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

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

Beej's Guide to Network Programming Using Internet Sockets

Brian "Beej" Hall

beej@beej.us

Version 2.3.20

October 8, 2005

1. Intro: 1.1. Audience; 1.2. Platform and Compiler; 1.3. Official Homepage; 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. structs and Data Handling: 3.1. Convert the Natives!; 3.2. IP Addresses and How to Deal With Them
4. System Calls or Bust: 4.1. socket()--Get the File Descriptor!; 4.2. bind()--What port am I on?; 4.3. connect()--Hey, you!; 4.4. listen()--Will somebody please call me?; 4.5. accept()--"Thank you for calling port 3490."; 4.6. send() and recv()--Talk to me, baby!; 4.7. sendto() and recvfrom()--Talk to me, DGRAM-style; 4.8. close() and shutdown()--Get outta my face!; 4.9. getpeername()--Who are you?; 4.10. gethostname()--Who am I?; 4.11. DNS--You say "whitehouse.gov", I say "63.161.169.137"
5. Client-Server Background: 5.1. A Simple Stream Server; 5.2. A Simple Stream Client; 5.3. Datagram Sockets
6. Slightly Advanced Techniques: 6.1. Blocking; 6.2. select()--Synchronous I/O Multiplexing; 6.3. Handling Partial send()s; 6.4. Son of Data Encapsulation
7. Common Questions
8. Man Pages: 8.1. accept(); 8.2. bind(); 8.3. connect(); 8.4. close(); 8.5. gethostname(); 8.6. gethostbyname(), gethostbyaddr(); 8.7. getpeername(); 8.8. errno; 8.9. fcntl(); 8.10. htons(), htonl(), ntohs(), ntohl(); 8.11. inet_ntoa(), inet_aton(); 8.12. listen(); 8.13. perror(), strerror(); 8.14. poll(); 8.15. recv(), recvfrom(); 8.16. select(); 8.17. setsockopt(), getsockopt(); 8.18. send(), sendto(); 8.19. shutdown(); 8.20. socket(); 8.21. struct sockaddr_in, struct in_addr
9. More References: 9.1. Books; 9.2. Web References; 9.3. RFCs

1. Intro

1.1. Audience

This document has been written as a tutorial, not a reference. It is probably at its best when read by individuals who are just starting out with socket programming and are looking for a foothold. It is certainly not the complete guide to sockets programming, by any means.

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

1.2. Platform and Compiler

The code contained within this document was compiled on a Linux PC using Gnu's gcc compiler. It should, however, build on just about any platform that uses gcc. Naturally, this doesn't apply if you're programming for Windows--see the section on Windows programming, below.

1.3. Official Homepage

This official location of this document is http://beej.us/guide/bgnet/.

1.4. Note for Solaris/SunOS Programmers

When compiling for Solaris or SunOS, you need to specify some extra command-line switches for 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 what that does, exactly, but some people seem to need it.

Another place that you might find problems is in the call to setsockopt(). The prototype differs from that on my Linux box, 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 above information--it's just what people have told me through email.

1.5. Note for Windows Programmers

I have a particular dislike for Windows, and encourage you to try Linux, BSD, or Unix instead. That being said, you can still use this stuff under Windows.

First, ignore pretty much all of the system header files I mention in here. All you need to include is:

#include

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

#include

{
WSADATA wsaData; // if this doesn't work
//WSAData wsaData; // then try this instead

if (WSAStartup(MAKEWORD(1, 1), &wsaData) != 0) {
fprintf(stderr, "WSAStartup failed. ");
exit(1);
}

You also have to tell your compiler to link in the Winsock library, usually called wsock32.lib or winsock32.lib or somesuch. Under VC++, this can be done through the Project menu, under Settings.... Click the Link tab, and look for the box titled "Object/library modules". Add "wsock32.lib" to that list.

Or so I hear.

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

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

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

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

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 library or 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 to digest...unfortunately a discussion about multithreading is beyond the scope 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 a response. I lead a pretty busy life and there are times when I just can't answer a question you have. When that's the case, I usually just delete the message. It's nothing personal; I just won't ever have the time to give the detailed answer you require.

