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WINSOCK 2
WINDOWS SOCKET: PROGRAM EXAMPLES PART 3
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Winsock Tutorial
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My Training Period: hours
Note: This is a continuation from previous Module. Machine’s OS is standalone Windows Xp Pro with SP2 except whenever mentioned. Compiler used was Visual C++ 2003 .Net 1.1. Beware the codes that span more than one line. Program examples have been tested for Non Destructive Test :o). All information compiled for Windows 2000 (NT5.0) above and...
Abilities
▪ Able to understand the basic of networking such as TCP/IP. ▪ Able to understand Winsock implementation and operations through the APIs and program examples. ▪ Able to gather, understand and use the Winsock functions, structures and macros in your programs. ▪ Able to build programs that use Microsoft C/Standard C programming language and Winsock APIs.
listen()
Return Values
If no error occurs, listen() returns zero. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError().
Remarks
To accept connections, a socket is first created with the socket() function and bound to a local address with the bind() function, a backlog for incoming connections is specified with listen(), and then the connections are accepted with the accept() function. Sockets that are connection oriented those of type SOCK_STREAM for example, are used with listen(). The socket s is put into passive mode where incoming connection requests are acknowledged and queued pending acceptance by the process. The listen() function is typically used by servers that can have more than one connection request at a time. If a connection request arrives and the queue is full, the client will receive an error with an indication of WSAECONNREFUSED. If there are no available socket descriptors, listen() attempts to continue to function. If descriptors become available, a later call to listen() or accept() will refill the queue to the current or most recent backlog, if possible, and resume listening for incoming connections. An application can call listen() more than once on the same socket. This has the effect of updating the current backlog for the listening socket. Should there be more pending connections than the new backlog value, the excess pending connections will be reset and dropped. For IrDA sockets the af_irda.h header file must be explicitly included.
Compatibility
The backlog parameter is limited (silently) to a reasonable value as determined by the underlying service provider. Illegal values are replaced by the nearest legal value. There is no standard provision to find out the actual backlog value.
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Program Example
// Microsoft Development Environment 2003 - Version 7.1.3088
// Copyright (r) 1987-2002 Microsoft Corporation. All Right Reserved
// Microsoft .NET Framework 1.1 - Version 1.1.4322
// Copyright (r) 1998-2002 Microsoft Corporation. All Right Reserved
//
// Run on Windows XP Pro machine, version 2002, SP 2
//
// <windows.h> already included
// WINVER = 0x0501 for Xp already defined in windows.h
#include <stdio.h>
#include <winsock2.h>
int main()
{
WORD wVersionRequested;
WSADATA wsaData;
int wsaerr;
// Using MAKEWORD macro, Winsock version request 2.2
wVersionRequested = MAKEWORD(2, 2);
wsaerr = WSAStartup(wVersionRequested, &wsaData);
if (wsaerr != 0)
{
/* Tell the user that we could not find a usable */
/* WinSock DLL.*/
printf("The Winsock dll not found!\n");
return 0;
}
else
{
printf("The Winsock dll found!\n");
printf("The status: %s.\n", wsaData.szSystemStatus);
}
/* Confirm that the WinSock DLL supports 2.2.*/
/* Note that if the DLL supports versions greater */
/* than 2.2 in addition to 2.2, it will still return */
/* 2.2 in wVersion since that is the version we */
/* requested. */
if (LOBYTE(wsaData.wVersion) != 2 || HIBYTE(wsaData.wVersion) != 2 )
{
/* Tell the user that we could not find a usable */
/* WinSock DLL.*/
printf("The dll do not support the Winsock version %u.%u!\n", LOBYTE(wsaData.wVersion), HIBYTE(wsaData.wVersion));
WSACleanup();
return 0;
}
else
{
printf("The dll supports the Winsock version %u.%u!\n", LOBYTE(wsaData.wVersion), HIBYTE(wsaData.wVersion));
printf("The highest version this dll can support: %u.%u\n", LOBYTE(wsaData.wHighVersion), HIBYTE(wsaData.wHighVersion));
}
//////////Create a socket////////////////////////
//Create a SOCKET object called m_socket.
SOCKET m_socket;
// Call the socket function and return its value to the m_socket variable.
// For this application, use the Internet address family, streaming sockets, and
// the TCP/IP protocol.
