GETSOCKOPT(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
getsockopt - get the socket options

SYNOPSIS

#include <sys/socket.h>

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

DESCRIPTION

The getsockopt() function manipulates options associated with a socket.

The getsockopt() function shall retrieve 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 shall be silently truncated. Otherwise, the object pointed to by the option_len argument shall 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 level identifier for the protocol controlling the option. For example, to indicate that an option is interpreted by the TCP (Transmission Control Protocol), set level to IPPROTO_TCP as defined in the <netinet/in.h> header.

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 shall store an int value. This is a Boolean option.

SO_ACCEPTCONN
    Reports whether socket listening is enabled. This option shall store an int value. This is a Boolean option.

SO_BROADCAST
    Reports whether transmission of broadcast messages is supported, if this is supported by the protocol. This option shall store 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 shall store 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 shall be broken and threads writing to that socket shall be notified with a SIGPIPE signal. This option shall store 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 shall store a linger structure.

SO_OOBINLINE
    Reports whether the socket leaves received out-of-band data (data marked urgent) inline. This option shall store an int value. This is a Boolean option.

SO_SNDBUF
    Reports send buffer size information. This option shall store an int value.

SO_RCVBUF
    Reports receive buffer size information. This option shall store an int value.

SO_ERROR
    Reports information about error status and clears it. This option shall store an int value.

SO_TYPE
    Reports the socket type. This option shall store an int value. Socket types are described in Socket Types .

SO_DONTROUTE
    Reports whether outgoing messages bypass the standard routing facilities. The destination shall 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 shall store 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 from that returned; for example, out-of-band data.) This option shall store 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 shall store 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 shall return with a partial count or errno set to [EAGAIN] or [EWOULDBLOCK] if no data was received. The default for this option is zero, which indicates that a receive operation shall 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 shall 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 shall store 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 shall return with a partial count or with errno set to [EAGAIN] or [EWOULDBLOCK] if no data was sent. The default for this option is zero, which indicates that a send operation shall not time out. The option shall store 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.

RETURN VALUE

Upon successful completion, getsockopt() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS

The getsockopt() function shall fail if:

EBADF
    The socket argument is not a valid file descriptor.

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 function.

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

bind(), close(), endprotoent(), setsockopt(), socket(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>, <netinet/in.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .