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发信人: mry (木日), 信区: Linux
标 题: Beej's Guide to Network Programming
发信站: BBS 水木清华站 (Tue Nov 16 19:31:29 1999)
《Unix Network Programming》虽然经典，不过太长了
初学者可以现读读这篇简单的，估计一天就差不多
========================================================
http://www.ecst.csuchico.edu/~beej/guide/net/
Beej's Guide to Network Programming
Using Internet Sockets
Version 1.5.3 (01-Nov-1997)
[http://www.ecst.csuchico.edu/~beej/guide/net]
----------------------------------------------------------------------------
Intro
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.
----------------------------------------------------------------------------
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... :-)
----------------------------------------------------------------------------
Platform and Compiler
Most of the code contained within this document was compiled on a Linux PC
using Gnu's gcc compiler. It was also found to compile on HPUX using gcc.
Note that every code snippet was not individually tested.
----------------------------------------------------------------------------
Contents:
1 What is a socket?
2 Two Types of Internet Sockets
3 Low level Nonsense and Network Theory
4 structs--Know these, or aliens will destroy the planet!
5 Convert the Natives!
6 IP Addresses and How to Deal With Them
7 socket()--Get the File Descriptor!
8 bind()--What port am I on?
9 connect()--Hey, you!
10 listen()--Will somebody please call me?
11 accept()--"Thank you for calling port 3490."
12 send() and recv()--Talk to me, baby!
13 sendto() and recvfrom()--Talk to me, DGRAM-style
14 close() and shutdown()--Get outta my face!
15 getpeername()--Who are you?
16 gethostname()--Who am I?
17 DNS--You say "whitehouse.gov", I say "198.137.240.100"
18 Client-Server Background
19 A Simple Stream Server
20 A Simple Stream Client
21 Datagram Sockets
22 Blocking
23 select()--Synchronous I/O Multiplexing. Cool!
24 More references
25 Disclaimer and Call for Help
----------------------------------------------------------------------------
1. 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 hell 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. 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, WWW browsers use the
HTTP protocol which uses stream sockets to get pages. Indeed, if you telnet
to a WWW site on port 80, and type "GET pagename", 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 with Internet routing only.
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.
----------------------------------------------------------------------------
3. 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.
[Encapsulated Protocols Image] 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 maybe
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 (was Network, Data Link, and Physical)
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.
----------------------------------------------------------------------------
4. 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 sockets interface, since
some of them are a real bitch 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 NBO, you
have to call a function (such as htons()) to change it from "Host Byte
Order". If I don't say "NBO", then you must leave the value in Host Byte
Order.
My First Struct(TM)--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.
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 bzero() or
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 socket() 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;
;
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.)
----------------------------------------------------------------------------
5. 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 if "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!
Remember: put your bytes in Network 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.
----------------------------------------------------------------------------
6. 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 "132.241.5.10" 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("132.241.5.10");
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.
All right, now you can convert string IP addresses to longs. 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 198.92.129.1 */
a2 = inet_ntoa(ina2.sin_addr); /* this is 132.241.5.10 */
printf("address 1: %s",a1);
printf("address 2: %s",a2);
will print:
address 1: 132.241.5.10
address 2: 132.241.5.10
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.)
----------------------------------------------------------------------------
7. 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 <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
But what are these arguments? First, domain should be set to "AF_INET", just
like in the struct sockaddr_in (above.) Next, the type argument tells the
kernel what kind of socket this is: SOCK_STREAM or SOCK_DGRAM. Finally, just
set protocol to "0". (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. See the getprotobyname() man
page.)
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.)
----------------------------------------------------------------------------
8. 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".) 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 <sys/types.h>
#include <sys/socket.h>
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 <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#define MYPORT 3490
main()
int sockfd;
struct sockaddr_in my_addr;
sockfd = socket(AF_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("132.241.5.10");
bzero(&(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 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! You can have any port
number above that, right up to 65535 (provided they aren't already being
used by another program.)
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), you can
simply call connect(), it'll check to see if the socket is unbound, and will
bind() it to an unused local port.
----------------------------------------------------------------------------
9. 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 "132.241.5.10" on port "23" (the standard telnet port.) Oh my
God! What do you do now?
