man2/getsockopt.2

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.\"     $Id: getsockopt.2,v 1.1 1999/05/24 14:57:04 freitag Exp $
.\"
.\" Modified Sat Jul 24 16:19:32 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Mon Apr 22 02:29:06 1996 by Martin Schulze (joey@infodrom.north.de)
.\" Modified Tue Aug 27 10:52:51 1996 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Thu Jan 23 13:29:34 1997 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Sun Mar 28 21:26:46 1999 by Andries Brouwer (aeb@cwi.nl)
.\" Modified 1999 by Andi Kleen <ak@muc.de>.
.\"     Removed most stuff because it is in socket.7 now.
.\"
.TH GETSOCKOPT 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
getsockopt, setsockopt \- get and set options on sockets
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.PP
.BI "int getsockopt(int " sockfd ", int " level ", int " optname ,
.BI "               void *restrict " optval ", socklen_t *restrict " optlen );
.BI "int setsockopt(int " sockfd ", int " level ", int " optname ,
.BI "               const void *" optval ", socklen_t " optlen );
.fi
.SH DESCRIPTION
.BR getsockopt ()
and
.BR setsockopt ()
manipulate options for the socket referred to by the file descriptor
.IR sockfd .
Options may exist at multiple
protocol levels; they are always present at the uppermost
socket level.
.PP
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,
.I level
is specified as
.BR 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
.B TCP
protocol,
.I level
should be set to the protocol number of
.BR TCP ;
see
.BR getprotoent (3).
.PP
The arguments
.I optval
and
.I optlen
are used to access option values for
.BR setsockopt ().
For
.BR getsockopt ()
they identify a buffer in which the value for the
requested option(s) are to be returned.
For
.BR getsockopt (),
.I optlen
is a value-result argument, initially containing the
size of the buffer pointed to by
.IR optval ,
and modified on return to indicate the actual size of
the value returned.
If no option value is to be supplied or returned,
.I optval
may be NULL.
.PP
.I Optname
and any specified options are passed uninterpreted to the appropriate
protocol module for interpretation.
The include file
.I <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.
.PP
Most socket-level options utilize an
.I int
argument for
.IR optval .
For
.BR setsockopt (),
the argument should be nonzero to enable a boolean option, or zero if the
option is to be disabled.
.PP
For a description of the available socket options see
.BR socket (7)
and the appropriate protocol man pages.
.SH RETURN VALUE
On success, zero is returned for the standard options.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.PP
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.
.SH ERRORS
.TP
.B EBADF
The argument
.I sockfd
is not a valid file descriptor.
.TP
.B EFAULT
The address pointed to by
.I optval
is not in a valid part of the process address space.
For
.BR getsockopt (),
this error may also be returned if
.I optlen
is not in a valid part of the process address space.
.TP
.B EINVAL
.I optlen
invalid in
.BR setsockopt ().
In some cases this error can also occur for an invalid value in
.IR optval
(e.g., for the
.B IP_ADD_MEMBERSHIP
option described in
.BR ip (7)).
.TP
.B ENOPROTOOPT
The option is unknown at the level indicated.
.TP
.B ENOTSOCK
The file descriptor
.I sockfd
does not refer to a socket.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008,
SVr4, 4.4BSD (these system calls first appeared in 4.2BSD).
.\" SVr4 documents additional ENOMEM and ENOSR error codes, but does
.\" not document the
.\" .BR SO_SNDLOWAT ", " SO_RCVLOWAT ", " SO_SNDTIMEO ", " SO_RCVTIMEO
.\" options
.SH NOTES
For background on the
.I socklen_t
type, see
.BR accept (2).
.SH BUGS
Several of the socket options should be handled at lower levels of the
system.
.SH SEE ALSO
.BR ioctl (2),
.BR socket (2),
.BR getprotoent (3),
.BR protocols (5),
.BR ip (7),
.BR packet (7),
.BR socket (7),
.BR tcp (7),
.BR udp (7),
.BR unix (7)