Sockets and intro to IO
multiplexing
Goals
We are going to study sockets programming as
means to introduce IO multiplexing problem.
We will revisit socket programming in lecture 3.
Socket Reference
Check out Beej’s guide to network programming
for free help. (http://beej.us/guide/bgnet)
For the homework you will also need Beej’s guide
for IPC.
Chapter 16 (16.1-16.5) of APUE
Before we begin
All the system calls described here are UNIX
system calls.
Microsoft windows actually has system calls with
exactly matching names and prototypes. (since
socket programming was pretty much borrowed
from UNIX)
Microsoft programming required different #include
and link options. it is beyond scope but covered in
beej’s guide.
Socket - OS view
Whenever a process want to open a
communication it open’s a socket.
Socket is literally one side of a communication. (1to-1 communication is two sockets)
In UNIX sockets are described by file descriptors
just like open files.
The types of sockets
There are many types of sockets.
We will deal with 2 types.
We will deal with 2 domains of sockets.
So as a total we will deal with 4 kinds of sockets.
There are LOTS more socket types.
Socket types
STREAM socket - data is transferred as an
ordered stream. no packets. (packets may be
stacked or broken on receives). no losses.
DGRAM (sort for datagram) sockets - data is
transferred as packets. (no breaking or stacking)
may not be in order. may have losses.
Domain of sockets
INTERNET - communication using
TCP/UDP/other internet protocol. can
communicate between hosts. has some
overhead. may have losses.
UNIX DOMAIN - communication using similar
commands between processes on the same host.
removes network overhead. no losses.
Summary
Internet stream sockets - Use TCP. allow stream
communications between processes on different or same hosts.
no losses.
Internet datagram sockets - Use UDP. allow datagram
communication between processes on different or same hosts.
prune to packet loss.
UDS - stream - allow stream communication on the same host.
no losses.
UDS - dgram - allow datagram communication on the same
host. no losses.
Socket(2)
•
SOCKET(2)
•
NAME
•
•
BSD System Calls Manual
SOCKET(2)
socket -- create an endpoint for communication
SYNOPSIS
•
#include <sys/socket.h>
•
int
•
socket(int domain, int type, int protocol)
The parameters
Domain - in our case we will deal with only TWO
domains AF_UNIX (for UDS) and AF_INET (for
internet)
Type - SOCK_STREAM or SOCK_DGRAM
protocol - in our case we will always use 0. (used
if there are multiple sockets of the same domain
and type)
Return value
is an int (socket file descriptors).
is just an int. (same as the int for file descriptor we
get for open) contains no info in itself.
it is actually an index for the OS for a place in the
file descriptors table (so the OS will know which fd
we want) - or as we said - just an int.
what socket do and does
not do
Socket create the OS “name” for a communication
end point.
Socket only create the end point of
communication not the communication itself.
Server side programming
The first thing we do in a TCP (AF_INET,
SOCK_STREAM) server is to create a listening
socket.
The listening socket will be used to create new
communication. (only for that!)
The listening socket will NOT be used for
communication with clients. we will have a new
socket for that.
Port
Port is a logical address within the machine for
communication.
your machine can have hundreds of processes
waiting for communications (there can also be an
multiple ports per process) on different ports.
lay man terms : IP is the address and port is the
mailbox number.
bind
•
BIND(2)
•
NAME
•
•
BSD System Calls Manual
BIND(2)
bind -- bind a name to a socket
SYNOPSIS
•
#include <sys/socket.h>
•
int
•
bind(int socket, const struct sockaddr *address, socklen_t address_len);
what bind do?
bind is actually attaching socket to a port.
if the port is available bind is successful. future
bind to the port will fail (unless the socket is
released)
The socket can now receive information on that
port.
Struct sockaddr
•
bind(int socket, const struct sockaddr *address, socklen_t address_len);
Struct sockaddr is “base class” for all
communication
We never use struct sockaddr.
we use struct sockaddr_in for internet sockets.
We use struct sockaddr_un for uds
using bind for TCP
•
my_addr.sin_family = AF_INET;
•
my_addr.sin_port = htons(MYPORT);
•
my_addr.sin_addr.s_addr = INADDR_ANY;
•
memset(my_addr.sin_zero, '\0', sizeof my_addr.sin_zero);
•
bind(sockfd, (struct sockaddr *)&my_addr, sizeof my_addr);
// short, network byte order
Endianess
how do bytes are ordered in an integer? (if our integer
consist of 4 bytes do we save it as ABCD or DCBA?)
There are two answers to that question. most significant
bit last or first.