Lucky for you, program, you're now perusing the section on connect()--how to
connect to a remote host. You read furiously onward, not wanting to
disappoint your user...
The connect() call is as follows:
#include <sys/types.h>
#include <sys/socket.h>
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 <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#define DEST_IP "132.241.5.10"
#define DEST_PORT 23
main()
int sockfd;
struct sockaddr_in dest_addr; /* will hold the destination addr */
sockfd = socket(AF_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);
bzero(&(dest_addr.sin_zero), 8); /* zero the rest of the struc
t */
/* don't forget to error check the connect()! */
connect(sockfd, (struct sockaddr *)&dest_addr, sizeof(struct sockadd
r));
.
.
.
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 kernel will choose a
local port for us, and the site we connect to will automatically get this
information from us. No worries.
----------------------------------------------------------------------------
10. 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().
----------------------------------------------------------------------------
11. 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 <sys/socket.h>
int accept(int sockfd, void *addr, int *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 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 <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#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(AF_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 */
bzero(&(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, &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 original sockfd in order to prevent more incoming connections on
the same port, if you so desire.
---------------------------------------------------------------------------
-
12. send() and recv()--Talk to me, baby!
These two functions are for communicating over stream sockets or connected
datagram sockets. If you want to use regular unconnected datagram sockets,
you'll need to see the section on sendto() and recvfrom(), below.
The send() call:
int send(int sockfd, const void *msg, int len, int flags);
sockfd is the socket descriptor you want to send data to (whether it's the
one returned by socket() or the one you got with accept().) msg is a pointer
to the data you want to send, and len is the length of that data in bytes.
Just set flags to 0. (See the send() man page for 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 actually sent out--this might be less
than the number you told it to send! See, sometimes you tell it to send a
whole gob of data and it 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
returned by send() doesn't match 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 probably manage to send the whole thing
all in one go. Again, -1 is returned on error, and errno is set to the error
number.
The recv() call is similar in many respects:
int recv(int sockfd, void *buf, int len, unsigned int flags);
sockfd is the socket descriptor to read from, buf is the buffer to read the
information into, len is the maximum length of the buffer, and flags can
again be set to 0. (See the recv() man page for flag information.)
recv() returns the number of bytes actually read into the buffer, or -1 on
error (with errno set, accordingly.)
There, that was easy, wasn't it? You can now pass data back and forth on
stream sockets! Whee! You're a Unix Network Programmer!
----------------------------------------------------------------------------
13. sendto() and recvfrom()--Talk to me, DGRAM-style
"This is all fine and dandy," I hear you saying, "but where does this leave
me with unconnected datagram sockets?" No problemo, amigo. We have just the
thing.
Since datagram sockets aren't connected to a remote host, guess which piece
of information we need to give before we send a packet? That's right! The
destination address! Here's the scoop:
int sendto(int sockfd, const void *msg, int len, unsigned int flags,
const struct sockaddr *to, int tolen);
As you can see, this call is basically the same as the call to send() with
the addition of two other pieces of information. to is a pointer to a
struct sockaddr (which you'll probably have as a struct sockaddr_in and cast
it at the last minute) which contains the destination IP address and port.
tolen can simply be set to sizeof(struct sockaddr).
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 to send!),
or -1 on error.
Equally similar are recv() and recvfrom(). The synopsis of recvfrom() is:
int recvfrom(int sockfd, void *buf, int len, unsigned int flags
struct sockaddr *from, int *fromlen);
Again, this is just like recv() with the addition of a couple fields. from
is a pointer to a local struct sockaddr that will be filled with the IP
address and port of the originating machine. fromlen is a pointer to a local
int that should be initialized to sizeof(struct sockaddr). When the function
returns, fromlen will contain the length of the address actually stored in
from.
recvfrom() returns the number of bytes received, or -1 on error (with errno
set accordingly.)
Remember, if you connect() a datagram socket, you can then simply use send()
and recv() for all your transactions. The socket itself is still a datagram
socket and the packets still use UDP, but the socket interface will
automatically add the destination and source information for you.
----------------------------------------------------------------------------
14. close() and shutdown()--Get outta my face!
