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       getsockopt, setsockopt - get and set options on sockets


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

       int  getsockopt(int  s, int level, int optname, void *optval, socklen_t

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


       Getsockopt  and  setsockopt()  manipulate the options associated with a
       socket.  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  socket  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  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.

       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 s 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().

                 The option is unknown at the level indicated.

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


       SVr4, 4.4BSD (these system  calls  first  appeared  in  4.2BSD).   SVr4
       documents  additional  ENOMEM  and  ENOSR  error  codes,  but  does not


       The  fifth  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),