GETSOCKOPT(2) Linux Programmer's Manual GETSOCKOPT(2)
getsockopt, setsockopt - get and set options on sockets
#include <sys/types.h> /* See NOTES */
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);
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 proto-
col 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
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 argu-
ment should be nonzero to enable a boolean option, or zero if the option is to be dis-
For a description of the available socket options see socket(7) and the appropriate proto-
col man pages.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EBADF The argument sockfd 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(). In some cases this error can also occur for an
invalid value in optval (e.g., for the IP_ADD_MEMBERSHIP option described in
The option is unknown at the level indicated.
ENOTSOCK The argument sockfd is a file, not a socket.
SVr4, 4.4BSD (these system calls first appeared in 4.2BSD), POSIX.1-2001.
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 system.
ioctl(2), socket(2), getprotoent(3), protocols(5), socket(7), tcp(7), unix(7)
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Linux 2008-12-03 GETSOCKOPT(2)