Whew! You've been send()'ing and recv()'ing data all day long, and you've
had it. You're ready to close the connection on your socket descriptor. This
is easy. You can just use the regular Unix file descriptor close() function:
close(sockfd);
This will prevent any more reads and writes to the socket. Anyone attempting
to read or write the socket on the remote end will receive an error.
Just in case you want a little more control over how the socket closes, you
can use the shutdown() function. It allows you to cut off communication in a
certain direction, or both ways (just like close() does.) Synopsis:
int shutdown(int sockfd, int how);
sockfd is the socket file descriptor you want to shutdown, and how is one of
the following:
* 0 - Further receives are disallowed
* 1 - Further sends are disallowed
* 2 - Further sends and receives are disallowed (like close())
shutdown() returns 0 on success, and -1 on error (with errno set
accordingly.)
If you deign to use shutdown() on unconnected datagram sockets, it will
simply make the socket unavailable for further send() and recv() calls
(remember that you can use these if you connect() your datagram socket.)
Nothing to it.
----------------------------------------------------------------------------
15. getpeername()--Who are you?
This function is so easy.
It's so easy, I almost didn't give it it's own section. But here it is
anyway.
The function getpeername() will tell you who is at the other end of a
connected stream socket. The synopsis:
#include <sys/socket.h>
int getpeername(int sockfd, struct sockaddr *addr, int *addrlen);
sockfd is the descriptor of the connected stream socket, addr is a pointer
to a struct sockaddr (or a struct sockaddr_in) that will hold the
information about the other side of the connection, and addrlen is a pointer
to an int, that should be initialized to sizeof(struct sockaddr).
The function returns -1 on error and sets errno accordingly.
Once you have their address, you can use inet_ntoa() or gethostbyaddr() to
print or get more information. No, you can't get their login name.
----------------------------------------------------------------------------
16. gethostname()--Who am I?
Even easier than getpeername() is the function gethostname(). It returns the
name of the computer that your program is running on. The name can then be
used by gethostbyname(), below, to determine the IP address of your local
machine.
What could be more fun? I could think of a few things, but they don't
pertain to socket programming. Anyway, here's the breakdown:
#include <unistd.h>
int gethostname(char *hostname, size_t size);
The arguments are simple: hostname is a pointer to an array of chars that
will contain the hostname upon the function's return, and size is the length
in bytes of the hostname array.
The function returns 0 on successful completion, and -1 on error, setting
errno as usual.
----------------------------------------------------------------------------
17. DNS--You say "whitehouse.gov", I say "198.137.240.100"
In case you don't know what DNS is, it stands for "Domain Name Service". In
a nutshell, you tell it what the human-readable address is for a site, and
it'll give you the IP address (so you can use it with bind(), connect(),
sendto(), or whatever you need it for.) This way, when someone enters:
$ telnet whitehouse.gov
telnet can find out that it needs to connect() to "198.137.240.100".
But how does it work? You'll be using the function gethostbyname():
#include <netdb.h>
struct hostent *gethostbyname(const char *name);
As you see, it returns a pointer to a struct hostent, the layout of which is
as follows:
struct hostent
char *h_name;
char **h_aliases;
int h_addrtype;
int h_length;
char **h_addr_list;
;
#define h_addr h_addr_list[0]
And here are the descriptions of the fields in the struct hostent:
* h_name - Official name of the host.
* h_aliases - A NULL-terminated array of alternate names for the host.
* h_addrtype - The type of address being returned; usually AF_INET.
* h_length - The length of the address in bytes.
* h_addr_list - A zero-terminated array of network addresses for the
host. Host addresses are in Network Byte Order.
* h_addr - The first address in h_addr_list.
gethostbyname() returns a pointer to the filled struct hostent, or NULL on
error. (But errno is not set--h_errno is set instead. See herror(), below.)
But how is it used? Sometimes (as we find from reading computer manuals),
just spewing the information at the reader is not enough. This function is
certainly easier to use than it looks.
