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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 .