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GETSOCKOPT(3P)             POSIX Programmer's Manual            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 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>:
       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 sup-
              ported 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 trans-
              mission of messages, if this is supported by the protocol.
       If the connected socket fails to respond to these messages, the connec-
       tion  shall be broken and threads writing to that socket shall be noti-
       fied 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 [EWOULD-
              BLOCK] 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 implementa-
              tions 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  Defi-
       nitions 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 .

IEEE/The Open Group                  2003                       GETSOCKOPT(3P)