getsockopt, setsockopt - get and set options on sockets


   #include <sys/types.h>          /* See NOTES */
   #include <sys/socket.h>

   int getsockopt(int sockfd, int level, int optname,
                  void *optval, socklen_t *optlen);
   int setsockopt(int sockfd, int level, int optname,
                  const void *optval, socklen_t optlen);


   getsockopt()   and  setsockopt()  manipulate  options  for  the  socket
   referred to by the  file  descriptor  sockfd.   Options  may  exist  at
   multiple  protocol  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 specified.  To manipulate options at
   the sockets API level, level is specified as SOL_SOCKET.  To manipulate
   options  at  any  other  level  the  protocol number of the appropriate
   protocol 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 arguments 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 argument, 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  argument  for  optval.   For
   setsockopt(),  the  argument  should  be  nonzero  to  enable a boolean
   option, or zero if the option is to be disabled.

   For a description of the available socket options see socket(7) and the
   appropriate protocol man pages.


   On success, zero is returned for the standard options.  On error, -1 is
   returned, and errno is set appropriately.

   Netfilter allows the programmer to define custom  socket  options  with
   associated  handlers;  for such options, the return value on success is
   the value returned by the handler.


   EBADF     The argument sockfd is not a valid file descriptor.

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

   EINVAL    optlen invalid in setsockopt().  In some cases this error can
             also occur for an invalid value  in  optval  (e.g.,  for  the
             IP_ADD_MEMBERSHIP option described in ip(7)).

             The option is unknown at the level indicated.

   ENOTSOCK  The file descriptor sockfd does not refer to a socket.


   POSIX.1-2001,  POSIX.1-2008,  SVr4,  4.4BSD  (these  system calls first
   appeared in 4.2BSD).


   POSIX.1 does not require  the  inclusion  of  <sys/types.h>,  and  this
   header  file  is not required on Linux.  However, some historical (BSD)
   implementations required this header file,  and  portable  applications
   are probably wise to include it.

   For background on the socklen_t type, see accept(2).


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


   ioctl(2), socket(2), getprotoent(3),  protocols(5),  ip(7),  packet(7),
   socket(7), tcp(7), udp(7), unix(7)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at

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