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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
       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 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 4.10 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				  2016-12-12			 GETSOCKOPT(2)