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WINSOCK 2

WINDOWS SOCKET: PROGRAM EXAMPLES PART 7

 

Winsock Tutorial   Winsock: Story 1 Winsock: Story 2 Winsock: Story 3 Winsock: Example 1 Winsock: Example 2 Winsock: Example 3 Winsock: Example 4 Winsock: Example 5 Winsock: Example 6 Winsock: Example 7 Winsock: Example 8 Winsock: Example 9 Winsock: Example 10 Winsock: Example 11 Winsock: Example 12 Winsock: Reference 1 Winsock: Reference 2 Winsock: Reference 3 Winsock: Reference 4	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...   The story was discussed at Winsock introduction story. Related functions, structures and macros used in the program examples have been dumped at Winsock structure & function 1 and Winsock structure & function 2. Other related and required information (if any) not available in no. 2 can be found at MSDN & Visual C++ online reference.   Abilities   ▪         Able to understand Winsock more implementation 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.   getsockopt()   Item Description Function getsockopt(). Use Retrieves a socket option. Prototype int getsockopt(   SOCKET s,   int level,   int optname,   char* optval,   int* optlen); Parameters s [in] Descriptor identifying a socket.   level [in] Level at which the option is defined. Example: SOL_SOCKET.   optname [in] Socket option for which the value is to be retrieved. Example: SO_ACCEPTCONN. The optname value must be a socket option defined within the specified level, or behavior is undefined.   optval [out] Pointer to the buffer in which the value for the requested option is to be returned.   optlen [in, out] Pointer to the size of the optval buffer, in bytes. Return value See below. Include file <winsock2.h> Library ws2_32.lib. Remark See below.   Table 1   Return Values   If no error occurs, getsockopt() returns zero. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError().   Error code Meaning WSANOTINITIALISED A successful WSAStartup() call must occur before using this function. WSAENETDOWN The network subsystem has failed. WSAEFAULT One of the optval or the optlen parameters is not a valid part of the user address space, or the optlen parameter is too small. WSAEINPROGRESS A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function. WSAEINVAL The level parameter is unknown or invalid. WSAENOPROTOOPT The option is unknown or unsupported by the indicated protocol family. WSAENOTSOCK The descriptor is not a socket.   Table 2   Remarks   The getsockopt() function retrieves the current value for a socket option associated with a socket of any type, in any state, and stores the result in optval. Options can exist at multiple protocol levels, but they are always present at the uppermost socket level. Options affect socket operations, such as the packet routing and OOB data transfer. The value associated with the selected option is returned in the buffer optval. The integer pointed to by optlen should originally contain the size of this buffer; on return, it will be set to the size of the value returned. For SO_LINGER, this will be the size of a LINGER structure. For most other options, it will be the size of an integer. The application is responsible for allocating any memory space pointed to directly or indirectly by any of the parameters it specified. If the option was never set with setsockopt(), then getsockopt() returns the default value for the option. The following options are supported for getsockopt(). The Type column identifies the type of data addressed by optval.

 

For level = SOL_SOCKET

 

 

For level = IPPROTO_TCP

 

 

For level = NSPROTO_IPX

 

Windows NT supports all IPX options. Windows Me/98/95 supports only the following options:

 

IPX_PTYPE
IPX_FILTERPTYPE
IPX_DSTYPE
IPX_RECVHDR
IPX_MAXSIZE
IPX_ADDRESS

 

 

BSD options not supported for getsockopt() are as shown in the following table.

 

 

When using the recv() function, if no data arrives during the period specified in SO_RCVTIMEO, the recv() function completes. In Windows versions prior to Windows 2000, any data received subsequently fails with WSAETIMEDOUT. In Windows 2000 and later, if no data arrives within the period specified in SO_RCVTIMEO the recv() function returns WSAETIMEDOUT, and if data is received, recv() returns SUCCESS. Calling getsockopt() with an unsupported option will result in an error code of WSAENOPROTOOPT being returned from WSAGetLastError().

 

SO_DEBUG

Windows Sockets service providers are encouraged (but not required) to supply output debug information if the SO_DEBUG option is set by an application. The mechanism for generating the debug information and the form it takes are beyond the scope of this document.

 

SO_ERROR

The SO_ERROR option returns and resets the per socket–based error code, which is different from the per thread based–error code that is handled using the WSAGetLastError() and WSASetLastError() function calls. A successful call using the socket does not reset the socket based error code returned by the SO_ERROR option.

 

SO_GROUP_ID

This option is reserved. This option is also exclusive to getsockopt(); the value should be null.

 

SO_GROUP_PRIORITY

This option is reserved. Group priority indicates the priority of the specified socket relative to other sockets within the socket group. Values are nonnegative integers, with zero corresponding to the highest priority. Priority values represent a hint to the underlying service provider about how potentially scarce resources should be allocated. For example, whenever two or more sockets are both ready to transmit data, the highest priority socket (lowest value for SO_GROUP_PRIORITY) should be serviced first, with the remainder serviced in turn according to their relative priorities.

The WSAENOPROTOOPT error code is indicated for non-group sockets or for service providers that do not support group sockets.

 

SO_KEEPALIVE

An application can request that a TCP/IP service provider enable the use of keep-alive packets on TCP connections by turning on the SO_KEEPALIVE socket option. A Windows Sockets provider need not support the use of keep-alive: if it does, the precise semantics are implementation-specific but should conform to section 4.2.3.6 of RFC 1122: Requirements for Internet Hosts - Communication Layers. If a connection is dropped as the result of keep-alives the error code WSAENETRESET is returned to any calls in progress on the socket, and any subsequent calls will fail with WSAENOTCONN. SO_KEEPALIVE is not supported on ATM sockets, and requests to enable the use of keep-alive packets on an ATM socket results in an error being returned by the socket.

 

SO_LINGER

SO_LINGER controls the action taken when unsent data is queued on a socket and a closesocket() is performed. See closesocket() for a description of the way in which the SO_LINGER settings affect the semantics of closesocket(). The application gets the current behavior by retrieving a LINGER structure (pointed to by the optval parameter).

 

SO_MAX_MSG_SIZE

This is a get-only socket option that indicates the maximum outbound (send) size of a message for message-oriented socket types (for example, SOCK_DGRAM) as implemented by a particular service provider. It has no meaning for byte stream oriented sockets. There is no provision to find out the maximum inbound–message size.

 

SO_PROTOCOL_INFO

This is a get-only option that supplies the WSAPROTOCOL_INFO structure associated with this socket.

 

SO_SNDBUF

When a Windows Sockets implementation supports the SO_RCVBUF and SO_SNDBUF options, an application can request different buffer sizes (larger or smaller). The call to setsockopt() can succeed even if the implementation did not provide the whole amount requested. An application must call this function with the same option to check the buffer size actually provided.

 

SO_REUSEADDR

By default, a socket cannot be bound (see bind()) to a local address that is already in use. On occasion, however, it can be necessary to reuse an address in this way. Because every connection is uniquely identified by the combination of local and remote addresses, there is no problem with having two sockets bound to the same local address as long as the remote addresses are different. To inform the Windows Sockets provider that a bind on a socket should not be disallowed because the desired address is already in use by another socket, the application should set the SO_REUSEADDR socket option for the socket before issuing the bind. Note that the option is interpreted only at the time of the bind(): it is therefore unnecessary (but harmless) to set the option on a socket that is not to be bound to an existing address, and setting or resetting the option after the bind() has no effect on this or any other socket. SO_REUSEADDR is not applicable for ATM sockets, and although requests to reuse and address do not result in an error, they have no affect on when an ATM socket is in use.

 

PVD_CONFIG

This option retrieves an opaque data structure object from the service provider associated with socket s. This object stores the current configuration information of the service provider. The exact format of this data structure is service provider specific.

 

TCP_NODELAY

The TCP_NODELAY option is specific to TCP/IP service providers. The Nagle algorithm is disabled if the TCP_NODELAY option is enabled (and vice versa). The Nagle algorithm (described in RFC 896) is very effective in reducing the number of small packets sent by a host. The process involves buffering send data when there is unacknowledged data already in flight or buffering send data until a full-size packet can be sent. It is highly recommended that Windows Sockets implementations enable the Nagle Algorithm by default because, for the vast majority of application protocols, the Nagle Algorithm can deliver significant performance enhancements. However, for some applications this algorithm can impede performance and setsockopt() with the same option can be used to turn it off. These are applications where many small messages are sent, and the time delays between the messages are maintained.

 

Program Example

 

The following code sample demonstrates the use of the getsockopt() function.

 

#include <stdio.h>

#include "winsock2.h"

 

void main()

{

  // Declare variables

  WSADATA wsaData;

  SOCKET ListenSocket;

  sockaddr_in service;

 

  // Initialize Winsock, request Winsock 2.2

  int iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);

 

  if(iResult != NO_ERROR)

    printf("Error at WSAStartup().\n");

  else

    printf("WSAStartup() is OK.\n");

 

  // Create a listening socket

  ListenSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

  if (ListenSocket == INVALID_SOCKET)

  {

    printf("Error at socket()\n");

    WSACleanup();

    return;

  }

  else

    printf("socket() is OK.\n");

 

  // Bind the socket to the local IP address and port 55555

  hostent* thisHost;

  char* ip;

  u_short port;

  port = 55555;

  thisHost = gethostbyname("");

  ip = inet_ntoa(*(struct in_addr *)*thisHost->h_addr_list);

 

  // Try running this program on machine using live IP...

  printf("\nIP address is: %s.\n", ip);

  printf("Hostname is: %s.\n", thisHost->h_name);

  printf("Address type: %i.\n\n", thisHost->h_addrtype);

 

  service.sin_family = AF_INET;

  service.sin_addr.s_addr = inet_addr(ip);

  service.sin_port = htons(port);