getsockopt(2) System Calls Manual getsockopt(2)
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 proto-
col 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 proto-
col 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 ac-
tual 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 appro-
priate 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 to indicate the error.
Netfilter allows the programmer to define custom socket options with as-
sociated 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.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.4BSD (first appeared in 4.2BSD).
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)
Linux man-pages 6.9.1 2024-05-02 getsockopt(2)
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