GETSOCKOPT(2)             (2020-04-11)              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 multiple protocol levels; they are always pre-
          sent 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 con-
          trolling 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

          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 dis-

          For a description of the available socket options see

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          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 han-

               The argument sockfd is not a valid file descriptor.

               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.

               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).

          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 system.

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

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          This page is part of release 5.10 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

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