##概论
##网络的历史 通信的两种方式:
- 有形介质
- 无形介质
- 1969年,ARPA,美国国防部高级研究计划局
- 1975年,Ethernet,Xerox公司与剑桥大学,是LAN的代表
- Novel LAN与Microsoft LAN两份天下
- 70年代,美国国防部提出TCP/IP
- 国际标准化组织ISO,提出OSI/RM(开放系统互连七层协议的参考模型)
##OSI模型
- Physical
- Data Link
- NetWork
- Transport
- Session
- Presentation
- Application
##Internet体系模型
Internet IP 地址有三种基本类型：
-
A类地址
其 W 的高端位为 0，允许有 126 个 A 类地址，分配给拥有大量主机的网络。
-
B类地址
由 W.X 表示网络 ID，其高端前二位为二进制的 10，它用于分配中等规模的网络，可有**16384(64*256)**个 B 类地址。
-
C类地址
其高端前三位为二进制 110，允许大约**200万(32*256*256)**个 C 类地址，每个网络只有 254 个主机，用于小型的局域网。
##客户/服务器模型
##Unix历史
##Linux的发展
##Linux和Unix的发展
#Unix/Linux模型
##Unix/Linux基本结构
外层的程序，诸如 shell 及编辑程序（vi ） ，是通过引用一组明确定义的系统调用而与内核交互的。这些系统调用通知内核为调用程序做各种操作，并在内核与调用程序之间交换数据。
然而， 它们最终都使用由内核提供的底层服务， 并通过系统调用 （SystemCall ）的集合利用这些服务。系统调用的集合及其实现系统调用的内部算法形成了内核的主体。简言之，内核提供了 UNIX/Linux 系统全部应用程序所依赖的服务，并且内核的定义了这些服务
我们看到了三个层次：用户、内核及硬件。
系统调用看起来象 C 程序中普通的函数调用，而库把这些函数调用映射成进入操作系统所需要的源语。然而，汇编语言程序可以不经过系统调用库而直接引用系统调用。程序常常使用像标准 I/O 库这样一些其它的库程序以提供对系统调用的更高级的使用。由于在编译期间把这些库连接到程序上，因此，以这里的观点来说，这些库是用户程序的一部分。
系统调用的集合分成与文件子系统交互作用的部分及与进程控制子系统交互作用的部分。
文件子系统管理文件，其中包括分配文件空间，管理空闲空间，控制对文件的存取，以及为用户检索数据。
进程通过一个特定的系统调用集合，比如通过系统调用open,close,read,write,stat,chown 以及 chmod 等与文件子系统交互。
#Beej's Guide to Network Programming Using Internet Sockets Beej's Guide to Network Programming Using Internet Sockets
#Sockets
##Two Type 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".
- 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.
How do stream sockets achieve this high level of data transmission quality? They use a protocol called "The Transmission Control Protocol", otherwise konwn as "TCP". TCP make 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". IP deals primarily with Internet routing and is not generally responsible for data integrity.
Datagram sockets also use IP for routing, but they don't use TCP; they use the "User Datagram Protocol", or "UDP"Why are they connectless? Well, basically, it's because you don't have to maintain an open connection as you do with strea 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 informations.
#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 showw some examples of how SOCK_DGRAM packets are built.
When another computer receives the packet, the hardware strips the Ethernet header, the kernel strips the IP and UDP header, the TFTP program strips the TFTP, and it finnally 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. 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.
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)
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.
#structs and Data Handling
First the easy one: a socket descriptor. A socket descriptor is the following type: int
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". 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.
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 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;
unsigned short int sin_port;
struct in_addr sin_addr;
unsigned char sin_zero[8];
}
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). A pointer to a struct sockaddr_in can be cast to a pointer to a struct sockaddr and vice-versa. The sin_port and sin_addr must be in Network Byte Order!
The question, how can the entire structure, struct in_addr sin_addr, be in Network Byte Order, requires careful examination of the structure struct in_addr:
struct in_addr{
unsigned long s_addr;
}
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.
##Convert the Natives
There are two types that you can convert: short and long. These functions work for the unsigned variation as well. The function needed starts with "h" for "host", follow it with "to", then "n" for "network", and "s" for "short": htons():
- htons() Host to Network Short
- htonl() Host to Network Long
- ntohs() Network to Host Short
- ntohl() Network to Host Long
Why do
sin_addrandsin_portneed to be in Network Byte Order in astruct sockaddr_in, butsin_familydoes not? The answer:sin_addrandsin_portget encapsulated in the packet at the IP and UDP layers, respectively. Thus, they must be in Network Byte Order. However, thesin_fmailyfield is only used by the kernel to determine what type of address the structure contains, so it must be in Host Byte Order. Also, sincesince_familydoes not get send out on the network, it can be in Host Byte Order.
