setsockopt

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NAME

       getsockopt, setsockopt - get and set options on sockets


SYNOPSIS

       #include <sys/types.h>
       #include <sys/socket.h>


       int  getsockopt(int  s, int level, int optname, void *opt-
       val, int *optlen);

       int setsockopt(int s, int level, int optname,  const  void
       *optval, int optlen);


DESCRIPTION

       Getsockopt  and  setsockopt manipulate the options associ-
       ated with a socket.  Options may exist at multiple  proto-
       col  levels;  they  are  always  present  at the uppermost
       socket level.

       When manipulating socket options the level  at  which  the
       option  resides  and the name of the option must be speci-
       fied.  To manipulate options at the socket level, level is
       specified  as  SOL_SOCKET  .  To manipulate options at any
       other level the protocol number of the appropriate  proto-
       col  controlling  the option is supplied.  For example, to
       indicate that an option is to be interpreted  by  the  TCP
       protocol,  level  should  be set to the protocol number of
       TCP; see getprotoent(3).

       The parameters optval and optlen are used to access option
       values  for  setsockopt.   For  getsockopt they identify a
       buffer in which the value for the requested option(s)  are
       to  be returned.  For getsockopt, optlen is a value-result
       parameter, initially containing the  size  of  the  buffer
       pointed  to  by optval, and modified on return to indicate
       the actual size of the value returned.  If no option value
       is to be supplied or returned, optval may be NULL.

       Optname and any specified options are passed uninterpreted
       to the appropriate  protocol  module  for  interpretation.
       The  include  file  sys/socket.h  contains definitions for
       socket level options, described below.  Options  at  other
       protocol  levels  vary  in  format  and  name; consult the
       appropriate entries in section 4 of the manual.

       Most socket-level options utilize  an  int  parameter  for
       optval.   For setsockopt, the parameter should be non-zero
       to enable a boolean option, or zero if the option is to be
       disabled.   SO_LINGER  uses  a  struct  linger  parameter,
       defined in sys/socket.h, which specifies the desired state
       of  the  option  and  the  linger  interval  (see  below).
       SO_SNDTIMEO and SO_RCVTIMEO use a struct  timeval  parame-
       ter, defined in sys/time.h.
       The  following options are recognized at the socket level.
       Except as noted, each may be examined with getsockopt  and
       set with setsockopt .

       SO_DEBUG
               enables recording of debugging information

       SO_REUSEADDR
               enables local address reuse

       SO_KEEPALIVE
               enables keep connections alive

       SO_DONTROUTE
               enables routing bypass for outgoing messages

       SO_LINGER
               linger on close if data present

       SO_BROADCAST
               enables permission to transmit broadcast messages

       SO_OOBINLINE
               enables reception of out-of-band data in band

       SO_SNDBUF
               set buffer size for output

       SO_RCVBUF
               set buffer size for input

       SO_SNDLOWAT
               set minimum count for output

       SO_RCVLOWAT
               set minimum count for input

       SO_SNDTIMEO
               set timeout value for output

       SO_RCVTIMEO
               set timeout value for input

       SO_TYPE get the type of the socket (get only)

       SO_ERROR
               get and clear error on the socket (get only)

       SO_DEBUG enables debugging in the underlying protocol mod-
       ules.  SO_REUSEADDR indicates that the rules used in vali-
       dating  addresses  supplied in a bind(2) call should allow
       reuse of local addresses.  SO_KEEPALIVE enables the  peri-
       odic  transmission  of  messages  on  a  connected socket.
       Should the  connected  party  fail  to  respond  to  these
       messages,  the  connection  is  considered broken and pro-
       cesses using the socket are notified via a SIGPIPE  signal
       when attempting to send data.  SO_DONTROUTE indicates that
       outgoing  messages  should  bypass  the  standard  routing
       facilities.   Instead, messages are directed to the appro-
       priate network interface according to the network  portion
       of the destination address.

       SO_LINGER  controls  the action taken when unsent messages
       are queued on socket and a close(2) is performed.  If  the
       socket promises reliable delivery of data and SO_LINGER is
       set, the system  will  block  the  process  on  the  close
       attempt  until it is able to transmit the data or until it
       decides it is unable to deliver the information (a timeout
       period,  termed  the  linger interval, is specified in the
       setsockopt  call  when  SO_LINGER   is   requested).    If
       SO_LINGER  is  disabled  and a close is issued, the system
       will process the close in a manner that allows the process
       to continue as quickly as possible.

