Ubuntu Manpage: getsockopt, setsockopt - get and set options on sockets

NAME

       getsockopt, setsockopt - get and set options on sockets

SYNOPSIS

       #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);

DESCRIPTION

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
disabled.

For a description of the available socket options see socket(7) and the appropriate
protocol man pages.

RETURN VALUE

       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.

ERRORS

       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)).

       ENOPROTOOPT
                 The option is unknown at the level indicated.

       ENOTSOCK  The file descriptor sockfd does not refer to a socket.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these system calls first appeared in 4.2BSD).

NOTES

       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).

BUGS

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

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.