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

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

Now that I've badgered you about how to write and not write me, I'd just like to let you know that I fully appreciate all the praise the guide has received over the years. 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 publically or privately. If you publically mirror the site and want me to link to it from the main page, drop me a line at beej@beej.us.

1.8. Note for Translators

If you want to translate the guide into another language, write me at beej@beej.us and I'll link to your translation from the main page.

Feel free to add your name and email address to the translation.

Sorry, but due to space constraints, I cannot host the translations myself.

1.9. Copyright and Distribution

This guide may be freely reprinted in any medium provided that its content is not altered, it is presented in its entirety, and this copyright notice remains intact.

Educators are especially encouraged to recommend or supply copies of this guide to their students.

This guide may be freely translated into any language, provided the translation is accurate, and the guide is reprinted in its entirety. The translation may also include the name and contact information for the translator.

The C source code presented in this document is hereby granted to the public domain.

Contact beej@beej.us for more information.

2. What is a socket?

You hear talk of "sockets" all the time, and perhaps you are wondering just what they are exactly. Well, they're this: 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 have been talking about is the fact that when Unix programs do any sort of I/O, they do it by reading or writing to a file descriptor. A file descriptor is simply an integer associated with an open file. But (and here's the catch), that file can be a network connection, a FIFO, a pipe, a terminal, a real on-the-disk file, or just about anything else. Everything in Unix is a file! So when you want to communicate with another program over the Internet you're gonna do it through 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, but I'm going to answer it anyway: You make a call to the socket() system routine. It returns the socket descriptor, and you communicate through it using the specialized send() and recv() (man send, man recv) socket calls.

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

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

2.1. Two Types of Internet Sockets

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

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

Stream sockets are reliable two-way connected communication streams. If you output two items into the socket in the order "1, 2", they will arrive in the order "1, 2" at the opposite end. They will also be error free. Any errors you do encounter are figments of your own deranged mind, and are not to be discussed here.

What uses stream sockets? Well, you may have heard of the telnet application, yes? It uses stream sockets. All the characters you type need to arrive in the same order you type them, right? Also, web browsers use the HTTP protocol which uses stream sockets to get pages. 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 back at you!

How do stream sockets achieve this high level of data transmission quality? They use a protocol called "The Transmission Control Protocol", otherwise known as "TCP" (see RFC-793 for extremely detailed info on 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 with Internet routing and is not generally responsible for data integrity.

Cool. What about Datagram sockets? Why are they called connectionless? What is the deal, here, anyway? Why are 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 data within 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" (see RFC-768.)

Why are they connectionless? Well, basically, it's because you don't have to maintain an open connection as you do with stream sockets. You just build a packet, slap an IP header on it with destination information, and send it out. No connection needed. They are generally used for packet-by-packet transfers of information. Sample applications: tftp, bootp, etc.

"Enough!" you may scream. "How do these programs even work if datagrams might get lost?!" Well, my human friend, each has it's own protocol on top of UDP. For example, the tftp protocol says that for each packet that gets sent, the recipient has to send back a packet that says, "I got it!" (an "ACK" packet.) If the sender of the original packet gets no reply in, say, five seconds, he'll re-transmit the packet until he finally gets an ACK. This acknowledgment procedure is very important when implementing SOCK_DGRAM applications.

2.2. Low level Nonsense and Network Theory

Since I just mentioned layering of protocols, it's time to talk about 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 Data Encapsulation! This is very very important. It's so important that you might just learn about it if you take the networks course here at Chico State ;-). Basically, it says this: a packet is born, the packet is wrapped ("encapsulated") in a header (and rarely a footer) by the first protocol (say, the TFTP protocol), then the whole thing (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 the Ethernet header, the kernel strips the IP and UDP headers, the TFTP program strips the TFTP header, and it finally has the data.

Now I can finally talk about the infamous Layered Network Model. This Network Model describes a system of network functionality that has many advantages over other models. For instance, you can write sockets programs that are exactly the same without caring how the data is physically transmitted (serial, thin Ethernet, AUI, whatever) because programs on lower levels deal with it for you. The actual network hardware and topology is transparent to the socket programmer.

