sendto(int s, const void *buf, size_t len,               int flags, const struct sockaddr *to,               socklen_t tolen);

Description

These functions send data to asocket. Generally speaking, send() is used for TCP SOCK_STREAM connected sockets, andsendto() is used for UDP SOCK_DGRAM unconnected datagramsockets. With the unconnected sockets, you must specify the destinationof a packet each time you send one, and that's why the last parametersof sendto() define where the packet is going.

With both send() and sendto(), theparameter s is the socket, buf is a pointer to thedata you want to send, len is the number of bytes you want tosend, and flags allows you to specify more information abouthow the data is to be sent. Set flags to zero if you want itto be "normal" data. Here are some of the commonly used flags, butcheck your local send() man pages for more details:

Return Value

Returns the number of bytes actually sent, or -1 onerror (and errno will be set accordingly.) Note that thenumber of bytes actually sent might be less than the number you asked itto send! See the section on handling partialsend()s for a helper function to get around this.

Also, if the socket has been closed by either side, the processcalling send() will get the signal SIGPIPE.(Unless send() was called with theMSG_NOSIGNAL flag.)

Example

int spatula_count = 3490;char *secret_message = "The Cheese is in The Toaster";int stream_socket, dgram_socket;struct sockaddr_in dest;int temp;// first with TCP stream sockets:// assume sockets are made and connected//stream_socket = socket(...//connect(stream_socket, ...// convert to network byte ordertemp = htonl(spatula_count);// send data normally:send(stream_socket, &temp, sizeof temp, 0);// send secret message out of band:send(stream_socket, secret_message, strlen(secret_message)+1, MSG_OOB);// now with UDP datagram sockets://getaddrinfo(...//dest = ...  // assume "dest" holds the address of the destination//dgram_socket = socket(...// send secret message normally:sendto(dgram_socket, secret_message, strlen(secret_message)+1, 0,        (struct sockaddr*)&dest, sizeof dest);

See Also

recv(),recvfrom()

9.22. shutdown()

Stop further sends and receives on a socket

Prototypes

#include int shutdown(int s, int how);

Description

That's it! I've had it! No moresend()s are allowed on this socket, but I still want torecv() data on it! Or vice-versa! How can I do this?

When you close() a socket descriptor, it closes bothsides of the socket for reading and writing, and frees the socketdescriptor. If you just want to close one side or the other, you canuse this shutdown() call.

As for parameters, s is obviously the socket you want toperform this action on, and what action that is can be specified withthe how parameter. How can be SHUT_RD toprevent further recv()s, SHUT_WR to prohibitfurther send()s, or SHUT_RDWR to doboth.

Note that shutdown() doesn't free up the socketdescriptor, so you still have to eventually close() thesocket even if it has been fully shut down.

This is a rarely used system call.

Return Value

Returns zero on success, or -1 on error (anderrno will be set accordingly.)

Example

int s = socket(PF_INET, SOCK_STREAM, 0);// ...do some send()s and stuff in here...// and now that we're done, don't allow any more sends()s:shutdown(s, SHUT_WR);

See Also

close()

9.23. socket()

Allocate a socket descriptor

Prototypes

#include #include int socket(int domain, int type, int protocol);

Description

Returns a new socket descriptor that you can useto do sockety things with. This is generally the first call in thewhopping process of writing a socket program, and you can use the resultfor subsequent calls to listen(), bind(),accept(), or a variety of other functions.

In usual usage, you get the values for these parameters from a callto getaddrinfo(), as shown in the example below. But youcan fill them in by hand if you really want to.

Return Value

The new socket descriptor to be used in subsequent calls, or-1 on error (and errno will be setaccordingly.)

