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 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.)
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.
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!
4.7.
"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, socklen_t 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, an
int deep-down, 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.
4.8.
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.)
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().
Nothing to it.
4.9.
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
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. (Ok, ok. If the other computer is running an ident daemon,
this is possible. This, however, is beyond the scope of this document. Check out
for more
info.)
4.10.
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
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.
4.11.
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
"63.161.169.137".
But how does it work? You'll be using the function
gethostbyname():
#include
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.
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.
Client-Server Interaction.
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.
5.1.
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.
:
(Note: a trailing backslash on a line means that the line is continued on the
next.)
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 socklen_t sin_size; struct sigaction sa; int yes=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), ' ', 8); // zero the rest of the struct
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
", 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!
", 14, 0) == -1) perror("send"); close(new_fd); exit(0); } close(new_fd); // parent doesn't need this }
return 0; }
In case you're curious, I have the code in one big main() function
for (I feel) syntactic clarity. Feel free to split it into smaller functions if
it makes you feel better.
(Also, this whole sigaction() thing might be new to you--that's ok.
The code that's there is responsible for reaping zombie processes that appear as
the fork()ed child processes exit. If you make lots of zombies and
don't reap them, your system administrator will become agitated.)
You can get the data from this server by using the client listed in the next
section.
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.
Notice that if you don't run the server before you run the client,
connect() returns "Connection refused". Very useful.
5.3.
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 :
/* ** listener.c -- a datagram sockets "server" demo */
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().
These aren't really advanced, but they're getting out of the more
basic levels we've already covered. In fact, if you've gotten this far, you
should consider yourself fairly accomplished in the basics of Unix network
programming! Congratulations!
So here we go into the brave new world of some of the more esoteric things
you might want to learn about sockets. Have at it!
6.1.
Blocking. You've heard about it--now what the heck 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():
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().
6.2.
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():
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(), 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 μ (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 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().
waits 2.5 seconds for something to appear on standard input:
/* ** select.c -- a select() demo */
#include #include #include #include
#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!
"); else printf("Timed out.
");
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 a
datagram socket--and you are right: it might be. Some Unices can use
select in this manner, and some can't. You should see what your local man page
says on the matter if you want to attempt it.
Some Unices update the time in your struct timeval to reflect the
amount of time still remaining before a timeout. But others do not. Don't rely
on that occurring if you want to be portable. (Use gettimeofday() if
you need to track time elapsed. It's a bummer, I know, but that's the way it
is.)
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.
And that, my friends, is a quick overview of the almighty select()
function.
But, by popular demand, here is an in-depth example. Unfortunately, the
difference between the dirt-simple example, above, and this one here is
significant. But have a look, then read the description that follows it.
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 */
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 socklen_t addrlen; int i, j;
FD_ZERO(&master); // clear the master and temp sets FD_ZERO(&read_fds);
// get the listener if ((listener = socket(PF_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); }
// 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
", 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
", 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().
But doesn't this mean that every time I get a new connection, I have to add
it to the master set? Yup! And every time a connection closes, I
have to remove it from the master set? Yes, it does.
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.
And that, my friends, is a less-than-simple overview of the almighty
select() function.
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.
You could write a function like this to do it, too:
#include #include
int sendall(int s, char *buf, int *len) { int total = 0; // how many bytes we've sent int bytesleft = *len; // how many we have left to send int n;
while(total < *len) { n = send(s, buf+total, bytesleft, 0); if (n == -1) { break; } total += n; bytesleft -= n; }
*len = total; // return number actually sent here
return n==-1?-1:0; // return -1 on failure, 0 on success }
In this example, s is the socket you 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.
The function returns -1 on error (and errno
is still set from the call to send().) Also, the number of bytes
actually sent is returned in len. This will be the same number
of bytes you asked it to send, unless there was an error. sendall()
will do it's best, huffing and puffing, to send the data out, but if there's an
error, it gets back to you right away.
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!
", 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!
6.4.
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.
Wow. That's vague.
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.
So far so good? "What's the problem?" you're asking.
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 ' '. 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).
Why did I choose the 8-byte and 128-byte limits for the fields? I pulled them
out of the air, assuming they'd be long enough. Maybe, though, 8 bytes is too
restrictive for your needs, and you can have a 30-byte name field, or whatever.
The choice is up to you.
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 to send() 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 "14 42 65 6E" from Benjamin, above, but that's
all we get in this call to recv()). We need to call recv()
over and over again until the packet is completely received.
But how? Well, 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 (because
that's how we defined the packet.)
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.
Once the packet is complete, you can do with it what you will. Use it, and
remove it from your work buffer.
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().
(Some of you readers will note that actually moving the partial second packet
to the beginning of the work buffer takes time, and the program can be coded to
not require this by using a circular buffer. Unfortunately for the rest of you,
a discussion on circular buffers is beyond the scope of this article. If you're
still curious, grab a data structures book and go from there.)
I never said it was easy. Ok, I did say it was easy. And it is; you just need
practice and pretty soon it'll come to you naturally. By Excalibur I swear
it!
