10 WinSock Programming
10.1 WinSock
10.1.1 Introduction
The Windows Sockets specification describes a common interface for networked
Windows programs. WinSock uses TCP/IP communications and provides for binary
and source code compatibility for different network types.
The Windows Sockets API (WinSock API, or WSA) is a library of functions that
implement the socket interface by the Berkley Software Distribution of UNIX. Win-
Sock augments the Berkley socket implementation by adding Windows-specific ex-
tensions to support the message-driven nature of Windows system.
The basic implementation normally involves:
• Opening a socket. This allows for multiple connections with multiple hosts. Each
socket has a unique identifier. It normally involves defining the protocol suite,
the socket type and the protocol name. The API call used for this is socket().
• Naming a socket. This involves assigning location and identity attributes to a
socket. The API call used for this is bind().
• Associate with another socket. This involves either listening for a connection or
actively seeking a connection. The API calls used in this are listen(), con-
nect()and accept().
• Send and receive between socket. The API calls used in this are send(),
sendto(), recv() and recvfrom().
• Close the socket. The API calls used in this are close() and shutdown().
10.1.2 Windows Sockets
The main WinSock API calls are:
socket(). Creates a socket.
accept(). Accepts a connection on a socket.
connect(). Establishes a connection to a peer.
bind(). Associates a local address with a socket.
listen(). Establishes a socket to listen for incoming connection.
send(). Sends data on a connected socket.
sendto(). Sends data on an unconnected socket.
recv(). Receives data from a connected socket.
recvfrom(). Receives data from an unconnected socket.
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shutdown(). Disables send or receive operations on a socket.
closesocket(). Closes a socket.
Figure 10.10 shows the operation of a connection of a client to a server. The server is
defined as the computer which waits for a connection, the client is the computer
which initially makes contact with the server.
On the server the computer initially creates a socket with the socket() function,
and this is bound to a name with the bind() function. After this the server listens for
a connection with the listen() function. When the client calls the connection()
function the server then accepts the connection with accept(). After this the server
and client can send and receive data with the send() or recv() functions. When the
data transfer is complete the closesocket() is used to close the socket.
socket()
The socket() function creates a socket. Its syntax is:
SOCKET socket ( int af, int type, int protocol)
where
af A value of PF_INET specifies the ARPA Internet address format specifi-
cation (others include AF_IPX for SPX/IPX and AF_APPLETALK for AppleTalk).
type Socket specification, which is typically either SOCK_STREAM or
SOCK_DGRAM. The SOCK_STREAM uses TCP and provides a sequenced, reliable,
two-way, connection-based stream. SOCK_DGRAM uses UDP and provides for
connectionless datagrams. This type of connection is not recommended. A
third type is SOCK_RAW, for types other than UDP or TCP, such as for ICMP.
protocol Defines the protocol to be used with the socket. If it is zero then the
caller does not wish to specify a protocol.
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Server Client
socket()
create a socket
s o c k e t ( )
c r e a t e a s o c k e t
bind()
give socket a
name connect()
connect to a socket
listen()
listen for
connections
accept()
accept a new
connection
send()/recv() send()/recv()
send/receive send/receive
data data
closesocket() closesocket()
close a socket close a socket
Figure 10.1 WinSock connection
If the socket function succeeds then the return value is a descriptor referencing the
new socket. Otherwise, it returns SOCKET_ERROR, and the specific error code can be
tested with WSAGetLastError. An example creation of a socket is given next:
SOCKET s;
s=socket(PF_INET,SOCK_STREAM,0);
if (s == INVALID_SOCKET)
{
cout << ″Socket error″
}
bind()
The bind() function associates a local address with a socket. It is before calls to the
connect or listen functions. When a socket is created with socket, it exists in a
name space (address family), but it has no name assigned. The bind function gives
the socket a local association (host address/port number). Its syntax is:
int bind(SOCKET s, const struct sockaddr FAR * addr, int namelen);
where
s A descriptor identifying an unbound socket.
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namelen The length of the addr.
addr The address to assign to the socket. The sockaddr structure is
defined as follows:
struct sockaddr
{
u_short sa_family;
char sa_data[14];
};
In the Internet address family, the sockadd_in structure is used by Windows Sockets
to specify a local or remote endpoint address to which to connect a socket. This is the
form of the sockaddr structure specific to the Internet address family and can be cast
to sockaddr. This structure can be filled with the sockaddr_in structure which has
the following form:
struct SOCKADDR_IN
{
short sin_family;
unsigned short sin_port;
struct in_addr sin_addr;
char sin_zero[8];
}
where
sin_family must be set to AF_INET.
sin_port IP port.
sin_addr IP address.
sin_zero Padding to make structure the same size as sockaddr.
If an application does not care what address is assigned to it, it may specify an Inter-
net address equal to INADDR_ANY, a port equal to 0, or both. An Internet address equal
to INADDR_ANY causes any appropriate network interface be used. A port value of 0
causes the Windows Sockets implementation to assign a unique port to the applica-
tion with a value between 1024 and 5000.
