GETSOCKOPT(2) Linux Programmer's Manual GETSOCKOPT(2)
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 re-
 ferred 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 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 ap-
 propriate 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.
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 ap-
 peared 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.
SEE ALSO
 ioctl(2), socket(2), getprotoent(3), protocols(5), ip(7), packet(7),
 socket(7), tcp(7), udp(7), unix(7)
COLOPHON
 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
 https://www.kernel.org/doc/man-pages/.
Linux 2020年04月11日 GETSOCKOPT(2)