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

Application
Presentation
Session
Transport
Network
Data Link
Physical

The Physical Layer is the hardware (serial, Ethernet, etc.). The Application Layer is just about as far from the physical layer as you can imagine--it's the place where users interact with the network.

Now, this model is so general you could probably use it as an automobile repair guide if you really wanted to. A layered model more consistent 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, ATM, or whatever)

At this point in time, you can probably see how these layers correspond 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 sockets is send() the data out. All you have to do for datagram sockets is encapsulate the packet in the method of your choosing and sendto() it out. The kernel builds the Transport Layer and Internet Layer on for you and the hardware does the Network Access Layer. Ah, modern technology.

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

3. `struct`s and Data Handling

Well, we're finally here. It's time to talk about programming. In this section, I'll cover various data types used by the sockets interface, 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 with me. Know this: there are two byte orderings: most significant byte (sometimes called an "octet") first, or least significant byte first. The former is called "Network Byte Order". Some machines store their numbers internally in Network Byte Order, some don't. When I say something has to be in Network Byte Order, you have to call a function (such as htons()) to change it from "Host Byte Order". If I don't say "Network Byte Order", then you must leave the value in Host Byte Order.

(For the curious, "Network Byte Order" is also known as "Big-Endian Byte Order".)

My First StructTM--struct sockaddr. This structure holds socket address information 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 for everything we do in this document. sa_data contains a destination address and port number for the socket. This is rather unwieldy since you don't want to tediously pack the address in the sa_data by hand.

To deal with struct sockaddr, programmers created a parallel structure: struct sockaddr_in ("in" for "Internet".)

struct sockaddr_in {
short int sin_family; // Address family
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 socket address. Note that sin_zero (which is included to pad the structure to the length of a struct sockaddr) should be set to all zeros with the function memset(). Also, and this is the important bit, a pointer to a struct sockaddr_in can be cast to a pointer to a struct sockaddr and vice-versa. So even though connect() wants a struct sockaddr*, you can still use a struct sockaddr_in and cast it at the last minute! Also, notice that sin_family corresponds to sa_family in a struct sockaddr and should be set to "AF_INET". Finally, the sin_port and sin_addr must be in Network Byte Order!

"But," you object, "how can the entire structure, struct in_addr sin_addr, be in Network Byte Order?" This question requires careful examination of the structure struct in_addr, one of the worst unions alive:

// Internet address (a structure for historical reasons)
struct in_addr {
unsigned long s_addr; // that's a 32-bit long, or 4 bytes
};

Well, it used to be a union, but now those days seem to be gone. Good riddance. So if you have declared ina to be of type struct sockaddr_in, then ina.sin_addr.s_addr references the 4-byte IP address (in Network Byte Order). Note that even if your system still uses the God-awful union for struct in_addr, you can still reference the 4-byte IP address in exactly the same way as I did above (this due to #defines.)

3.1. Convert the Natives!

We've now been lead right into the next section. There's been too much talk about this Network to Host Byte Order conversion--now is the time for action!

All righty. There are two types 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 to Network Byte Order. Start with "h" for "host", follow it with "to", then "n" for "network", and "s" for "short": h-to-n-s, or htons() (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 a stolh() ("Short to Long Host") function--not at this party, anyway. But there are:

htons() -- "Host to Network Short"
htonl() -- "Host to Network Long"
ntohs() -- "Network to Host Short"
ntohl() -- "Network to Host Long"

Now, you may think you're wising up to this. You might think, "What do I do if I have to change byte order on a char?" Then you might think, "Uh, never mind." You might also think that since your 68000 machine already uses network byte order, you don't have to call htonl() on your IP addresses. You would be right, BUT if you try to port to a machine that has reverse network byte order, your program will fail. Be portable! This is a Unix world! (As much as Bill Gates would like to think otherwise.) Remember: put your bytes in Network Byte Order before you put them on the network.

