#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 multi-
       ple  protocol  levels;  they are always present at the uppermost socket

       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 pro-
       tocol 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  getsock-
       opt(), 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; con-
       sult the appropriate entries in section 4 of the manual.

       Most socket-level options utilize an int argument for optval.  For set-
       sockopt(),  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 descriptor.

       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.

       The  optlen  argument of getsockopt() and setsockopt() is in reality an
       int [*] (and this is what 4.x BSD and  libc4  and  libc5  have).   Some
       POSIX  confusion resulted in the present socklen_t, also used by glibc.
       See also 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 4.04 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

Linux                             2015-12-28                     GETSOCKOPT(2)
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