That?s right! Everything in Unix is a file! What this means 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 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.
To get a file descriptor for a network connection 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.
Since the socket() system routine returns a file descriptor, you can talk to it using read() and write() routines, however send() and recv() are a specialized type of routines which give you a greater control over what you send and how you read it.
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.
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. 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.
Datagram sockets also use IP for routing, but they don't use TCP; they use the "User Datagram Protocol", or "UDP" (see RFC-768.) 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. Hence, Datagram sockets are less reliable, and should only be used if you are an expert.
Datagram sockets are connectionless 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. 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.
intJust a regular int.
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.
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_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 "But," you object, "how can the entire structure,
/* 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
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"). But there are:
A final point: why do sin_addr and sin_port need to
be in Network Byte Order in a
First, let's say you have a
ina.sin_addr.s_addr = inet_addr("155.1.1.1");
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! 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
printf("%s",inet_ntoa(ina.sin_addr));
That will print the IP address. Note that inet_ntoa() takes a
char *a1, *a2;
.
.
a1 = inet_ntoa(ina1.sin_addr); /* this is 198.92.129.1 */
a2 = inet_ntoa(ina2.sin_addr); /* this is 155.1.1.1 */
printf("address 1: %s\n",a1);
printf("address 2: %s\n",a2);
will print:
address 1: 155.1.1.1 address 2: 155.1.1.1If you need to save the address, strcpy() it to your own character array. For more reference, see DNS, below.
#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 man page on socket(). Also, there's a "better" way to get the protocol. See the man page on getprotobyname().
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 man page on perror())
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 5000
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("155.1.1.1");
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 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 overboard 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.
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 "155.1.1.1"
#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 struct */
/* don't forget to error check the connect()! */
connect(sockfd, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr));
.
.
.
Again, be sure to check the return value from connect()--it'll
return -1 on error and set the variable errno.
Also, notice that we didn't call bind(). Basically, we don't care about our local port number; we only care where we're going. The kernel will choose a local port for us, and the site we connect to will automatically get this information from us. No worries.
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().
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.
The accept() returns -1 and sets errno if an error occurs.
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 5000 /* 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.
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 man page on send() for more information concerning flags.)
Some sample code might be:
char *msg = "Konrad 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 man page on recv() 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, and you can now pass data back and forth on stream sockets! You're a Unix Network Programmer!
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.
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:
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.).
#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.
#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.
$ telnet whitehouse.govtelnet 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:
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 is 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\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().
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.
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 5000where remotehostname is the name of the machine you're running it on. (Note: a trailing backslash on a line means that the line is continued on the next.)
Here is a simple server source code:
#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 5000 /* 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)) \
== -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, \
&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 */
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 */
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.
All this client does is connect to the host you specify on the command line, port 5000. It gets the string that the server sends.
Here is a simple client source code:
#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 5000 /* 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 */
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, \
sizeof(struct sockaddr)) == -1) {
perror("connect");
exit(1);
}
if ((numbytes=recv(sockfd, buf, MAXDATASIZE, 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.
listener sits on a machine waiting for an incoming packet on port 5555. The 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 5555 /* 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)) \
== -1) {
perror("bind");
exit(1);
}
addr_len = sizeof(struct sockaddr);
if ((numbytes=recvfrom(sockfd, buf, MAXBUFLEN, 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);
}
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 talker.c code:
#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 5555 /* 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 */
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, \
(struct sockaddr *)&their_addr, sizeof(struct sockaddr))) == -1) {
perror("recvfrom");
exit(1);
}
printf("sent %d bytes to %s\n",numbytes,inet_ntoa(their_addr.sin_addr));
close(sockfd);
return 0;
}
However, once you call connect(), you don't have to use recvfrom()
or sendto(), you can use the previously discussed send() and recv() functions.
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().
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:
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, timeoutmight
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!\n");
else
printf("Timed out.\n");
}
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.
Unix Network Programming by W. Richard Stevens.
Published by Prentice Hall. ISBN 0-13-949876-1.
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_Internet/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/)
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)