A final point: why do sin_addr and sin_port need to be in Network Byte Order in a struct sockaddr_in, but sin_family does not? The answer: sin_addr and sin_port get encapsulated in the packet at the IP and UDP layers, respectively. Thus, they must be in Network Byte Order. However, the sin_family field is only used by the kernel to determine what type of address the structure contains, so it must be in Host Byte Order. Also, since sin_family does not get sent out on the network, it can be in Host Byte Order.

3.2. IP Addresses and How to Deal With Them

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

First, let's say you have a struct sockaddr_in ina, and you have an IP address "10.12.110.57" that you want to store into it. The function you want to use, inet_addr(), converts an IP address in numbers-and-dots notation into an unsigned long. The assignment can be made as follows:

ina.sin_addr.s_addr = inet_addr("10.12.110.57");

Notice that inet_addr() returns the address in Network Byte Order already--you don't have to call htonl(). Swell!

Now, the above code snippet isn't very robust because there is no error checking. See, inet_addr() returns -1 on error. Remember binary numbers? (unsigned)-1 just happens to correspond to the IP address 255.255.255.255! That's the broadcast address! Wrongo. Remember to do your error checking properly.

Actually, there's a cleaner interface you can use instead of inet_addr(): it's called inet_aton() ("aton" means "ascii to network"):

#include
#include
#include

int inet_aton(const char *cp, struct in_addr *inp);

And here's a sample usage, while packing a struct sockaddr_in (this example will make more sense to you when you get to the sections on bind() and connect().)

struct sockaddr_in my_addr;

my_addr.sin_family = AF_INET; // host byte order
my_addr.sin_port = htons(MYPORT); // short, network byte order
inet_aton("10.12.110.57", &(my_addr.sin_addr));
memset(&(my_addr.sin_zero), '', 8); // zero the rest of the struct

inet_aton(), unlike practically every other socket-related function, returns non-zero on success, and zero on failure. And the address is passed back in inp.

Unfortunately, not all platforms implement inet_aton() so, although its use is preferred, the older more common inet_addr() is used in this guide.

All right, now you can convert string IP addresses to their binary representations. What about the other way around? What if you have a struct in_addr and you want to print it in numbers-and-dots notation? In this case, you'll want to use the function inet_ntoa() ("ntoa" means "network to ascii") like this:

printf("%s", inet_ntoa(ina.sin_addr));

That will print the IP address. Note that inet_ntoa() takes a struct in_addr as an argument, not a long. Also notice that it returns a pointer to a char. This points to a statically stored char array within inet_ntoa() so that each time you call inet_ntoa() it will overwrite the last IP address you asked for. For example:

char *a1, *a2;

a1 = inet_ntoa(ina1.sin_addr); // this is 192.168.4.14
a2 = inet_ntoa(ina2.sin_addr); // this is 10.12.110.57
printf("address 1: %s ",a1);
printf("address 2: %s ",a2);

will print:

address 1: 10.12.110.57
address 2: 10.12.110.57

If you need to save the address, strcpy() it to your own character array.

That's all on this topic for now. Later, you'll learn to convert a string like "whitehouse.gov" into its corresponding IP address (see DNS, below.)

4. System Calls or Bust

This is the section where we get into the system calls that allow you to access the network functionality of a Unix box. When you call one of these functions, the kernel takes over and does all the work for you automagically.

The place most people get stuck around here is what order to call these things in. In that, the man pages are no use, as you've probably discovered. Well, to help with that dreadful situation, I've tried to lay out the system calls in the following sections in exactly (approximately) the same order that 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 the Internet like the Son of Jon Postel!

4.1. `socket()`--Get the File Descriptor!

I guess I can put it off no longer--I have to talk about the socket() system call. Here's the breakdown:

#include
#include

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

But what are these arguments? First, domain should be set to "PF_INET". Next, the type argument tells the kernel what kind of socket this is: SOCK_STREAM or SOCK_DGRAM. Finally, just set protocol to "0" to have socket() choose the correct protocol based on the type. (Notes: there are many more domains than I've listed. There are many more types than I've listed. See the socket() man page. Also, there's a "better" way to get the protocol, but specifying 0 works in 99.9% of all cases. See the getprotobyname() man page if you're curious.)

socket() simply returns to you a socket descriptor that you can use in later system calls, or -1 on error. The global variable errno is set to the error's value (see the perror() man page.)

