GETSOCKOPT(2)              Linux Programmer's Manual             GETSOCKOPT(2)

       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 re-
       ferred 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  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 ap-
       propriate  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 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  ap-
       peared 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)

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Linux                             2017-09-15                     GETSOCKOPT(2)
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