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


       EBADF     The argument sockfd is not a valid 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 argument sockfd is a file, not a socket.


       SVr4,   4.4BSD   (these   system   calls  first  appeared  in  4.2BSD),


       POSIX.1-2001 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), socket(7), tcp(7),


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