##IP Addresses and How to Deal With Them
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().
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 is a cleaner interface you can use instead of inet_addr(): it's called inet_aton() ("aton" means "ascii to network"):
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int inet_aton(const char *cp, struct in_addr *inp);
And here's a sample usage, while packing a struct sockaddr_in
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), '\0', 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, althought its use is preferred, the older more common inet_addr() is used in this guide.
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")
printf("%s", inet_ntoa(ina.sin_addr));
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.
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\n",a1);
printf("address 2: %s\n",a2);
will print
address 1: 10.12.110.57
address 2: 10.12.110.57
#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.
##socket()--Get the File Descriptor!
#include <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
First, domain should be set to "AF_INET", just like in the struct sockaddr_in. 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 socketet() choose the correct protocol based on the type.
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.
Once 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 referenced by their protocol family (what the "PF" in "PF_INET" stands for). That didn't happen. Oh well. So the correct thing to do is to use AF_INET in your struct sockaddr_in and PF_INET in your call to socket(). But practically speaking, you can use AF_INET everywhere.
##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.
#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).
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
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.
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 bit a of 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.
##connect()--Hey, you!
#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).
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.
##listen()--Will somebody please call me?
You want to wait for incoming connections and handle them in some way. The process is two step: first you listen(), then you accept()
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.
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 ort. So if you're going to be listening for incoming connections, the sequence of system calls you'll make is:
socket();
bind();
listen();
##accept()--"Thank you for calling port 3490"
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!
#include <sys/socket.h>
int accept(int sockfd, void *addr, int *addrlen);
sockfd is the listen()ing socket descriptor. addr will usually be a pointer to a local struct sockaddr_in. This is where the information about the incomming connection will go. 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.
accept() returns -1 and sets errno if an error occurs.
##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.
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.)
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 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.
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.)
Wait! recv() can return 0. This can mean only one thing: the remote side has closed the connection on you! A return value of 0 is recv()'s way of letting you know this has occurred.
##sendto() and recvfrom()--Talk to me, DGRAM-style
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.
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.
##close() and shutdown()--Get outta my face!
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.
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.)
It's important to note that shutdown() doesn't actually close the file descriptor--it just changes its usability. To free a socket descriptor, you need to use close().
##getpeername()--Who are you?
#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.
##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.
#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.
##DNS--You say "whitehouse.gov", I say "198.137.240.92"
$ telnet whitehouse.gov
telnet can find out that it needs to connect() to "198.137.240.92".
#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 inh_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.)
With `gethostbyname(), you can't use perror() to print error message (since errno is not used). Instead, call herror().
/*
** getip.c -- a hostname lookup demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int main(int argc, char *argv[])
{
struct hostent *h;
if (argc != 2) { // error check the command line
fprintf(stderr,"usage: getip address\n");
exit(1);
}
if ((h=gethostbyname(argv[1])) == NULL) { // get the host info
herror("gethostbyname");
exit(1);
}
printf("Host name : %s\n", h->h_name);
printf("IP Address : %s\n", 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().
The only possible weirdness might be in the printing of the IP address, above.
h->h_addris a char*, butinet_ntoa()wants astruct in_addrpassed to it. So I casth->h_addrto astruct in_addr*, then dereference it to get at the data.
#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 23 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.
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.
##A Simple Stream Server
All this server does is send the string "Hello, World!\n" 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.
/*
** server.c -- a stream socket server demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/wait.h>
#include <signal.h>
#define MYPORT 3490 // the port users will be connecting to
#define BACKLOG 10 // how many pending connections queue will hold
void sigchld_handler(int s)
{
while(wait(NULL) > 0);
}
int main(void)
{
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;
struct sigaction sa;
int yes=1;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
exit(1);
}
if (setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) {
perror("setsockopt");
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; // automatically fill with my IP
memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct
if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr))
== -1) {
perror("bind");
exit(1);
}
if (listen(sockfd, BACKLOG) == -1) {
perror("listen");
exit(1);
}
sa.sa_handler = sigchld_handler; // reap all dead processes
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if (sigaction(SIGCHLD, &sa, NULL) == -1) {
perror("sigaction");
exit(1);
}
while(1) { // main accept() loop
sin_size = sizeof(struct sockaddr_in);
if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr,
&sin_size)) == -1) {
perror("accept");
continue;
}
printf("server: got connection from %s\n",
inet_ntoa(their_addr.sin_addr));
if (!fork()) { // this is the child process
close(sockfd); // child doesn't need the listener
if (send(new_fd, "Hello, world!\n", 14, 0) == -1)
perror("send");
close(new_fd);
exit(0);
}
close(new_fd); // parent doesn't need this
}
return 0;
}
##A Simple Stream Client
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.