Network order tends to be MSB first. hosts tend to be
MSB last(depending on vendor. the function htons will
rearrange bytes if needed (or nothing)
Listen(2)
After attaching a the socket to a port using bind
we call listen(2)
listen is a system call that tells the OS to open a
queue for new communications.
man 2 listen
•
LISTEN(2)
•
NAME
•
•
BSD System Calls Manual
listen -- listen for connections on a socket
SYNOPSIS
•
#include <sys/socket.h>
•
int
•
listen(int socket, int backlog)
LISTEN(2)
what does listen do
allocate memory for queue.(enough for backlog
connections)
Tells the OS to put incoming connections in the
queue.
Accept(2)
Accept is a BLOCKING function.
Accept tells the machine to wait until a new
connection is available.
Accept is used to create a connection with an
incoming client
Accept return NEW FILE DESCRIPTOR for the
new connection!
man 2 accept
•
ACCEPT(2)
•
NAME
•
•
BSD System Calls Manual
accept -- accept a connection on a socket
SYNOPSIS
•
#include <sys/socket.h>
•
int
•
ACCEPT(2)
accept(int socket, struct sockaddr *restrict address, socklen_t *restrict
address_len);
Blocking vs. non-Blocking
functions
Some system calls can take a long time and we
are dependent on some external condition. for
example when we receive communication or wait
for clients to connect.
We call those clients Blocking.
Blocking functions are functions
Communication
we communicate using send(2) and recv(2)
function that do just that.
•
SEND(2)
•
NAME
•
•
BSD System Calls Manual
SEND(2)
man 2 send
send, sendmsg, sendto -- send a message from a socket
SYNOPSIS
•
#include <sys/socket.h>
•
ssize_t
send(int socket, const void *buffer, size_t length, int flags);
•
ssize_t
sendmsg(int socket, const struct msghdr *message, int flags);
•
ssize_t
sendto(int socket, const void *buffer, size_t length, int flags,
•
•
const struct sockaddr *dest_addr, socklen_t dest_len);
DESCRIPTION
•
Send(), sendto(), and sendmsg() are used to transmit a message to another
•
socket. Send() may be used only when the socket is in a connected state,
•
while sendto() and sendmsg() may be used at any time.
man 2 recv
•
RECV(2)
•
NAME
•
•
•
•
BSD System Calls Manual
RECV(2)
recv, recvfrom, recvmsg -- receive a message from a socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
•
#include <sys/socket.h>
•
ssize_t
•
recv(int socket, void *buffer, size_t length, int flags);
coding a TCP client
Client calls socket just like a server.
However, client does not need well known port
and uses connect(2) to create communication
instead of bind(2)+listen(2)+accept(2).
Once connection is open we use send(2)/recv(2)
normally.
•
SYNOPSIS
•
#include <sys/types.h>
•
#include <sys/socket.h>
•
int
•
connect(int socket, const struct sockaddr *address,
•
man 2 connect
socklen_t address_len);
•
CONNECT(2)
•
NAME
•
•
BSD System Calls Manual
connect -- initiate a connection on a socket
SYNOPSIS
CONNECT(2)
one more thing
to terminate a connection we use close(2) just like
closing a file.
Coding silly client and silly
server
Silly client connects to server and send’s “hello
world\n”.
Silly server wants to receive connections from silly
clients and print what silly client sends.
silly client
•
int main() {
• struct sockaddr_in sin;
•
int sockfd=socket(AF_INET,SOCK_STREAM,0);
•
sin.sin_family=AF_INET;
•
sin.sin_port=htons(1234);
•
sin.sin_addr.s_addr=inet_addr("127.0.0.1");
•
memset(&sin.sin_zero,0,sizeof(sin.sin_zero));
•
connect(sockfd, &sin, sizeof(sin));
•
send(sockfd, "hello world\n",12,0);
•
close(sockfd);
•
}
silly server
•
int main()
•
{
•
struct sockaddr_in sin,theirsin;
•
int len, bufsize=1000 ,newfd;
•
char buf[1000];
•
int sockfd=socket(AF_INET,SOCK_STREAM,0);
•
sin.sin_family=AF_INET;
•
sin.sin_port=htons(1234);
•
sin.sin_addr.s_addr=inet_addr("127.0.0.1");
•
memset(&sin.sin_zero,0,sizeof(sin.sin_zero));
silly server 2
• bind (sockfd, &sin, sizeof(sin));
• listen (sockfd, 10);
•
newfd=accept(sockfd, &theirsin, &len);
•
int numbytes=recv(newfd, buf, bufsize ,0);
•
buf[numbytes]='\0';
•
printf ("client says : %s",buf);
•
close(newfd);
•
close(sockfd);
•
}
multiple clients?
Silly server support only the first client.
Even if we will loop and accept new connections,
since RECEIVE and ACCEPT are blocking we
cannot (at least until we learn something) do them
both
Multi tasking and IO
multiplexing
In the 2nd half of the lecture we will discuss doing
things in parallel.
Either handling multiple I/O (I/O multiplexing) or
doing several things together (for example
handling I/O while we do some complex math)