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

NAME

       getsockopt, setsockopt - get and set options on sockets

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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);

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 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.

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.

STANDARDS

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

NOTES

       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.