// using AF_INET family, TCP socket type and protocol of the AF_INET - IPv4
m_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
// Check for errors to ensure that the socket is a valid socket.
if (m_socket == INVALID_SOCKET)
{
printf("Error at socket(): %ld\n", WSAGetLastError());
WSACleanup();
return 0;
}
else
{
printf("socket() is OK!\n");
}
////////////////bind()//////////////////////////////
// Create a sockaddr_in object and set its values.
sockaddr_in service;
// AF_INET is the Internet address family.
service.sin_family = AF_INET;
// "127.0.0.1" is the local IP address to which the socket will be bound.
service.sin_addr.s_addr = inet_addr("127.0.0.1");
// 55555 is the port number to which the socket will be bound.
service.sin_port = htons(55555);
// Call the bind function, passing the created socket and the sockaddr_in structure as parameters.
// Check for general errors.
if (bind(m_socket, (SOCKADDR*)&service, sizeof(service)) == SOCKET_ERROR)
{
printf("bind() failed: %ld.\n", WSAGetLastError());
closesocket(m_socket);
return 0;
}
else
{
printf("bind() is OK!\n");
}
// Call the listen function, passing the created socket and the maximum number of allowed
// connections to accept as parameters. Check for general errors.
if (listen( m_socket, 1) == SOCKET_ERROR)
printf("listen(): Error listening on socket %ld.\n", WSAGetLastError());
else
{
printf("listen() is OK, I'm waiting for connections...\n");
}
return 0;
}
Figure 1
Another example that demonstrates the use of the listen() function.
Program Example
#include <stdio.h>
#include <winsock2.h>
int main()
{
// Initialize Winsock
WSADATA wsaData;
int iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
if (iResult != NO_ERROR)
printf("Server: Error at WSAStartup().\n");
else
printf("Server: WSAStartup() is OK!\n");
// Create a SOCKET for listening for
// incoming connection requests.
SOCKET ListenSocket;
ListenSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (ListenSocket == INVALID_SOCKET)
{
printf("Server: Error at socket(): %ld\n", WSAGetLastError());
WSACleanup();
return 0;
}
else
printf("Server: socket() is OK.\n");
// The sockaddr_in structure specifies the address family,
// IP address, and port for the socket that is being bound.
sockaddr_in service;
service.sin_family = AF_INET;
service.sin_addr.s_addr = inet_addr("127.0.0.1");
service.sin_port = htons(55555);
if (bind(ListenSocket, (SOCKADDR*)&service, sizeof(service)) == SOCKET_ERROR)
{
printf("Server: bind() failed.\n");
closesocket(ListenSocket);
return 0;
}
else
printf("Server: bind() is OK.\n");
// Listen for incoming connection requests
// on the created socket
if (listen(ListenSocket, 1) == SOCKET_ERROR)
printf("Server: listen(): Error listening on socket.\n");
printf("Server: I'm listening on socket, waiting for connection...\n");
WSACleanup();
return 0;
}
Figure 2
At this time our server is listening for connection on port 55555 but it still can’t accept (establish) connection from clients or other servers.
accept()
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Item |
Description |
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Function |
accept(). |
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Use |
Permits an incoming connection attempt on a socket. |
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Prototype |
SOCKET accept(SOCKET s, struct sockaddr* addr, int* addrlen); |
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Parameters |
s [in] Descriptor that identifies a socket that has been placed in a listening state with the listen() function. The connection is actually made with the socket that is returned by accept.
addr [out] Optional pointer to a buffer that receives the address of the connecting entity, as known to the communications layer. The exact format of the addr parameter is determined by the address family that was established when the socket from the sockaddr() structure was created.
addrlen [in, out] Optional pointer to an integer that contains the length of addr. |
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Return value |
Se below. |
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Include file |
<winsock2.h> |
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Library |
ws2_32.lib |
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Remark |
See below. |
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Table 3 |
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Return Values
If no error occurs, accept() returns a value of type SOCKET that is a descriptor for the new socket. This returned value is a handle for the socket on which the actual connection is made. Otherwise, a value of INVALID_SOCKET is returned, and a specific error code can be retrieved by calling WSAGetLastError(). The integer referred to by addrlen initially contains the amount of space pointed to by addr. On return it will contain the actual length in bytes of the address returned.
