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       getsockopt, setsockopt - get and set options on sockets


       Standard C library (libc, -lc)


       #include <sys/socket.h>

       int getsockopt(int sockfd, int level, int optname,
                      void optval[restrict *.optlen],
                      socklen_t *restrict optlen);
       int setsockopt(int sockfd, int level, int optname,
                      const void optval[.optlen],
                      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

       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 to indicate the error.

       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

              The option is unknown at the level indicated.

              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).


       For background on the socklen_t type, see accept(2).


       Several of the socket options should be handled at lower levels of the system.


       ioctl(2),  socket(2),  getprotoent(3),  protocols(5), ip(7), packet(7), socket(7), tcp(7),
       udp(7), unix(7)