Here's an example program:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
struct hostent *h;
if (argc != 2) /* error check the command line */
fprintf(stderr,"usage: getip address");
exit(1);
if ((h=gethostbyname(argv[1])) == NULL) /* get the host info */
herror("gethostbyname");
exit(1);
printf("Host name : %s", h->h_name);
printf("IP Address : %s",inet_ntoa(*((struct in_addr *)h->h_addr)));
return 0;
With gethostbyname(), you can't use perror() to print error message (since
errno is not used). Instead, call herror().
It's pretty straightforward. You simply pass the string that contains the
machine name ("whitehouse.gov") to gethostbyname(), and then grab the
information out of the returned struct hostent.
The only possible weirdness might be in the printing of the IP address,
above. h->h_addr is a char *, but inet_ntoa() wants a struct in_addr passed
to it. So I cast h->h_addr to a struct in_addr *, then dereference it to get
at the data.
----------------------------------------------------------------------------
18. Client-Server Background
It's a client-server world, baby. Just about everything on the network deals
with client processes talking to server processes and vice-versa. Take
telnet, for instance. When you connect to a remote host on port 24 with
telnet (the client), a program on that host (called telnetd, the server)
springs to life. It handles the incoming telnet connection, sets you up with
a login prompt, etc.
[Client-Server Relationship]
Figure 2. The Client-Server Relationship.
The exchange of information between client and server is summarized in
Figure 2.
Note that the client-server pair can speak SOCK_STREAM, SOCK_DGRAM, or
anything else (as long as they're speaking the same thing.) Some good
examples of client-server pairs are telnet/telnetd, ftp/ftpd, or
bootp/bootpd. Every time you use ftp, there's a remote program, ftpd, that
serves you.
Often, there will only be one server on a machine, and that server will
handle multiple clients using fork(). The basic routine is: server will wait
for a connection, accept() it, and fork() a child process to handle it. This
is what our sample server does in the next section.
----------------------------------------------------------------------------
19. A Simple Stream Server
All this server does is send the string "Hello, World!" out over a stream
connection. All you need to do to test this server is run it in one window,
and telnet to it from another with:
$ telnet remotehostname 3490
where remotehostname is the name of the machine you're running it on.
The server code: (Note: a trailing backslash on a line means that the line
is continued on the next.)
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/wait.h>
#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;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
perror("socket");
exit(1);
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 */
bzero(&(my_addr.sin_zero), 8); /* zero the rest of the struct
*/
if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr
)) \par
== -1)
perror("bind");
exit(1);
if (listen(sockfd, BACKLOG) == -1)
perror("listen");
exit(1);
while(1) /* main accept() loop */
sin_size = sizeof(struct sockaddr_in);
if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, \pa
r &sin_size)) == -
1)
perror("accept");
continue;
printf("server: got connection from %s", \par
inet_ntoa(their_addr.sin_addr));
if (!fork()) /* this is the child process */
if (send(new_fd, "Hello, world!", 14, 0) == -1)
perror("send");
close(new_fd);
exit(0);
close(new_fd); /* parent doesn't need this */
while(waitpid(-1,NULL,WNOHANG) > 0); /* clean up child processes
*/
In case you're curious, I have the code in one big main() function for (I
feel) syntactic clarity. Feel free to split it into smaller functions if it
makes you feel better.
You can also get the string from this server by using the client listed in
the next section.
----------------------------------------------------------------------------
20. A Simple Stream Client
This guy's even easier than the server. All this client does is connect to
the host you specify on the command line, port 3490. It gets the string that
the server sends.
The client source:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#define PORT 3490 /* the port client will be connecting to */
#define MAXDATASIZE 100 /* max number of bytes we can get at once */
int main(int argc, char *argv[])
int sockfd, numbytes;
char buf[MAXDATASIZE];
struct hostent *he;
struct sockaddr_in their_addr; /* connector's address information */
if (argc != 2)
fprintf(stderr,"usage: client hostname");
exit(1);
if ((he=gethostbyname(argv[1])) == NULL) /* get the host info */
herror("gethostbyname");
exit(1);
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
perror("socket");
exit(1);
their_addr.sin_family = AF_INET; /* host byte order */
their_addr.sin_port = htons(PORT); /* short, network byte order *
/
their_addr.sin_addr = *((struct in_addr *)he->h_addr);
bzero(&(their_addr.sin_zero), 8); /* zero the rest of the struct
*/
if (connect(sockfd, (struct sockaddr *)&their_addr, \par
sizeof(struct sockaddr)) == -1)
perror("connect");
exit(1);
if ((numbytes=recv(sockfd, buf, MAXDATASIZE, 0)) == -1)
perror("recv");
exit(1);
buf[numbytes] = '\fs21 0';
printf("Received: %s",buf);
close(sockfd);
return 0;
Notice that if you don't run the server before you run the client, connect()
returns "Connection refused". Very useful.