If no error occur then it returns a zero value. Otherwise, it returns
INVALID_SOCKET, and the specific error code can be tested with WSAGetLastError.
If an application needs to bind to an arbitrary port outside of the range 1024 to
5000 then the following outline code can be used:
#include <windows.h>
#include <winsock.h>
int main(void)
{
SOCKADDR_IN sin;
SOCKET s;
s = socket(AF_INET,SOCK_STREAM,0);
if (s == INVALID_SOCKET)
{
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// Socket failed
}
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(100); // port=100
if (bind(s, (LPSOCKADDR)&sin, sizeof (sin)) == 0)
{
// Bind failed
}
return(0);
}
The Windows Sockets htons function converts an unsigned short (u_short) from
host byte order to network byte order.
connect()
The connect() function establishes a connection with a peer. If the specified socket
is unbound then unique values are assigned to the local association by the system and
the socket is marked as bound. Its syntax is:
int connect (SOCKET s, const struct sockaddr FAR * name,
int namelen)
where
s Descriptor identifying an unconnected socket.
name Name of the peer to which the socket is to be connected.
namelen Name length.
If no error occur then it returns a zero value. Otherwise, it returns SOCKET_ERROR, and
the specific error code can be tested with WSAGetLastError.
listen()
The listen() function establishes a socket which listens for an incoming connection.
The sequence to create and accept a socket is:
• socket(). Creates a socket.
• listen(). This creates a queue for incoming connections and is typically used by
a server that can have more than one connection at a time.
• accept(). These connections are then accepted with accept.
The syntax of listen() is:
int listen (SOCKET s, int backlog)
where
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s Describes a bound, unconnected socket.
backlog Defines the queue size for the maximum number of pending con-
nections may grow (typically a maximum of 5).
If no error occur then it returns a zero value. Otherwise, it returns SOCKET_ERROR, and
the specific error code can be tested with WSAGetLastError.
#include <windows.h>
#include <winsock.h>
int main(void)
{
SOCKADDR_IN sin;
SOCKET s;
s = socket(AF_INET,SOCK_STREAM,0);
if (s == INVALID_SOCKET)
{
// Socket failed
}
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(100); // port=100
if (bind(s, (struct sockaddr FAR *)&sin, sizeof (sin)) ==
SOCKET_ERROR)
{
// Bind failed
}
if (listen(s,4)==SOCKET_ERROR)
{
// Listen failed
}
return(0);
}
accept()
The accept() function accepts a connection on a socket. It extracts any pending con-
nections from the queue and creates a new socket with the same properties as the
specified socket. Finally, it returns a handle to the new socket. Its syntax is:
SOCKET accept(SOCKET s, struct sockaddr FAR *addr, int FAR *addrlen );
where
s Descriptor identifying a socket that is in listen mode.
addr Pointer to a buffer that receives the address of the connecting entity, as
known to the communications layer.
addrlen Pointer to an integer which contains the length of the address addr.
If no error occur then it returns a zero value. Otherwise, it returns INVALID_SOCKET,
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and the specific error code can be tested with WSAGetLastError.
#include <windows.h>
#include <winsock.h>
int main(void)
{
SOCKADDR_IN sin;
SOCKET s;
int sin_len;
s = socket(AF_INET,SOCK_STREAM,0);
if (s == INVALID_SOCKET)
{
// Socket failed
}
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(100); // port=100
if (bind(s, (struct sockaddr FAR *)&sin, sizeof (sin)) ==
SOCKET_ERROR)
{
// Bind failed
}
if (listen(s,4)<0)
{
// Listen failed
}
sin_len = sizeof(sin);
s=accept(s,(struct sockaddr FAR *) & sin,(int FAR *) &sin_len);
if (s==INVALID_SOCKET)
{
// Accept failed
}
return(0);
}
send()
The send() function sends data to a connected socket. Its syntax is:
int send (SOCKET s, const char FAR *buf, int len, int flags)
where
s Connected socket descriptor.
buf Transmission data buffer.
len Buffer length.
flags Calling flag.
The flags parameter influences the behavior of the function. These can be:
MSG_DONTROUTE
Specifies that the data should not be subject to routing.
MSG_OOB Send out-of-band data.
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If send() succeeds then the return value is the number of characters set (which can
be less than the number indicated by len). Otherwise, it returns SOCKET_ERRO, and the
specific error code can be tested with WSAGetLastError.
#include <windows.h>
#include <winsock.h>
#include <string.h>
#define STRLENGTH 100
int main(void)
{
SOCKADDR_IN sin;
SOCKET s;
int sin_len;
char sendbuf[STRLENGTH];
s = socket(AF_INET,SOCK_STREAM,0);
if (s == INVALID_SOCKET)
{
// Socket failed
}
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(100); // port=100
if (bind(s, (struct sockaddr FAR *)&sin, sizeof (sin)) ==
SOCKET_ERROR)
{
// Bind failed
}
if (listen(s,4)<0)
{
// Listen failed
}
sin_len = sizeof(sin);
s=accept(s,(struct sockaddr FAR *) & sin,(int FAR *) &sin_len);
if (s<0)
{
// Accept failed
}
while (1)
{
// get message to send and put into sendbuff
send(s,sendbuf,strlen(sendbuf),80);
}
return(0);
}
recv()
The recv() function receives data from a socket. It waits until data arrives and its
syntax is:
int recv(SOCKET s, char FAR *buf, int len, int flags)
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where
s Connected socket descriptor.
buf Incoming data buffer.
len Buffer length.
flags Specifies the method by which the data is received.