(This PF_INET thing is a close relative of the AF_INET that you used when initializing the sin_family field in your struct sockaddr_in. In fact, they're so closely related that they actually have the same value, and many programmers will call socket() and pass AF_INET as the first argument instead of PF_INET. Now, get some milk and cookies, because it's times for a story. Once upon a time, a long time ago, it was thought that maybe a address family (what the "AF" in "AF_INET" stands for) might support several protocols that were referred to by their protocol family (what the "PF" in "PF_INET" stands for). That didn't happen. And they all lived happily ever after, The End. So the most correct thing to do is to use AF_INET in your struct sockaddr_in and PF_INET in your call to socket().)

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

4.2. `bind()`--What port am I on?

Once you have a socket, you might have to associate that socket with a port on your local machine. (This is commonly done if you're going to listen() for incoming connections on a specific port--MUDs do this when they tell you to "telnet to x.y.z port 6969".) The port number is used by the kernel to match an incoming packet to a certain process's socket descriptor. If you're going to only be doing a connect(), this may be unnecessary. Read it anyway, just for kicks.

Here is the synopsis for the bind() system call:

#include
#include

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

sockfd is the socket file descriptor returned by socket(). my_addr is a pointer to a struct sockaddr that contains information about your address, namely, port and IP address. addrlen can be set to sizeof(struct sockaddr).

Whew. That's a bit to absorb in one chunk. Let's have an example:

#include
#include
#include
#include
#include

#define MYPORT 3490

main()
{
int sockfd;
struct sockaddr_in my_addr;

sockfd = socket(PF_INET, SOCK_STREAM, 0); // do some error checking!

my_addr.sin_family = AF_INET; // host byte order
my_addr.sin_port = htons(MYPORT); // short, network byte order
my_addr.sin_addr.s_addr = inet_addr("10.12.110.57");
memset(&(my_addr.sin_zero), '', 8); // zero the rest of the struct

// don't forget your error checking for bind():
bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr));
.
.
.

There are a few things to notice here: my_addr.sin_port is in Network Byte Order. So is my_addr.sin_addr.s_addr. Another thing to watch out for is that the header files might differ from system to system. To be sure, you should check your local man pages.

Lastly, on the topic of bind(), I should mention that some of the process of getting your own IP address and/or port can be automated:

my_addr.sin_port = 0; // choose an unused port at random
my_addr.sin_addr.s_addr = INADDR_ANY; // use my IP address

See, by setting my_addr.sin_port to zero, you are telling bind() to choose the port for you. Likewise, by setting my_addr.sin_addr.s_addr to INADDR_ANY, you are telling it to automatically fill in the IP address of the machine the process is running on.

If you are into noticing little things, you might have seen that I didn't put INADDR_ANY into Network Byte Order! Naughty me. However, I have inside info: INADDR_ANY is really zero! Zero still has zero on bits even if you rearrange the bytes. However, purists will point out that there could be a parallel dimension where INADDR_ANY is, say, 12 and that my code won't work there. That's ok with me:

my_addr.sin_port = htons(0); // choose an unused port at random
my_addr.sin_addr.s_addr = htonl(INADDR_ANY); // use my IP address

Now we're so portable you probably wouldn't believe it. I just wanted to point that out, since most of the code you come across won't bother running INADDR_ANY through htonl().

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

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

Sometimes, you might notice, you try to rerun a server and bind() fails, claiming "Address already in use." What does that mean? Well, a little bit of a socket that was connected is still hanging around in the kernel, and it's hogging 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, like this:

int yes=1;
//char yes='1'; // Solaris people use this

// lose the pesky "Address already in use" error message
if (setsockopt(listener,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) {
perror("setsockopt");
exit(1);
}

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

4.3. `connect()`--Hey, you!