Example

struct addrinfo hints, *res;int sockfd;// first, load up address structs with getaddrinfo():memset(&hints, 0, sizeof hints);hints.ai_family = AF_UNSPEC;     // AF_INET, AF_INET6, or AF_UNSPEChints.ai_socktype = SOCK_STREAM; // SOCK_STREAM or SOCK_DGRAMgetaddrinfo("www.example.com", "3490", &hints, &res);// make a socket using the information gleaned from getaddrinfo():sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);

See Also

accept(),bind(),getaddrinfo(),listen()

9.24. struct sockaddr and pals

Structures for handling internet addresses

Prototypes

include // All pointers to socket address structures are often cast to pointers// to this type before use in various functions and system calls:struct sockaddr {    unsigned short    sa_family;    // address family, AF_xxx    char              sa_data[14];  // 14 bytes of protocol address};// IPv4 AF_INET sockets:struct sockaddr_in {    short            sin_family;   // e.g. AF_INET, AF_INET6    unsigned short   sin_port;     // e.g. htons(3490)    struct in_addr   sin_addr;     // see struct in_addr, below    char             sin_zero[8];  // zero this if you want to};struct in_addr {    unsigned long s_addr;          // load with inet_pton()};// IPv6 AF_INET6 sockets:struct sockaddr_in6 {    u_int16_t       sin6_family;   // address family, AF_INET6    u_int16_t       sin6_port;     // port number, Network Byte Order    u_int32_t       sin6_flowinfo; // IPv6 flow information    struct in6_addr sin6_addr;     // IPv6 address    u_int32_t       sin6_scope_id; // Scope ID};struct in6_addr {    unsigned char   s6_addr[16];   // load with inet_pton()};// General socket address holding structure, big enough to hold either// struct sockaddr_in or struct sockaddr_in6 data:struct sockaddr_storage {    sa_family_t  ss_family;     // address family    // all this is padding, implementation specific, ignore it:    char      __ss_pad1[_SS_PAD1SIZE];    int64_t   __ss_align;    char      __ss_pad2[_SS_PAD2SIZE];};

Description

These are thebasic structures for all syscalls and functions that deal with internetaddresses. Often you'll use getaddinfo() to fill thesestructures out, and then will read them when you have to.

In memory, the struct sockaddr_in and structsockaddr_in6 share the same beginning structure as struct sockaddr, and you can freelycast the pointer of one type to the other without any harm, except thepossible end of the universe.

Just kidding on that end-of-the-universe thing...if the universe doesend when you cast a struct sockaddr_in* to a structsockaddr*, I promise you it's pure coincidence and you shouldn'teven worry about it.

So, with that in mind, remember that whenever a function says ittakes a struct sockaddr* you can cast your structsockaddr_in*, struct sockaddr_in6*, or structsockadd_storage* to that type with ease and safety.

struct sockaddr_in is the structure used with IPv4addresses (e.g. "192.0.2.10"). It holds an address family(AF_INET), a port in sin_port, and an IPv4address in sin_addr.

There's also this sin_zero field in structsockaddr_in which some people claim must be set to zero. Otherpeople don't claim anything about it (the Linux documentation doesn'teven mention it at all), and setting it to zero doesn't seem to beactually necessary. So, if you feel like it, set it to zero usingmemset().

Now, that struct in_addr is a weird beast on differentsystems. Sometimes it's a crazy union with all kinds of#defines and other nonsense. But what you should do is onlyuse the s_addr field in this structure, because many systemsonly implement that one.

struct sockadd_in6 and struct in6_addr arevery similar, except they're used for IPv6.

struct sockaddr_storage is a struct you can pass toaccept() or recvfrom() when you're trying towrite IP version-agnostic code and you don't know if the new address isgoing to be IPv4 or IPv6. The struct sockaddr_storagestructure is large enough to hold both types, unlike the original smallstruct sockaddr.

Example

// IPv4:struct sockaddr_in ip4addr;int s;ip4addr.sin_family = AF_INET;ip4addr.sin_port = htons(3490);inet_pton(AF_INET, "10.0.0.1", &ip4addr.sin_addr);s = socket(PF_INET, SOCK_STREAM, 0);bind(s, (struct sockaddr*)&ip4addr, sizeof ip4addr);

// IPv6:struct sockaddr_in6 ip6addr;int s;ip6addr.sin6_family = AF_INET6;ip6addr.sin6_port = htons(4950);inet_pton(AF_INET6, "2001:db8:8714:3a90::12", &ip6addr.sin6_addr);s = socket(PF_INET6, SOCK_STREAM, 0);bind(s, (struct sockaddr*)&ip6addr, sizeof ip6addr);

See Also

accept(),bind(),connect(),inet_aton(),inet_ntoa()

10. More References

You've come this far, and now you're screaming for more! Whereelse can you go to learn more about all this stuff?

