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getsockopt, setsockopt - get and set options on sockets
#include <sys/types.h> /* See NOTES */
int getsockopt(int s, int level, int optname,
void *optval, socklen_t *optlen);
int setsockopt(int s, int level, int optname,
const void *optval, socklen_t optlen);
getsockopt() and setsockopt() manipulate the options associated with a
socket. 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 socket 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 parameters 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 parameter, 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 parameter for optval. For
setsockopt(), the parameter 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.
On success, zero is returned. On error, -1 is returned, and errno is
EBADF The argument s is not a valid 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().
The option is unknown at the level indicated.
ENOTSOCK The argument s is a file, not a socket.
SVr4, 4.4BSD (these system calls first appeared in 4.2BSD),
POSIX.1-2001 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.
The optlen argument of getsockopt() and setsockopt() is in reality an
int [*] (and this is what 4.x BSD and libc4 and libc5 have). Some
POSIX confusion resulted in the present socklen_t, also used by glibc.
See also accept(2).
Several of the socket options should be handled at lower levels of the
ioctl(2), socket(2), getprotoent(3), protocols(5), socket(7), tcp(7),
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