Technical Reference: Communications, Volume 2

getsockopt Subroutine

Purpose

Gets options on sockets

Library

Standard C Library (libc.a)

Syntax

#include <sys/socket.h>

int getsockopt (Socket, Level, OptionName, OptionValue, OptionLength) int Socket, Level, OptionName; void * OptionValue; socklen_t * OptionLength;

Description

The getsockopt subroutine allows an application program to query socket options. The calling program specifies the name of the socket, the name of the option, and a place to store the requested information. The operating system gets the socket option information from its internal data structures and passes the requested information back to the calling program.

Options can exist at multiple protocol levels. They are always present at the uppermost socket level. When retrieving socket options, specify the level where the option resides and the name of the option.

All applications containing the getsockopt subroutine must be compiled with _BSD set to a specific value. Acceptable values are 43 and 44. In addition, all socket applications must include the BSD libbsd.a library.

Parameters

Socket	Specifies the unique socket name.
Level	Specifies the protocol level where the option resides. Options can be retrieved at the following levels: Socket level Specifies the Level parameter as the SOL_SOCKET option. Other levels Supplies the appropriate protocol number for the protocol controlling the option. For example, to indicate that an option will be interpreted by the TCP protocol, set the Level parameter to the protocol number of TCP, as defined in the netinet/in.h file.
OptionName	Specifies a single option. The OptionName parameter and any specified options are passed uninterpreted to the appropriate protocol module for interpretation. The sys/socket.h file contains definitions for socket level options. The netinet/tcp.h file contains definitions for TCP protocol level options. Socket-level options can be enabled or disabled; they operate in a toggle fashion. The sys/atmsock.h file contains definitions for ATM protocol level options. The following list defines socket protocol level options found in the sys/socket.h file: SO_DEBUG Specifies the recording of debugging information. This option enables or disables debugging in the underlying protocol modules. SO_BROADCAST Specifies whether transmission of broadcast messages is supported. The option enables or disables broadcast support. SO_CKSUMREV Enables performance enhancements in the protocol layers. If the protocol supports this option, enabling causes the protocol to defer checksum verification until the user's data is moved into the user's buffer (on recv, recvfrom, read, or recvmsg thread). This can cause applications to be awakened when no data is available, in the case of a checksum error. In this case, EAGAIN is returned. Applications that set this option must handle the EAGAIN error code returned from a receive call. SO_REUSEADDR Specifies that the rules used in validating addresses supplied by a bind subroutine should allow reuse of a local port. A particular IP address can only be bound once to the same port. This option enables or disables reuse of local ports. SO_REUSEADDR allows an application to explicitly deny subsequent bind subroutine to the port/address of the socket with SO_REUSEADDR set. This allows an application to block other applications from binding with the bind subroutine. SO_REUSEPORT Specifies that the rules used in validating addresses supplied by a bind subroutine should allow reuse of a local port/address combination. Each binding of the port/address combination must specify the SO_REUSEPORT socket option. This option enables or disables the reuse of local port/address combinations. SO_KEEPALIVE Monitors the activity of a connection by enabling or disabling the periodic transmission of ACK messages on a connected socket. The idle interval time can be designated using the TCP/IP no command. Broken connections are discussed in "Understanding Socket Types and Protocols" in AIX 5L Version 5.2 Communications Programming Concepts.
	SO_DONTROUTE Indicates outgoing messages should bypass the standard routing facilities. Does not apply routing on outgoing messages. Directs messages to the appropriate network interface according to the network portion of the destination address. This option enables or disables routing of outgoing messages. SO_LINGER Lingers on a close subroutine if data is present. This option controls the action taken when an unsent messages queue exists for a socket, and a process performs a close subroutine on the socket. If the SO_LINGER option is set, the system blocks the process during the close subroutine until it can transmit the data or until the time expires. If the SO_LINGER option is not specified, and a close subroutine is issued, the system handles the call in a way that allows the process to continue as quickly as possible. The sys/socket.h file defines the linger structure that contains the l_linger value for specifying linger time interval. If linger time is set to anything but 0, the system tries to send any messages queued on the socket. The maximum value that l_linger can be set to is 65535. SO_OOBINLINE Leaves received out-of-band data (data marked urgent) in line. This option enables or disables the receipt of out-of-band data. SO_SNDBUF Retrieves buffer size information. SO_RCVBUF Retrieves buffer size information. SO_SNDLOWAT Retrieves send buffer low-water mark information. SO_RCVLOWAT Retrieves receive buffer low-water mark information.
	SO_SNDTIMEO Retrieves time-out information. This option is settable, but currently not used. SO_RCVTIMEO Retrieves time-out information. This option is settable, but currently not used. SO_ERROR Retrieves information about error status and clears. The following list defines TCP protocol level options found in the netinet/tcp.h file: TCP_RFC1323 Indicates whether RFC 1323 is enabled or disabled on the specified socket. A non-zero OptionValue returned by the getsockopt subroutine indicates the RFC is enabled. TCP_NODELAY Specifies whether TCP should follow the Nagle algorithm for deciding when to send data. By default TCP will follow the Nagle algorithm. To disable this behavior, applications can enable TCP_NODELAY to force TCP to always send data immediately. A non-zero OptionValue returned by the getsockopt subroutine indicates TCP_NODELAY is enabled. For example, TCP_NODELAY should be used when there is an appliciation using TCP for a request/response.
	The following list defines ATM protocol level options found in the sys/atmsock.h file: SO_ATM_PARM Retrieves all ATM parameters. This socket option can be used instead of using individual sockets options described below. It uses the connect_ie structure defined in sys/call_ie.h file. SO_ATM_AAL_PARM Retrieves ATM AAL (Adaptation Layer) parameters. It uses the aal_parm structure defined in sys/call_ie.h file. SO_ATM_TRAFFIC_DES Retrieves ATM Traffic Descriptor values. It uses the traffic_desc structure defined in sys/call_ie.h file. SO_ATM_BEARER Retrieves ATM Bearer capability information. It uses the bearer structure defined in sys/call_ie.h file. SO_ATM_BHLI Retrieves ATM Broadband High Layer Information. It uses the bhli structure defined in sys/call_ie.h file. SO_ATM_BLLI Retrieves ATM Broadband Low Layer Information. It uses the blli structure defined in sys/call_ie.h file. SO_ATM_QoS Retrieves ATM Ouality Of Service values. It uses the qos_parm structure defined in sys/call_ie.h file. SO_ATM_TRANSIT_SEL Retrieves ATM Transit Selector Carrier. It uses the transit_sel structure defined in sys/call_ie.h file. SO_ATM_MAX_PEND Retrieves the number of outstanding transmit buffers that are permitted before an error indication is returned to applications as a result of a transmit operation. This option is only valid for non best effort types of virtual circuits. SO_ATM_CAUSE Retrieves cause for the connection failure. It uses the cause_t structure defined in the sys/call_ie.h file.
OptionValue	Specifies a pointer to the address of a buffer. The OptionValue parameter takes an integer parameter. The OptionValue parameter should be set to a nonzero value to enable a Boolean option or to a value of 0 to disable the option. The following options enable and disable in the same manner: SO_DEBUG SO_REUSEADDR SO_KEEPALIVE SO_DONTROUTE SO_BROADCAST SO_OOBINLINE TCP_RFC1323
OptionLength	Specifies the length of the OptionValue parameter. The OptionLength parameter initially contains the size of the buffer pointed to by the OptionValue parameter. On return, the OptionLength parameter is modified to indicate the actual size of the value returned. If no option value is supplied or returned, the OptionValue parameter can be 0.