----------------------------------------------------------------------------
21. Datagram Sockets
I really don't have that much to talk about here, so I'll just present a
couple of sample programs: talker.c and listener.c.
listener sits on a machine waiting for an incoming packet on port 4950.
talker sends a packet to that port, on the specified machine, that contains
whatever the user enters on the command line.
Here is the source for listener.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/wait.h>
#define MYPORT 4950 /* the port users will be connecting to */
#define MAXBUFLEN 100
main()
int sockfd;
struct sockaddr_in my_addr; /* my address information */
struct sockaddr_in their_addr; /* connector's address information */
int addr_len, numbytes;
char buf[MAXBUFLEN];
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
perror("socket");
exit(1);
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 */
bzero(&(my_addr.sin_zero), 8); /* zero the rest of the struct
*/
if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr
)) \par
== -1)
perror("bind");
exit(1);
addr_len = sizeof(struct sockaddr);
if ((numbytes=recvfrom(sockfd, buf, MAXBUFLEN, 0, \par
(struct sockaddr *)&their_addr, &addr_len)) == -1)
perror("recvfrom");
exit(1);
printf("got packet from %s",inet_ntoa(their_addr.sin_addr));
printf("packet is %d bytes long",numbytes);
buf[numbytes] = '\fs21 0';
printf("packet contains "%s"",buf);
close(sockfd);
Notice that in our call to socket() we're finally using SOCK_DGRAM. Also,
note that there's no need to listen() or accept(). This is one of the perks
of using unconnected datagram sockets!
Next comes the source for talker.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/wait.h>
#define MYPORT 4950 /* the port users will be connecting to */
int main(int argc, char *argv[])
int sockfd;
struct sockaddr_in their_addr; /* connector's address information */
struct hostent *he;
int numbytes;
if (argc != 3)
fprintf(stderr,"usage: talker hostname message");
exit(1);
if ((he=gethostbyname(argv[1])) == NULL) /* get the host info */
herror("gethostbyname");
exit(1);
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
perror("socket");
exit(1);
their_addr.sin_family = AF_INET; /* host byte order */
their_addr.sin_port = htons(MYPORT); /* short, network byte order *
/
their_addr.sin_addr = *((struct in_addr *)he->h_addr);
bzero(&(their_addr.sin_zero), 8); /* zero the rest of the struct
*/
if ((numbytes=sendto(sockfd, argv[2], strlen(argv[2]), 0, \par
(struct sockaddr *)&their_addr, sizeof(struct sockaddr))) == -1)
perror("recvfrom");
exit(1);
printf("sent %d bytes to %s",numbytes,inet_ntoa(their_addr.sin_addr)
);
close(sockfd);
return 0;
And that's all there is to it! Run listener on some machine, then run talker
on another. Watch them communicate! Fun G-rated excitement for the entire
nuclear family!
Except for one more tiny detail that I've mentioned many times in the past:
connected datagram sockets. I need to talk about this here, since we're in
the datagram section of the document. Let's say that talker calls connect()
and specifies the listener's address. From that point on, talker may only
sent to and receive from the address specified by connect(). For this
reason, you don't have to use sendto() and recvfrom(); you can simply use
send() and recv().
----------------------------------------------------------------------------
22. Blocking
Blocking. You've heard about it--now what the hell is it? In a nutshell,
"block" is techie jargon for "sleep". You probably noticed that when you run
listener, above, it just sits there until a packet arrives. What happened is
that it called recvfrom(), there was no data, and so recvfrom() is said to
"block" (that is, sleep there) until some data arrives.
