getsockopt() -- get value of socket option

SYNOPSIS

#include <sys/socket.h>

int getsockopt(int s, int level, int optname, void *optval, socklen_t *optlen);

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 may exist at multiple protocol levels, but they are always present at the uppermost socket' level. Options affect socket operations, such as the routing of packets, out-of-band data transfer, and so on.

The level argument specifies the protocol level at which the option resides. To retrieve options at the socket level, specify the level argument as SOL_SOCKET. To retrieve options at other levels, supply the appropriate protocol number for the protocol controlling the option. For example, to indicate that an option is to be interpreted by the TCP (Transport Control Protocol), set level to the protocol number of TCP.

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 is set to the size of the value returned. For SO_LINGER, this is the size of a struct linger; for most other options it is 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 an option has not been set with setsockopt(), getsockopt() returns the default value for the option.

The following options are supported for getsockopt():

SO_DEBUG

Reports whether debugging information is being recorded. This option stores an int value in the optval argument. This is a BOOL option.

SO_ACCEPTCONN

Reports whether socket listening is enabled. This option stores an int value in the optval argument. This is a BOOL option.

SO_BROADCAST

Reports whether transmission of broadcast messages is supported, if this is supported by the protocol. This option stores an int value in the optval argument. This is a BOOL option.

SO_REUSEADDR

Reports whether the rules used in validating addresses supplied to bind() should allow reuse of local addresses, if this is supported by the protocol. This option stores an int value in the optval argument. This is a BOOL option.

SO_KEEPALIVE

Reports whether connections are kept active with periodic transmission of messages, if this is supported by the protocol.

If the connected socket fails to respond to these messages, the connection is broken and processes writing to that socket are notified with an ENETRESET errno. This option stores an int value in the optval argument. This is a BOOL option.

SO_LINGER

Reports whether the socket lingers on close() if data is present. If SO_LINGER is set, the system blocks the process during close() until it can transmit the data or until the end of the interval indicated by the l_linger member, whichever comes first. If SO_LINGER is not specified, and close() is issued, the system handles the call in a way that allows the process to continue as quickly as possible. This option stores a linger structure in the optval argument.

SO_OOBINLINE

Reports whether the socket leaves received out-of-band data (data marked urgent) in line. This option stores an int value in optval argument. This is a BOOL option.

SO_SNDBUF

Reports send buffer size information. This option stores an int value in the optval argument.

SO_RCVBUF

Reports receive buffer size information. This option stores an int value in the optval argument.

SO_ERROR

Reports information about error status and clears it. This option stores an int value in the optval argument.

SO_TYPE

Reports the socket type. This option stores an int value in the optval argument.

SO_DONTROUTE

Reports whether outgoing messages bypass the standard routing facilities. The destination must be on a directly-connected network, and messages are directed to the appropriate network interface according to the destination address. The effect, if any, of this option depends on what protocol is in use. This option stores an int value in the optval argument. This is a BOOL option.

SO_MAX_MSG_SIZE

Maximum size of a message for message-oriented socket types (for example, SOCK_DGRAM). Has no meaning for stream-oriented sockets. This option stores an int value in the optval argument.

TCP_NODELAY

Specifies whether the Nagle algorithm used by TCP for send coalescing is disabled. This option stores an int value in the optval argument. This is a BOOL option.

For boolean options, a zero value indicates that the option is disabled and a non-zero value indicates that the option is enabled.

PARAMETERS

s: Is a socket descriptor.
level: Identifies the level at which the option exists. Choose between the socket level or the underlying protocol. Socket level is indicated by the symbolic constant SOL_SOCKET. Any other level requires the protocol number of the appropriate protocol controlling the option.
optname: Specifies a single option to which the request applies.
optval: Points to the buffer where the option is returned. Can be 0 if no option value is to be supplied or returned.
optlen: Initially contains the buffer size pointed to by optval. Upon return, contains the actual size of the returned value.

RETURN VALUES

If successful, getsockopt() returns a zero. If a failure occurs, it returns a value of -1 and sets errno to one of the following values:

EBADF: The parameter s is not a valid descriptor.
ENOPROTOOPT: The option is unknown at the level indicated.
ENOTSOCK: The parameter s is a file, not a socket.

getsockopt()
get value of socket option

Function

SYNOPSIS

DESCRIPTION

PARAMETERS

RETURN VALUES

CONFORMANCE

MULTITHREAD SAFETY LEVEL

PORTING ISSUES

AVAILABILITY

SEE ALSO

getsockopt() get value of socket option

Function

getsockopt()
get value of socket option