NAME

getsockopt - get the socket options

SYNOPSIS

#include <sys/socket.h>

int getsockopt(int socket, int level, int option_name, 
     void *option_value, socklen_t *option_len);

DESCRIPTION

The getsockopt() function retrieves the value for the option specified by the option_name argument for the socket specified by the socket argument. If the size of the option value is greater than option_len, the value stored in the object pointed to by the option_value argument will be silently truncated. Otherwise, the object pointed to by the option_len argument will be modified to indicate the actual length of the value.

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 will be interpreted by the TCP (Transport Control Protocol), set level to the protocol number of TCP, as defined in the <netinet/in.h> header, or as determined by using getprotobyname() function.

The socket in use may require the process to have appropriate privileges to use the getsockopt() function.

The option_name argument specifies a single option to be retrieved. It can be one of the following values defined in <sys/socket.h>:

SO_DEBUG

Reports whether debugging information is being recorded. This option stores an int value. This is a boolean option.

SO_ACCEPTCONN

Reports whether socket listening is enabled. This option stores an int value.

SO_BROADCAST

Reports whether transmission of broadcast messages is supported, if this is supported by the protocol. This option stores an int value. This is a boolean 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. This is a boolean 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 a SIGPIPE signal. This option stores an int value.

This is a boolean 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.

SO_OOBINLINE

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

SO_SNDBUF

Reports send buffer size information. This option stores an int value.

SO_RCVBUF

Reports receive buffer size information. This option stores an int value.

SO_ERROR

Reports information about error status and clears it. This option stores an int value.

SO_TYPE

Reports the socket type. This option stores an int value.

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. This is a boolean option.

SO_RCVLOWAT

Reports the minimum number of bytes to process for socket input operations. The default value for SO_RCVLOWAT is 1. If SO_RCVLOWAT is set to a larger value, blocking receive calls normally wait until they have received the smaller of the low water mark value or the requested amount. (They may return less than the low water mark if an error occurs, a signal is caught, or the type of data next in the receive queue is different than that returned, e.g. out of band data). This option stores an int value. Note that not all implementations allow this option to be retrieved.

SO_RCVTIMEO

Reports the timeout value for input operations. This option stores a timeval structure with the number of seconds and microseconds specifying the limit on how long to wait for an input operation to complete. If a receive operation has blocked for this much time without receiving additional data, it returns with a partial count or errno set to [EAGAIN] or [EWOULDBLOCK] if no data were received. The default for this option is zero, which indicates that a receive operation will not time out. Note that not all implementations allow this option to be retrieved.

SO_SNDLOWAT

Reports the minimum number of bytes to process for socket output operations. Non-blocking output operations will process no data if flow control does not allow the smaller of the send low water mark value or the entire request to be processed. This option stores an int value. Note that not all implementations allow this option to be retrieved.

SO_SNDTIMEO

Reports the timeout value specifying the amount of time that an output function blocks because flow control prevents data from being sent. If a send operation has blocked for this time, it returns with a partial count or with errno set to [EAGAIN] or [EWOULDBLOCK] if no data were sent. The default for this option is zero, which indicates that a send operation will not time out. The option stores a timeval structure. Note that not all implementations allow this option to be retrieved.

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

Options at other protocol levels vary in format and name.

The socket in use may require the process to have appropriate privileges to use the getsockopt() function.

RETURN VALUE

Upon successful completion, getsockopt() returns 0. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

The getsockopt() function will fail if:

[EBADF]

The socket argument is not a valid file descriptor.

[EFAULT]

The option_value or option_len parameter can not be accessed or written.

[EINVAL]

The specified option is invalid at the specified socket level.

[ENOPROTOOPT]

The option is not supported by the protocol.

[ENOTSOCK]

The socket argument does not refer to a socket.

The getsockopt() function may fail if:

[EACCES]

The calling process does not have the appropriate privileges.

[EINVAL]

The socket has been shut down.

[ENOBUFS]

Insufficient resources are available in the system to complete the call.

[ENOSR]

There were insufficient STREAMS resources available for the operation to complete.

SEE ALSO

bind(), close(), endprotoent(), setsockopt(), socket(), <sys/socket.h>.

CHANGE HISTORY

First released in Issue 4.