10.1. Books

For old-school actualhold-it-in-your-hand pulp paper books, try some of the followingexcellent books. I used to be an affiliate with a very popular internetbookseller, but their new customer tracking system is incompatible witha print document. As such, I get no more kickbacks. If you feelcompassion for my plight, paypal a donation tobeej@beej.us. :-)

Unix Network Programming, volumes 1-2 by W.Richard Stevens. Published by Prentice Hall. ISBNs for volumes 1-2:0131411551,0130810819.

Internetworking with TCP/IP, volumes I-III byDouglas E. Comer and David L. Stevens. Published by Prentice Hall.ISBNs for volumes I, II, and III:0131876716,0130319961,0130320714.

TCP/IP Illustrated, volumes 1-3 by W.Richard Stevens and Gary R. Wright. Published by Addison Wesley. ISBNsfor volumes 1, 2, and 3 (and a 3-volume set):0201633469,020163354X,0201634953,(0201776316).

TCP/IP Network Administration by CraigHunt. Published by O'Reilly & Associates, Inc. ISBN0596002971.

Advanced Programming in the UNIXEnvironment by W. Richard Stevens. Published by AddisonWesley. ISBN0201433079.

10.2. Web References

On the web:

BSD Sockets: A Quick AndDirty Primer (Unix system programming info,too!)

The Unix SocketFAQ

Intro toTCP/IP

TCP/IPFAQ

The WinsockFAQ

And here are some relevant Wikipedia pages:

BerkeleySockets

Internet Protocol(IP)

Transmission Control Protocol(TCP)

User Datagram Protocol(UDP)

Client-Server

Serialization (packing and unpackingdata)

10.3. RFCs

RFCs—the realdirt! These are documents that describe assigned numbers, programmingAPIs, and protocols that are used on the Internet. I've included linksto a few of them here for your enjoyment, so grab a bucket of popcornand put on your thinking cap:

RFC 1—The First RFC;this gives you an idea of what the "Internet" was like just as it wascoming to life, and an insight into how it was being designed from theground up. (This RFC is completely obsolete, obviously!)

RFC 768—The UserDatagram Protocol (UDP)

RFC791—The Internet Protocol (IP)

RFC793—The Transmission Control Protocol(TCP)

RFC 854—The TelnetProtocol

RFC 959—File TransferProtocol (FTP)

RFC1350—The Trivial File Transfer Protocol(TFTP)

RFC1459—Internet Relay Chat Protocol(IRC)

RFC1918—Address Allocation for PrivateInternets

RFC2131—Dynamic Host Configuration Protocol(DHCP)

RFC2616—Hypertext Transfer Protocol(HTTP)

RFC2821—Simple Mail Transfer Protocol(SMTP)

RFC3330—Special-Use IPv4 Addresses

RFC3493—Basic Socket Interface Extensions forIPv6

RFC3542—Advanced Sockets Application ProgramInterface (API) for IPv6

RFC3849—IPv6 Address Prefix Reserved forDocumentation

RFC3920—Extensible Messaging and Presence Protocol(XMPP)

RFC3977—Network News Transfer Protocol(NNTP)

RFC4193—Unique Local IPv6 UnicastAddresses

RFC4506—External Data Representation Standard(XDR)

The IETF has a nice online tool for searching and browsing RFCs.