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Error code |
Meaning |
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WSANOTINITIALISED |
A successful WSAStartup() call must occur before using this function. |
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WSAECONNRESET |
An incoming connection was indicated, but was subsequently terminated by the remote peer prior to accepting the call. |
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WSAEFAULT |
The addrlen parameter is too small or addr is not a valid part of the user address space. |
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WSAEINTR |
A blocking Windows Sockets 1.1 call was canceled through WSACancelBlockingCall(). |
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WSAEINVAL |
The listen() function was not invoked prior to accept. |
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WSAEINPROGRESS |
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function. |
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WSAEMFILE |
The queue is nonempty upon entry to accept and there are no descriptors available. |
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WSAENETDOWN |
The network subsystem has failed. |
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WSAENOBUFS |
No buffer space is available. |
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WSAENOTSOCK |
The descriptor is not a socket. |
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WSAEOPNOTSUPP |
The referenced socket is not a type that supports connection-oriented service. |
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WSAEWOULDBLOCK |
The socket is marked as non-blocking and no connections are present to be accepted. |
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Table 4 |
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Remarks
The accept() function extracts the first connection on the queue of pending connections on socket s. It then creates and returns a handle to the new socket. The newly created socket is the socket that will handle the actual connection; it has the same properties as socket s, including the asynchronous events registered with the WSAAsyncSelect() or WSAEventSelect() functions. The accept() function can block the caller until a connection is present if no pending connections are present on the queue, and the socket is marked as blocking. If the socket is marked as non-blocking and no pending connections are present on the queue, accept() returns an error as described in the following. After the successful completion of accept() returns a new socket handle, the accepted socket cannot be used to accept more connections. The original socket remains open and listens for new connection requests.
The parameter addr is a result parameter that is filled in with the address of the connecting entity, as known to the communications layer. The exact format of the addr parameter is determined by the address family in which the communication is occurring. The addrlen is a value-result parameter; it should initially contain the amount of space pointed to by addr; on return it will contain the actual length (in bytes) of the address returned. The accept() function is used with connection-oriented socket types such as SOCK_STREAM. If addr and/or addrlen are equal to NULL, then no information about the remote address of the accepted socket is returned.
Program Example
// Microsoft Development Environment 2003 - Version 7.1.3088
// Copyright (r) 1987-2002 Microsoft Corporation. All Right Reserved
// Microsoft .NET Framework 1.1 - Version 1.1.4322
// Copyright (r) 1998-2002 Microsoft Corporation. All Right Reserved
//
// Run on Windows XP Pro machine, version 2002, SP 2
//
// <windows.h> already included
// WINVER = 0x0501 for Xp already defined in windows.h
#include <stdio.h>
#include <winsock2.h>
int main()
{
WORD wVersionRequested;
WSADATA wsaData;
int wsaerr;
// Using MAKEWORD macro, Winsock version request 2.2
wVersionRequested = MAKEWORD(2, 2);
wsaerr = WSAStartup(wVersionRequested, &wsaData);
if (wsaerr != 0)
{
/* Tell the user that we could not find a usable */
/* WinSock DLL.*/
printf("Server: The Winsock dll not found!\n");
return 0;
}
else
{
printf("Server: The Winsock dll found!\n");
printf("Server: The status: %s.\n", wsaData.szSystemStatus);
}
/* Confirm that the WinSock DLL supports 2.2.*/
/* Note that if the DLL supports versions greater */
/* than 2.2 in addition to 2.2, it will still return */
/* 2.2 in wVersion since that is the version we */
/* requested. */
if (LOBYTE(wsaData.wVersion) != 2 || HIBYTE(wsaData.wVersion) != 2 )
{
/* Tell the user that we could not find a usable */
/* WinSock DLL.*/
printf("Server: The dll do not support the Winsock version %u.%u!\n", LOBYTE(wsaData.wVersion), HIBYTE(wsaData.wVersion));
WSACleanup();
return 0;
}
else
{
printf("Server: The dll supports the Winsock version %u.%u!\n", LOBYTE(wsaData.wVersion), HIBYTE(wsaData.wVersion));
printf("Server: The highest version this dll can support: %u.%u\n", LOBYTE(wsaData.wHighVersion), HIBYTE(wsaData.wHighVersion));
}
//////////Create a socket////////////////////////