       The option SO_BROADCAST requests permission to send broad-
       cast datagrams on the socket.  Broadcast was a  privileged
       operation  in earlier versions of the system.  With proto-
       cols  that  support  out-of-band  data,  the  SO_OOBINLINE
       option  requests  that  out-of-band  data be placed in the
       normal data input queue  as  received;  it  will  then  be
       accessible  with  recv  or  read calls without the MSG_OOB
       flag.  Some protocols always behave as if this  option  is
       set.   SO_SNDBUF  and  SO_RCVBUF are options to adjust the
       normal  buffer  sizes  allocated  for  output  and   input
       buffers,  respectively.   The buffer size may be increased
       for high-volume connections, or may be decreased to  limit
       the  possible backlog of incoming data.  The system places
       an absolute limit on these values.

       SO_SNDLOWAT is an option to set the minimum count for out-
       put operations.  Most output operations process all of the
       data supplied by the call, delivering data to the protocol
       for  transmission  and blocking as necessary for flow con-
       trol.  Nonblocking output operations will process as  much
       data  as  permitted subject to flow control without block-
       ing, but will process no data if  flow  control  does  not
       allow  the  smaller  of  the  low  water mark value or the
       entire request to be  processed.   A  select(2)  operation
       testing  the ability to write to a socket will return true
       only if the low water mark amount could be processed.  The
       default  value for SO_SNDLOWAT is set to a convenient size
       for network efficiency, often  1024.   SO_RCVLOWAT  is  an
       option  to set the minimum count for input operations.  In
       general, receive calls will  block  until  any  (non-zero)
       amount  of  data  is received, then return with smaller of
       the amount available or the amount requested.  The default
       value  for  SO_SNDLOWAT  is 1.  If SO_SNDLOWAT 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.  Receive calls may  still  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 than that returned.

       SO_SNDTIMEO is an option to set a timeout value for output
       operations.  It accepts a struct  timeval  parameter  with
       the number of seconds and microseconds used to limit waits
       for output operations to complete.  If  a  send  operation
       has  blocked for this much time, it returns with a partial
       count or with the error EWOULDBLOCK if no data were  sent.
       In  the  current  implementation,  this timer is restarted
       each time additional data are delivered to  the  protocol,
       implying that the limit applies to output portions ranging
       in size from the low water mark to the high water mark for
       output.   SO_RCVTIMEO  is an option to set a timeout value
       for input operations.  It accepts a struct timeval parame-
       ter  with  the  number of seconds and microseconds used to
       limit waits for input operations to complete.  In the cur-
       rent  implementation,  this  timer  is restarted each time
       additional data are received by the protocol, and thus the
       limit  is  in  effect  an  inactivity timer.  If a receive
       operation has been blocked  for  this  much  time  without
       receiving  additional  data, it returns with a short count
       or with the error EWOULDBLOCK if no data were received.

       Finally, SO_TYPE and SO_ERROR are options used  only  with
       setsockopt.   SO_TYPE returns the type of the socket, such
       as SOCK_STREAM; it is  useful  for  servers  that  inherit
       sockets on startup.  SO_ERROR returns any pending error on
       the socket and clears the error status.  It may be used to
       check  for asynchronous errors on connected datagram sock-
       ets or for other asynchronous errors.


RETURN VALUE

       On success, zero is returned.  On error, -1  is  returned,
       and errno is set appropriately.


ERRORS

       EBADF   The argument s is not a valid descriptor.

       ENOTSOCK
               The argument s is a file, not a socket.

       ENOPROTOOPT
               The option is unknown at the level indicated.

       EFAULT  The address pointed to by optval is not in a valid
               part of the process address space.   For  getsock-
               opt,  this error may also be returned if optlen is
               not in a valid part of the process address  space.



HISTORY

       These system calls appeared in BSD 4.2.


BUGS

       Several  of  the socket options should be handled at lower
       levels of the system.


SEE ALSO

       ioctl(2), socket(2), getprotoent(3), protocols(5)
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