Index

     255.255.255.255: 7.6, 9.13

     accept(): 5.5, 5.6, 9.1
     Address already in use: 5.3, 8.0
     AF_INET: 3.3, 5.2, 8.0
     AF_INET6: 3.3
     asynchronous I/O: 9.11

     Bapper: 7.6
     bind(): 5.3, 8.0, 9.2
          implicit: 5.3, 5.4
     blah blah blah: 2.2
     blocking: 7.1
     books: 10.1
     broadcast: 7.6
     byte ordering: 3.2, 3.3, 7.4, 9.12

     client:
          datagram: 6.3
          stream: 6.2
     client/server: 6.0
     close(): 5.9, 9.4
     closesocket(): 1.5, 5.9, 9.4
     compilers:
          gcc: 1.2
     compression: 8.0
     connect(): 2.1, 5.3, 5.3, 5.4, 9.3
          on datagram sockets: 5.8, 6.3, 9.3
     Connection refused: 6.2
     CreateProcess(): 1.5, 8.0
     CreateThread(): 1.5
     CSocket: 1.5
     Cygwin: 1.5

     data encapsulation: 2.2, 7.3
     DHCP: 10.3
     disconnected network: see private network.
     DNS:
     domain name service: see DNS.
     donkeys: 7.3

     EAGAIN: 9.21
     email to Beej: 1.6
     encryption: 8.0
     EPIPE: 9.4
     errno: 9.10, 9.16
     Ethernet: 2.2
     EWOULDBLOCK: 7.1, 9.1
     Excalibur: 7.5
     external data representation standard: see XDR.

     F_SETFL: 9.11
     fcntl(): 7.1, 9.1, 9.11
     FD_CLR(): 7.2, 9.19
     FD_ISSET(): 7.2, 9.19
     FD_SET(): 7.2, 9.19
     FD_ZERO(): 7.2, 9.19
     file descriptor: 2.0
     firewall: 3.4.1, 7.6, 8.0
          poking holes in: 8.0
     footer: 2.2
     fork(): 1.5, 6.0, 8.0
     FTP: 10.3

     getaddrinfo(): 3.3, 4.0, 5.1
     gethostbyaddr(): 5.10, 9.7
     gethostbyname(): 5.11, 9.6, 9.7
     gethostname(): 5.11, 9.6
     getnameinfo(): 4.0, 5.10
     getpeername(): 5.10, 9.9
     getprotobyname(): 9.23
     getsockopt(): 9.20
     gettimeofday(): 7.2
     goat: 8.0
     goto: 8.0

     header: 2.2
     header files: 8.0
     herror(): 9.7
     hstrerror(): 9.7
     htonl(): 3.2, 9.12, 9.12
     htons(): 3.2, 3.3, 7.4, 9.12, 9.12
     HTTP: 10.3
     HTTP protocol: 2.1

     ICMP: 8.0
     IEEE-754: 7.4
     INADDR_ANY:
     INADDR_BROADCAST: 7.6
     inet_addr(): 3.4, 9.13
     inet_aton(): 3.4, 9.13
     inet_ntoa(): 3.4, 9.13
     inet_ntoa(): 3.4, 5.10
     inet_pton(): 3.4
     Internet Control Message Protocol: see ICMP.
     Internet protocol: see IP.
     Internet Relay Chat: see IRC.
     ioctl(): 8.0
     IP: 2.1, 2.2, 3.0, 3.4, 5.3, 5.8, 5.11, 10.3
     IP address: 9.2, 9.6, 9.7, 9.9
     IPv4: 3.1
     IPv6: 3.1, 3.3, 3.4.1, 4.0
     IRC: 7.4, 10.3
     ISO/OSI: 2.2

     layered network model: see ISO/OSI.
     Linux: 1.5
     listen(): 5.3, 5.5, 9.15
          backlog: 5.5
          with select(): 7.2
     lo: see loopback device.
     localhost: 8.0
     loopback device: 8.0

     man pages: 9.0
     Maximum Transmission Unit: see MTU.
     mirroring: 1.7
     MSG_DONTROUTE: 9.21
     MSG_DONTWAIT: 9.21
     MSG_NOSIGNAL: 9.21
     MSG_OOB: 9.18, 9.21
     MSG_PEEK: 9.18
     MSG_WAITALL: 9.18
     MTU: 8.0

