GETSOCKOPT(P) POSIX Programmer's Manual GETSOCKOPT(P)
getsockopt - get the socket options
int getsockopt(int socket, int level, int option_name,
void *restrict option_value, socklen_t *restrict option_len);
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>:
Reports whether debugging information is being recorded. This option shall store an int value. This is a Boolean option.
Reports whether socket listening is enabled. This option shall store an int value. This is a Boolean option.
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.
Reports whether the rules used in validating addresses supplied to bind() should allow reuse of local addresses, if this is sup-
ported by the protocol. This option shall store an int value. This is a Boolean option.
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.
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.
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.
Reports send buffer size information. This option shall store an int value.
Reports receive buffer size information. This option shall store an int value.
Reports information about error status and clears it. This option shall store an int value.
Reports the socket type. This option shall store an int value. Socket types are described in Socket Types .
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.
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.
Reports the timeout value for input operations. This option shall store a timeval structure with the number of seconds and microsec-
onds 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.
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.
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 [EWOULD-
BLOCK] 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.
Upon successful completion, getsockopt() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.
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.
The option is not supported by the protocol.
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.
Insufficient resources are available in the system to complete the function.
The following sections are informative.
bind() , close() , endprotoent() , setsockopt() , socket() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>,
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
ogy -- 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 .
IEEE/The Open Group 2003 GETSOCKOPT(P)