getsockopt  CB_E0O

The Windows Sockets getsockopt function retrieves a socket option.

int getsockopt (

Parameters

s

[in] A descriptor identifying a socket.

level

[in] The level at which the option is defined; the supported levels include SOL_SOCKET and IPPROTO_TCP. (See annex for more protocol-specific levels.)

optname

[in] The socket option for which the value is to be retrieved.

optval

[out] A pointer to the buffer in which the value for the requested option is to be returned.

optlen

[in/out] A pointer to the size of the optval buffer.

Remarks

getsockopt 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 out-of-band 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 struct linger; 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 identifies the type of data addressed by optval.

level = SOL_SOCKET

level = IPPROTO_TCP

BSD options not supported for getsockopt are:

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 specification.

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 is a get-only socket option which indicates the identifier of the group this socket belongs to. Note that socket group IDs are unique across all processes for a give service provider. If this socket is not a group socket, the value is NULL.

SO_GROUP_PRIORITY

Group priority indicates the priority of the specified socket relative to other sockets within the socket group. Values are non-negative 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 which 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_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 struct linger (pointed to by the optval argument) with the following elements:

struct linger { 

    u_short    l_onoff;

    u_short    l_linger;

}

SO_MAX_MSG_SIZE

This is a get-only socket option which indicates the maximum 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

SO_PROTOCOL_INFO

This is a get-only option which supplies the WSAPROTOCOL_INFO structure associated with this socket. See WSAEnumProtocols for more information about this structure.

SO_RCVBUF

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, although 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 which is already in use. On occasion, however, it may be necessary to "re-use" an address in this way. Since 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 which 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.

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 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 by essentially buffering send data if there is unacknowledged data already "in flight" or until a full-size packet can be sent. It is highly recommended that Windows Sockets implementations enable the Nagle Algorithm by default, and 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, which need to be received by the peer with the time delays between the messages maintained.

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 Codes

See Also