/*
** client.c -- a stream socket client demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.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\n");
exit(1);
}
if ((he=gethostbyname(argv[1])) == NULL) { // get the host info
perror("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);
memset(&(their_addr.sin_zero), '\0', 8); // zero the rest of the struct
if (connect(sockfd, (struct sockaddr *)&their_addr,
sizeof(struct sockaddr)) == -1) {
perror("connect");
exit(1);
}
if ((numbytes=recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror("recv");
exit(1);
}
buf[numbytes] = '\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.
##Datagram Sockets
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.
/*
** listener.c -- a datagram sockets "server" demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define MYPORT 4950 // the port users will be connecting to
#define MAXBUFLEN 100
int main(void)
{
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; // automatically fill with my IP
memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct
if (bind(sockfd, (struct sockaddr *)&my_addr,
sizeof(struct sockaddr)) == -1) {
perror("bind");
exit(1);
}
addr_len = sizeof(struct sockaddr);
if ((numbytes=recvfrom(sockfd,buf, MAXBUFLEN-1, 0,
(struct sockaddr *)&their_addr, &addr_len)) == -1) {
perror("recvfrom");
exit(1);
}
printf("got packet from %s\n",inet_ntoa(their_addr.sin_addr));
printf("packet is %d bytes long\n",numbytes);
buf[numbytes] = '\0';
printf("packet contains \"%s\"\n",buf);
close(sockfd);
return 0;
}
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!
/*
** talker.c -- a datagram "client" demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.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\n");
exit(1);
}
if ((he=gethostbyname(argv[1])) == NULL) { // get the host info
perror("gethostbyname");
exit(1);
}
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
perror("socket");
exit(1);
}
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);
memset(&(their_addr.sin_zero), '\0', 8); // zero the rest of the struct
if ((numbytes=sendto(sockfd, argv[2], strlen(argv[2]), 0,
(struct sockaddr *)&their_addr, sizeof(struct sockaddr))) == -1) {
perror("sendto");
exit(1);
}
printf("sent %d bytes to %s\n", numbytes,
inet_ntoa(their_addr.sin_addr));
close(sockfd);
return 0;
}
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().
#Slightly Advanced Techniques
##Blocking
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 follwing section on select().
##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.
#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 which is ready for reading. You can test them with the macro FD_ISSET(), blow.
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 setFD_SET(int fd, fd_set *set)-- addsfdto the setFD_CLR(int fd, fd_set *set)-- removesfdfrom the setFD_ISSET(int fd, fd_set *set)-- tests to see iffdis 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 struct 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.
struct timeval {
int tv_sec; // seconds
int tv_usec; // microseconds
};
Yay! We have a microsecond resolution timer! Well, don't count on it. Standard Unix timeslice is around 100 milliseconds, so you might have to wait 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().
/*
** select.c -- a select() demo
*/
#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#define STDIN 0 // file descriptor for standard input
int main(void)
{
struct timeval tv;
fd_set readfds;
tv.tv_sec = 2;
tv.tv_usec = 500000;
FD_ZERO(&readfds);
FD_SET(STDIN, &readfds);
// don't care about writefds and exceptfds:
select(STDIN+1, &readfds, NULL, NULL, &tv);
if (FD_ISSET(STDIN, &readfds))
printf("A key was pressed!\n");
else
printf("Timed out.\n");
return 0;
}
If you're on a line buffered terminal, the key you hit should be RETURN or it will time out anyway.
Now, some of you might think this is a great way to wait for data on datagram socket--and you are right: it might be. Some unices can use select in this manneer, and some can't. You should see what your local man page says on the matter if you want to attempt it.
What happens if a socket in the read set closes the connection? Well, in that case, select() returns with that socket descriptor set as "ready to read". When you actually do recv() from it, recv() will return 0. That's how you know the client has closed the connection.
One more 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.
This program acts like a simple multi-user chat server. Start it running in one window, then telnet to it ("telnet hostname 9034") from multiple other windows. When you type something in one telnet session, it should appear in all the others.