Let's just pretend for a few minutes that you're a telnet application. Your user commands you (just like in the movie TRON) to get a socket file descriptor. You comply and call socket(). Next, the user tells you to connect to "10.12.110.57" on port "23" (the standard telnet port.) Yow! What do you do now?

Lucky for you, program, you're now perusing the section on connect()--how to connect to a remote host. So read furiously 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 socket file descriptor, as returned by the socket() call, serv_addr is a struct sockaddr containing the destination port and IP address, and addrlen can be set to sizeof(struct sockaddr).

Isn't this starting to make more sense? Let's have an example:

#include
#include
#include
#include

#define DEST_IP "10.12.110.57"
#define DEST_PORT 23

main()
{
int sockfd;
struct sockaddr_in dest_addr; // will hold the destination addr

sockfd = socket(PF_INET, SOCK_STREAM, 0); // do some error checking!

dest_addr.sin_family = AF_INET; // host byte order
dest_addr.sin_port = htons(DEST_PORT); // short, network byte order
dest_addr.sin_addr.s_addr = inet_addr(DEST_IP);
memset(&(dest_addr.sin_zero), '', 8); // zero the rest of the struct

// don't forget to error check the connect()!
connect(sockfd, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr));
.
.
.

Again, be sure to check the return value from connect()--it'll return -1 on error and set the variable errno.

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

4.4. `listen()`--Will somebody please call me?

Ok, time for a change of pace. What if you don't want to connect to a remote host. Say, just for kicks, that you want to wait for incoming connections and handle them in some way. The process is two step: first you listen(), then you accept() (see below.)

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

int listen(int sockfd, int backlog);

sockfd is the usual socket file descriptor from the socket() system call. backlog is the number of connections allowed on the incoming queue. What does that mean? Well, incoming connections are going to wait in this queue until you accept() them (see below) and this is the limit on how many can queue up. Most systems silently limit this number to about 20; you can probably get away with setting it to 5 or 10.

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

Well, as you can probably imagine, we need to call bind() before we call listen() or the kernel will have us listening on a random port. Bleah! So if you're going to be listening for incoming connections, the sequence of system calls you'll make is:

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

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

4.5. `accept()`--"Thank you for calling port 3490."

Get ready--the accept() call is kinda weird! What's going to happen is this: someone far far away will try to connect() to your machine on a port that you are listen()ing on. Their connection will be queued up waiting to be accept()ed. You call accept() and you tell it to get the pending connection. It'll return to you a brand new socket file descriptor to use for this single connection! That's right, suddenly you have two socket file descriptors for the price of one! The original one is still listening on your port and the newly created one is finally ready 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 the listen()ing socket descriptor. Easy enough. addr will usually be a pointer to a local struct sockaddr_in. This is where the information about the incoming connection will go (and with it you can determine which host is calling you from which port). addrlen is a local integer variable that should be set to sizeof(struct sockaddr_in) before its address is passed to accept(). Accept will not put more than that many bytes into addr. If it puts fewer in, it'll change the value of addrlen to reflect that.

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

Like before, this is a bunch to absorb in one chunk, so here's a sample 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 hold

main()
{
int sockfd, new_fd; // listen on sock_fd, new connection on new_fd
struct sockaddr_in my_addr; // my address information
struct sockaddr_in their_addr; // connector's address information
int sin_size;

sockfd = socket(PF_INET, SOCK_STREAM, 0); // do some error checking!

my_addr.sin_family = AF_INET; // host byte order
my_addr.sin_port = htons(MYPORT); // short, network byte order
my_addr.sin_addr.s_addr = INADDR_ANY; // auto-fill with my IP
memset(&(my_addr.sin_zero), '', 8); // zero the rest of the struct

// don't forget your error checking for these calls:
bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr));

listen(sockfd, BACKLOG);

sin_size = sizeof(struct sockaddr_in);
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
.
.
.

Again, note that we will use the socket descriptor new_fd for all send() and recv() calls. If you're only getting one single connection ever, you can close() the listening sockfd in order to prevent more incoming connections on the same port, if you so desire.