Options at other protocol levels vary in format and name.

IP level (IPPROTO_IP level) options are defined as follows:

IP_DONTFRAG	Get current IP_DONTFRAG option value.
IP_FINDPMTU	Get current PMTU value.
IP_PMTUAGE	Get current PMTU time out value.

In the case of TCP protocol sockets:

IP_DONTGRAG	Not supported.
IP_FINDPMTU	Get current PMTU value.
IP_PMTUAGE	Not supported.

IPV6 level (IPPROTO_IPV6 level) options are defined as follows:

IPV6_V6ONLY	Determines whether the socket is restricted to IPV6 communications only.
	Option Type:	int (boolean interpretation)
IPV6_UNICAST_HOPS	Allows the user to determine the outgoing hop limit value for unicast IPV6 packets.
	Option Type:	int
IPV6_MULTICAST_HOPS	Allows the user to determine the outgoing hop limit value for multicast IPV6 packets.
	Option Type:	int
IPV6_MULTICAST_IF	Allows the user to determine the interface being used for outgoing multicast packets.
	Option Type:	unsigned int
IPV6_MULTICAST_LOOP	If a multicast datagram is sent to a group that the sending host belongs, a copy of the datagram is looped back by the IP layer for local delivery (if the option is set to 1). If the option is set to 0, a copy is not looped back.
	Option Type:	unsigned int

ICMPV6 level (IPPROTO_ICMPV6 level) options are defined as follows:

ICMP6_FILTER	Allows the user to filter ICMPV6 messages by the ICMPV6 type field. If no filter was set, the default kernel filter will be returned.
	Option Type:	The icmp6_filter structure defined in the netinet/icmp6.h file.

Return Values

Upon successful completion, the getsockopt subroutine returns a value of 0.

If the getsockopt subroutine is unsuccessful, the subroutine handler performs the following actions:

Returns a value of -1 to the calling program.
Moves an error code, indicating the specific error, into the errno global variable.

Upon successful completion of the IPPROTO_IP option IP_PMTUAGE the returns are:

OptionValue 0 if PMTU discovery is not enabled.
OptionValue -1 if PMTU discovery is not complete.
Positive non-zero OptionValue if PMTU is available.

Upon successful completion of TCP protocol sockets option IP_FINDPMTU the returns are:

OptionValue 0 if PMTU discovery (tcp_pmtu_discover) is not enabled.
OptionValue -1 if PMTU discovery is not complete/not available.
Positive non-zero OptionValue if PMTU is available.

Error Codes

The getsockopt subroutine is unsuccessful if any of the following errors occurs:

EBADF	The Socket parameter is not valid.
ENOTSOCK	The Socket parameter refers to a file, not a socket.
ENOPROTOOPT	The option is unknown.
EFAULT	The address pointed to by the OptionValue parameter is not in a valid (writable) part of the process space, or the OptionLength parameter is not in a valid part of the process address space.

Examples

The following program fragment illustrates the use of the getsockopt subroutine to determine an existing socket type:

#include <sys/types.h>
#include <sys/socket.h>
int type, size;
size = sizeof(int);
if(getsockopt(s, SOL_SOCKET, SO_TYPE, (char*)&type,&size)<0){
.
.
.
}

Related Information

The no command.

The bind subroutine, close subroutine, endprotoent subroutine, getprotobynumber subroutine, getprotoent subroutine, setprotoent subroutine, setsockopt subroutine, socket subroutine.

Sockets Overview, Understanding Socket Options, and Understanding Socket Types and Protocols in AIX 5L Version 5.2 Communications Programming Concepts.