If recv() succeeds then the return value is the number of bytes received (a zero iden-
tifies that the connection has been closed). Otherwise, it returns SOCKET_ERRO, and
the specific error code can be tested with WSAGetLastError.
The flags parameter may have one of the following values:
MSG_PEEK Peek at the incoming data. Any received data is copied into the
buffer, but not removed from the input queue.
MSG_OOB Process out-of-band data.
#include <windows.h>
#include <winsock.h>
#define STRLENGTH 100
int main(void)
{
SOCKADDR_IN sin;
SOCKET s;
int sin_len,status;
char recmsg[STRLENGTH];
s = socket(AF_INET,SOCK_STREAM,0);
if (s == INVALID_SOCKET)
{
// Socket failed
}
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(100); // port=100
if (bind(s, (struct sockaddr FAR *)&sin, sizeof (sin)) ==
SOCKET_ERROR)
{
// Bind failed
}
if (listen(s,4)<0)
{
// Listen failed
}
sin_len = sizeof(sin);
s=accept(s,(struct sockaddr FAR *) & sin,(int FAR *) &sin_len);
if (s<0)
{
// Accept failed
}
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while (1)
{
status=recv(s,recmsg,STRLENGTH,80);
if (status==SOCKET_ERROR)
{
// no socket
break;
}
recmsg[status]=NULL; // terminate string
if (status)
{
// szMsg contains received string
}
else
{
break;
// connection broken
}
}
return(0);
}
shutdown()
The shutdown() function disables send or receive operations on a socket and does not
close any opened sockets. Its syntax is:
int shutdown(SOCKET s, int how);
where
s Socket descriptor.
how Flag that identifies operation types that will no longer be allowed.
These are:
0 – Disallows subsequent receives.
1 – Disallows subsequent sends.
2 – Disables send and receive.
If no error occur then it returns a zero value. Otherwise, it returns INVALID_SOCKET,
and the specific error code can be tested with WSAGetLastError.
closesocket()
The closesocket() function closes a socket. Its syntax is:
int closesocket (SOCKET s);
where
s Socket descriptor.
If no error occur then it returns a zero value. Otherwise, it returns INVALID_SOCKET,
176 Mastering Networks
and the specific error code can be tested with WSAGetLastError.
10.2 TCP/IP services reference
Port Service Comment Port Service Comment
1 TCPmux 7 echo
9 discard Null 11 systat Users
13 daytime 15 netstat
17 qotd Quote 18 msp Message send protocol
19 chargen ttytst source 21 ftp
23 telnet 25 smtp Mail
37 time Timserver 39 rlp Resource location
42 nameserver IEN 116 43 whois Nicname
53 domain DNS 57 mtp Deprecated
67 bootps BOOTP server 67 bootps
68 bootpc BOOTP client 69 tftp
70 gopher Internet Gopher 77 rje Netrjs
79 finger 80 www WWW HTTP
87 link Ttylink 88 kerberos Kerberos v5
95 supdup 101 hostnames
102 iso-tsap ISODE 105 csnet-ns CSO name server
107 rtelnet Remote Telnet 109 pop2 POP version 2
110 pop3 POP version 3 111 sunrpc
113 auth Rap ID 115 sftp
117 uucp-path 119 nntp USENET
123 ntp Network Timel 137 netbios-ns NETBIOS Name Service
138 netbios-dgm NETBIOS 139 netbios-ssn NETBIOS session
143 imap2 161 snmp SNMP
162 snmp-trap SNMP trap 163 cmip-man ISO management over IP
164 cmip-agent 177 xdmcp X Display Manager
178 nextstep NeXTStep 179 bgp BGP
191 prospero 194 irc Internet Relay Chat
199 smux SNMP Multiplexer 201 at-rtmp AppleTalk routing
202 at-nbp AppleTalk name binding 204 at-echo AppleTalk echo
206 at-zis AppleTalk zone information 210 z3950 NISO Z39.50 database
213 ipx IPX 220 imap3 Interactive Mail Access
372 ulistserv UNIX Listserv 512 exec Comsat 513 login
513 who Whod 514 shell No passwords used
514 syslog 515 printer Line printer spooler
517 talk 518 ntalk
520 route RIP 525 timed Timeserver
526 tempo Newdate 530 courier Rpc
531 conference Chat 532 netnews Readnews
533 netwall Emergency broadcasts 540 uucp Uucp daemon
543 klogin Kerberized ‘rlogin’ (v5) 544 kshell Kerberized ‘rsh’ (v5)
TCP/UDP 177