Lots of functions block. accept() blocks. All the recv*() functions block.
The reason they can do this is because they're allowed to. When you first
create the socket descriptor with socket(), the kernel sets it to blocking.
If you don't want a socket to be blocking, you have to make a call to
fcntl():
#include <unistd.h>
#include <fcntl.h>
.
.
sockfd = socket(AF_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-blocking socket and there's
no data there, it's not allowed to block--it will return -1 and errno will
be set to EWOULDBLOCK.
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 elegant solution for
checking to see if there's data waiting to be read comes in the following
section on select().
----------------------------------------------------------------------------
23. select()--Synchronous I/O Multiplexing
This function is somewhat strange, but it's very useful. Take the following
situation: you are a server and you want to listen for incoming connections
as well as keep reading from the connections you already have.
No problem, you say, just an accept() and a couple of recv()s. Not so fast,
buster! What if you're blocking on an accept() call? How are you going to
recv() data at the same time? "Use non-blocking sockets!" No way! You don't
want to be a CPU hog. What, then?
select() gives you the power to monitor several sockets at the same time.
It'll tell you which ones are ready for reading, which are ready for
writing, and which sockets have raised exceptions, if you really want to
know that.
Without any further ado, I'll offer the synopsis of select():
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int numfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
The function monitors "sets" of file descriptors; in particular readfds,
writefds, and exceptfds. If you want to see if you can read from standard
input and some socket descriptor, sockfd, just add the file descriptors 0
and sockfd to the set readfds. The parameter numfds should be set to the
values of the highest file descriptor plus one. In this example, it should
be set to sockfd+1, since it is assuredly higher than standard input (0).
When select() returns, readfds will be modified to reflect which of the file
descriptors you selected is ready for reading. You can test them with the
macro FD_ISSET(), below.
Before progressing much further, I'll talk about how to manipulate these
sets. Each set is of the type fd_set. The following macros operate on this
type:
* FD_ZERO(fd_set *set) - clears a file descriptor set
* FD_SET(int fd, fd_set *set) - adds fd to the set
* FD_CLR(int fd, fd_set *set) - removes fd from the set
* FD_ISSET(int fd, fd_set *set) - tests to see if fd is in the set
Finally, what is this weirded out struct timeval? Well, sometimes you don't
want to wait forever for someone to send you some data. Maybe every 96
seconds you want to print "Still Going..." to the terminal even though
nothing has happened. This time structure allows you to specify a timeout
period. If the time is exceeded and select() still hasn't found any ready
file descriptors, 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 to wait, and set tv_usec to the
number of microseconds to wait. Yes, that's microseconds, not milliseconds.
There are 1,000 microseconds in a millisecond, and 1,000 milliseconds in a
second. Thus, there are 1,000,000 microseconds in a second. Why is it
"usec"? The "u" is supposed to look like the Greek letter Mu that we use for
"micro". Also, when the function returns, timeout might be updated to show
the time still remaining. This depends on what flavor of Unix you're
running.
Yay! We have a microsecond resolution timer! Well, don't count on it.
Standard Unix timeslice is 100 milliseconds, so you'll probably have to wait
at least that long, no matter how small you set your struct timeval.
Other things of interest: If you set the fields in your struct timeval to 0,
select() will timeout immediately, effectively polling all the file
descriptors in your sets. If you set the parameter timeout to NULL, it will
never timeout, and will wait until the first file descriptor is ready.
Finally, if you don't care about waiting for a certain set, you can just set
it to NULL in the call to select().
The following code snippet waits 2.5 seconds for something to appear on
standard input:
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#define STDIN 0 /* file descriptor for standard input */
main()
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!");
else
printf("Timed out.");
If you're on a line buffered terminal, the key you hit should be RETURN or
it will time out anyway.
One final note of interest about select(): if you have a socket that is
listen()'ing, you can check to see if there is a new connection by putting
that socket's file descriptor in the readfds set.
And that, my friends, is a quick overview of the almighty select() function.
----------------------------------------------------------------------------
24. More References
You've come this far, and now you're screaming for more! Where else can you
go to learn more about all this stuff?
Try the following man pages, for starters:
* socket()
* bind()
* connect()
* listen()
* accept()
* send()
* recv()
* sendto()
* recvfrom()
* close()
* shutdown()
* getpeername()
* getsockname()
* gethostbyname()
* gethostbyaddr()
* getprotobyname()
* fcntl()
* select()
* perror()
Also, look up the following books:
Internetworking with TCP/IP, volumes I-III by Douglas E. Comer and
David L. Stevens. Published by Prentice Hall. Second edition ISBNs:
0-13-468505-9, 0-13-472242-6, 0-13-474222-2. There is a third edition
of this set which covers IPv6 and IP over ATM.
Using C on the UNIX System by David A. Curry. Published by O'Reilly &
Associates, Inc. ISBN 0-937175-23-4.
TCP/IP Network Administration by Craig Hunt. Published by O'Reilly &
Associates, Inc. ISBN 0-937175-82-X.
TCP/IP Illustrated, volumes 1-3 by W. Richard Stevens and Gary R.
Wright. Published by Addison Wesley. ISBNs: 0-201-63346-9,
0-201-63354-X, 0-201-63495-3.
Unix Network Programming by W. Richard Stevens. Published by Prentice
Hall. ISBN 0-13-949876-1.
On the web:
BSD Sockets: A Quick And Dirty Primer
(http://sci173x.mrs.umn.edu/~bentlema/unix/sockets.html)
Client-Server Computing
(http://pandonia.canberra.edu.au/ClientServer/socket.html)
Intro to TCP/IP (gopher)
(gopher://gopher-chem.ucdavis.edu/11/Index/Internet_aw/Intro_the_Intern
et/intro.to.ip/)
Internet Protocol Frequently Asked Questions (France)
(http://web.cnam.fr/Network/TCP-IP/)
The Unix Socket FAQ
(http://www.auroraonline.com/sock-faq/)
RFCs--the real dirt:
RFC-768 -- The User Datagram Protocol (UDP)
(ftp://nic.ddn.mil/rfc/rfc768.txt)
RFC-791 -- The Internet Protocol (IP)
(ftp://nic.ddn.mil/rfc/rfc791.txt)
RFC-793 -- The Transmission Control Protocol (TCP)
(ftp://nic.ddn.mil/rfc/rfc793.txt)
RFC-854 -- The Telnet Protocol
(ftp://nic.ddn.mil/rfc/rfc854.txt)
RFC-951 -- The Bootstrap Protocol (BOOTP)
(ftp://nic.ddn.mil/rfc/rfc951.txt)
RFC-1350 -- The Trivial File Transfer Protocol (TFTP)
(ftp://nic.ddn.mil/rfc/rfc1350.txt)
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25. Disclaimer and Call for Help
Well, that's the lot of it. Hopefully at least some of the information
contained within this document has been remotely accurate and I sincerely
hope there aren't any glaring errors. Well, sure, there always are.
So, if there are, that's tough for you. I'm sorry if any inaccuracies
contained herein have caused you any grief, but you just can't hold me
accountable. See, I don't stand behind a single word of this document,
legally speaking. This is my warning to you: the whole thing could be a load
of crap.
But it's probably not. After all, I've spent many many hours messing with
this stuff, and implemented several TCP/IP network utilities for Windows
(including Telnet) as summer work. I'm not the sockets god; I'm just some
guy.
By the way, if anyone has any constructive (or destructive) criticism about
this document, please send mail to beej@ecst.csuchico.edu and I'll try to
make an effort to set the record straight.
In case you're wondering why I did this, well, I did it for the money. Hah!
No, really, I did it because a lot of people have asked me socket-related
questions and when I tell them I've been thinking about putting together a
socket page, they say, "cool!" Besides, I feel that all this hard-earned
knowledge is going to waste if I can't share it with others. WWW just
happens to be the perfect vehicle. I encourage others to provide similar
information whenever possible.
Enough of this--back to coding! ;-)
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Copyright * 1995, 1996 by Brian "Beej" Hall. This guide may be reprinted in
any medium provided that its content is not altered, it is presented in its
entirety, and this copyright notice remains intact. Contact
beej@ecst.csuchico.edu for more information.
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