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 spec-
ified 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 indi-
cate 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 privi-
leges 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 trans-
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
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
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 [EWOULD-
BLOCK] if no data was received. The default for this option is
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.
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 following sections are informative.
-- 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)
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