     NAT: 3.4.1
     netstat: 8.0, 8.0
     network address translation: see NAT.
     NNTP: 10.3
     non-blocking sockets: 7.1, 9.1, 9.11, 9.21
     ntohl(): 3.2, 9.12, 9.12
     ntohs(): 3.2, 9.12, 9.12

     O_ASYNC: see asynchronous I/O.
     O_NONBLOCK: see non-blocking sockets.
     OpenSSL: 8.0
     out-of-band data: 9.18, 9.21

     packet sniffer: 8.0
     Pat: 7.6
     perror(): 9.10, 9.16
     PF_INET: 8.0, 9.23
     ping: 8.0
     poll(): 7.2, 9.17
     port: 5.8, 9.2, 9.9
     ports: 5.3, 5.3
     private network: 3.4.1
     promiscuous mode: 8.0

     raw sockets: 2.1, 8.0
     read(): 2.0
     recv(): 2.0, 2.0, 5.7, 9.18
          timeout: 8.0
     recvfrom(): 5.8, 9.18
     recvtimeout(): 8.0
     references: 10.1
          web-based: 10.2
     RFCs: 10.3
     route: 8.0

     SA_RESTART: 8.0
     Secure Sockets Layer: see SSL.
     security: 8.0
     select(): 1.5, 7.1, 7.2, 8.0, 8.0, 9.19
          with listen(): 7.2
     send(): 2.0, 2.0, 2.2, 5.7, 9.21
     sendall(): 7.3, 7.5
     sendto(): 2.2, 9.21
     serialization: 7.4
     server:
          datagram: 6.3
          stream: 6.1
     setsockopt(): 5.3, 7.6, 8.0, 8.0, 9.20
     shutdown(): 5.9, 9.22
     sigaction(): 6.1, 8.0
     SIGIO: 9.11
     SIGPIPE: 9.4, 9.21
     SIGURG: 9.18, 9.21
     SMTP: 10.3
     SO_BINDTODEVICE: 9.20
     SO_BROADCAST: 7.6, 9.20
     SO_RCVTIMEO: 8.0
     SO_REUSEADDR: 5.3, 8.0, 9.20
     SO_SNDTIMEO: 8.0
     SOCK_DGRAM: see socket;datagram.
     SOCK_RAW: 9.23
     SOCK_STREAM: see socket;stream.
     socket: 2.0
          datagram: 2.1, 2.1, 2.2, 5.8, 9.18, 9.20, 9.21, 9.23
          raw: 2.1
          stream: 2.1, 2.1, 9.1, 9.18, 9.21, 9.23
          types: 2.0, 2.1
     socket descriptor: 2.0, 3.3
     socket(): 2.0, 5.2, 9.23
     SOL_SOCKET: 9.20
     Solaris: 1.4, 9.20
     SSL: 8.0
     strerror(): 9.10, 9.16
     struct addrinfo: 3.3
     struct hostent: 9.7
     struct in_addr: 9.24
     struct pollfd: 9.17
     struct sockaddr: 3.3, 5.8, 9.18, 9.24
     struct sockaddr_in: 3.3, 9.1, 9.24
     struct timeval: 7.2, 9.19
     SunOS: 1.4, 9.20

     TCP: 2.1, 9.23, 10.3
     gcc: 2.1, 10.3
     TFTP: 2.2, 10.3
     timeout, setting: 8.0
     translations: 1.8
     transmission control protocol: see TCP.
     TRON: 5.4

     UDP: 2.1, 2.2, 7.6, 9.23, 10.3
     user datagram protocol: see UDP.

     Vint Cerf: 3.1

     Windows: 1.5, 5.9, 8.0, 9.4, 9.20
     Winsock: 1.5, 5.9
     Winsock FAQ: 1.5
     write(): 2.0
     WSACleanup(): 1.5
     WSAStartup(): 1.5

     XDR: 7.4, 10.3
     XMPP: 10.3

     zombie process: 6.1