int getsockopt(int socket, intlevel, intoption_name,
void *restrictoption_value, socklen_t *restrictoption_len);
The getsockopt() function manipulates options associated with
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>
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
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
allow reuse of local addresses, if this is supported by the protocol.
This option shall store an int value. This is a
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
continue as quickly as possible. This option shall store a linger
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
Reports receive buffer size information. This option shall store an
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 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.
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 [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
For Boolean options, a zero value indicates that the option is disabled
and a non-zero value indicates that the option is
Upon successful completion, getsockopt() shall return 0; otherwise,
-1 shall be returned and errno set to indicate
The getsockopt() function shall fail if:
The socket argument is not a valid file descriptor.
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:
The calling process does not have the appropriate privileges.
The socket has been shut down.
Insufficient resources are available in the system to complete the
The following sections are informative.
bind() , close() , endprotoent() , setsockopt()
, socket() , the Base Definitions volume of IEEE Std 1003.1-2001,
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