/*
** selectserver.c -- a cheezy multiperson chat server
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define PORT 9034 // port we're listening on
int main(void)
{
fd_set master; // master file descriptor list
fd_set read_fds; // temp file descriptor list for select()
struct sockaddr_in myaddr; // server address
struct sockaddr_in remoteaddr; // client address
int fdmax; // maximum file descriptor number
int listener; // listening socket descriptor
int newfd; // newly accept()ed socket descriptor
char buf[256]; // buffer for client data
int nbytes;
int yes=1; // for setsockopt() SO_REUSEADDR, below
int addrlen;
int i, j;
FD_ZERO(&master); // clear the master and temp sets
FD_ZERO(&read_fds);
// get the listener
if ((listener = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
exit(1);
}
// lose the pesky "address already in use" error message
if (setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes,
sizeof(int)) == -1) {
perror("setsockopt");
exit(1);
}
// bind
myaddr.sin_family = AF_INET;
myaddr.sin_addr.s_addr = INADDR_ANY;
myaddr.sin_port = htons(PORT);
memset(&(myaddr.sin_zero), '\0', 8);
if (bind(listener, (struct sockaddr *)&myaddr, sizeof(myaddr)) == -1) {
perror("bind");
exit(1);
}
// listen
if (listen(listener, 10) == -1) {
perror("listen");
exit(1);
}
// add the listener to the master set
FD_SET(listener, &master);
// keep track of the biggest file descriptor
fdmax = listener; // so far, it's this one
// main loop
for(;;) {
read_fds = master; // copy it
if (select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1) {
perror("select");
exit(1);
}
// run through the existing connections looking for data to read
for(i = 0; i <= fdmax; i++) {
if (FD_ISSET(i, &read_fds)) { // we got one!!
if (i == listener) {
// handle new connections
addrlen = sizeof(remoteaddr);
if ((newfd = accept(listener, (struct sockaddr *)&remoteaddr,
&addrlen)) == -1) {
perror("accept");
} else {
FD_SET(newfd, &master); // add to master set
if (newfd > fdmax) { // keep track of the maximum
fdmax = newfd;
}
printf("selectserver: new connection from %s on "
"socket %d\n", inet_ntoa(remoteaddr.sin_addr), newfd);
}
} else {
// handle data from a client
if ((nbytes = recv(i, buf, sizeof(buf), 0)) <= 0) {
// got error or connection closed by client
if (nbytes == 0) {
// connection closed
printf("selectserver: socket %d hung up\n", i);
} else {
perror("recv");
}
close(i); // bye!
FD_CLR(i, &master); // remove from master set
} else {
// we got some data from a client
for(j = 0; j <= fdmax; j++) {
// send to everyone!
if (FD_ISSET(j, &master)) {
// except the listener and ourselves
if (j != listener && j != i) {
if (send(j, buf, nbytes, 0) == -1) {
perror("send");
}
}
}
}
}
} // it's SO UGLY!
}
}
}
return 0;
}
Notice I have two file descriptor sets in the code: master and read_fds. The first, master, holds all the socket descriptors that are currently connected, as well as the socket descriptor that is listening for new connections.
The reason I have the master set is that select() actually changes the set you pass into it to reflect which sockets are ready to read. Since I have to keep track of the connections from one call of select() to the next, I must store these safely away somewhere. At the last minute, I copy the master into the read_fds, and then call select().
Notice I check to see when the listener socket is ready to read. When it is, it means I have a new connection pending, and I accept() it and add it to the master set. Similarly, when a client connection is ready to read, and recv() returns 0, I know the client has closed the connection, and I must remove it from the master set.
If the client recv() returns non-zero, though, I know some data has been received. So I get it, and then go through the master list and send that data to all the rest of the connected clients.
##Handling Partial send()s
Remember back in the section about send(), above, when I said that send() might not send all the bytes you asked it to? That is, you want it to send 512 bytes, but it returns 412. What happened to the remaining 100 bytes?
Well, they're still in your little buffer waiting to be sent out. Due to circumstances beyond your control, the kernel decided not to send all the data out in one chunk, and now, my friend, it's up to you to get the data out there.
#include <sys/types.h>
#include <sys/socket.h>
int sendall(int s, char *buf, int *len)
{
int total = 0; // how many bytes we've sent
int bytesleft = *len; // how many we have left to send
int n;
while(total < *len) {
n = send(s, buf+total, bytesleft, 0);
if (n == -1) { break; }
total += n;
bytesleft -= n;
}
*len = total; // return number actually sent here
return n==-1?-1:0; // return -1 on failure, 0 on success
}
In this example, s is the socket you want to send the data to, buf is the buffer containing the data, and len is a pointer to an int containing the number of bytes in the buffer.
For completeness, here's a sample call to the function:
char buf[10] = "Beej!";
int len;
len = strlen(buf);
if (sendall(s, buf, &len) == -1) {
perror("sendall");
printf("We only sent %d bytes because of the error!\n", len);
}
What happens on the receiver's end when part of a packet arrives? If the packets are variable length, how does the receiver know when one packet ends and another begins? Yes, real-world scenarios are a royal pain in the donkeys. You probably have to encapsulate (remember that from the data encapsulation section way back there at the beginning?) Read on for details!
##Son of Data Encapsulation
What does it really mean to encapsulate data, anyway? In the simplest case, it means you'll stick a header on there with either some identifying information or a packet length, or both.
What should your header look like? Well, it's just some binary data that represents whatever you feel is necessary to complete your project.
Okay. For instance, let's say you have a multi-user chat program that uses SOCK_STREAMs. When a user types ("says") something, two pieces of information need to be transmitted to the server: what was said and who said it.
The problem is that the messages can be of varying lengths. One person named "tom" might say, "Hi", and another person named "Benjamin" might say, "Hey guys what is up?"
So you send() all this stuff to the clients as it comes in. Your outgoing data stream looks like this:
t o m H i B e n j a m i n H e y g u y s w h a t i s u p ?
And so on. How does the client know when one message starts and another stops? You could, if you wanted, make all messages the same length and just call the sendall() we implemented, above. But that wastes bandwidth! We don't want to send() 1024 bytes just so "tom" can say "Hi".
So we encapsulate the data in a tiny header and packet structure. Both the client and server know how to pack and unpack (sometimes referred to as "marshal" and "unmarshal") this data. Don't look now, but we're starting to define a protocol that describes how a client and server communicate!
In this case, let's assume the user name is a fixed length of 8 characters, padded with '\0'. And then let's assume the data is variable length, up to a maximum of 128 characters. Let's have a look a sample packet structure that we might use in this situation:
len(1 byte, unsigned) -- The total length of the packet, counting the 8-byte user name and chat data.name(8 bytes) -- The user's name, NUL-padded if necessary.chatdata(n-bytes) -- The data itself, no more than 128 bytes. The length of the packet should be calculated as the length of this data plus 8 (the length of the name field, above).
Using the above packet definition, the first packet would consist of the following information (in hex and ASCII):
0A 74 6F 6D 00 00 00 00 00 48 69
(length) T o m (padding) H i
And the second is similar:
14 42 65 6E 6A 61 6D 69 6E 48 65 79 20 67 75 79 73 20 77 ...
(length) B e n j a m i n H e y g u y s w ...
The length is stored in Network Byte Order, of course. In this case, it's only one byte so it doesn't matter, but generally speaking you'll want all your binary integers to be stored in Network Byte Order in your packets.
-
When you're sending this data, you should be safe and use a command similar to
sendall(), above, so you know all the data is sent, even if it takes multiple calls tosend()to get it all out. -
Likewise, when you're receiving this data, you need to do a bit of extra work. To be safe, you should assume that you might receive a partial packet (like maybe we receive "00 14 42 65 6E" from Benjamin, above, but that's all we get in this call to
recv()). We need to callrecv()over and over agin until the packet is completely received.we know the number of bytes we need to receive in total for the packet to be complete, since that number is tacked on the front of the packet. We also know the maximum packet size is 1+8+128, or 137 bytes
What you can do is declare an array big enough for two packets. This is your work array where you will reconstruct packets as they arrive.
Every time you
recv()data, you'll feed it into the work buffer and check to see if the packet is complete. That is, the number of bytes in the buffer is greater than or equal to the length specified in the header (+1, because the length in the header doesn't include the byte for the length itself.) If the number of bytes in the buffer is less than 1, the packet is not complete, obviously. You have to make a special case for this, though, since the first byte is garbage and you can't rely on it for the correct packet length.Whew! Are you juggling that in your head yet? Well, here's the second of the one-two punch: you might have read past the end of one packet and onto the next in a single recv() call. That is, you have a work buffer with one complete packet, and an incomplete part of the next packet! Bloody heck. (But this is why you made your work buffer large enough to hold two packets--in case this happened!)
Since you know the length of the first packet from the header, and you've been keeping track of the number of bytes in the work buffer, you can subtract and calculate how many of the bytes in the work buffer belong to the second (incomplete) packet. When you've handled the first one, you can clear it out of the work buffer and move the partial second packed down the to front of the buffer so it's all ready to go for the next recv().