- NetBSD Manual Pages
SOCKET(2) NetBSD System Calls Manual SOCKET(2)
Powered by man-cgi (2021-06-01).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
socket -- create an endpoint for communication
Standard C Library (libc, -lc)
socket(int domain, int type, int protocol);
socket() creates an endpoint for communication and returns a descriptor.
The domain parameter specifies a communications domain within which com-
munication will take place; this selects the protocol family which should
be used. These families are defined in the include file <sys/socket.h>.
The currently understood formats are:
PF_LOCAL local (previously UNIX) domain protocols
PF_INET ARPA Internet protocols
PF_INET6 IPv6 (Internet Protocol version 6) protocols
PF_NS Xerox Network Systems protocols
PF_APPLETALK AppleTalk protocols
PF_BLUETOOTH Bluetooth protocols
PF_CAN CAN bus protocols
The socket has the indicated type, which specifies the semantics of com-
munication. Currently defined types are:
The following flags can be or'ed to the type to condition the returned
file descriptor: The following flags are valid:
SOCK_CLOEXEC Set the close on exec property.
SOCK_NONBLOCK Sets non-blocking I/O.
SOCK_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.
A SOCK_STREAM type provides sequenced, reliable, two-way connection based
byte streams. An out-of-band data transmission mechanism may be sup-
ported. A SOCK_DGRAM socket supports datagrams (connectionless, unreli-
able messages of a fixed (typically small) maximum length). A
SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connec-
tion-based data transmission path for datagrams of fixed maximum length;
a consumer may be required to read an entire packet with each read system
call. This facility is protocol specific, and presently implemented only
for PF_NS. SOCK_RAW sockets provide access to internal network protocols
and interfaces. The types SOCK_RAW, which is available only to the
super-user, and SOCK_RDM, which is planned, but not yet implemented, are
not described here.
The protocol specifies a particular protocol to be used with the socket.
Normally only a single protocol exists to support a particular socket
type within a given protocol family. However, it is possible that many
protocols may exist, in which case a particular protocol must be speci-
fied in this manner. The protocol number to use is particular to the
communication domain in which communication is to take place; see
Sockets of type SOCK_STREAM are full-duplex byte streams. A stream
socket must be in a connected state before any data may be sent or
received on it. A connection to another socket is created with a
connect(2) call. Once connected, data may be transferred using read(2)
and write(2) calls or some variant of the send(2) and recv(2) calls.
When a session has been completed a close(2) may be performed. Out-of-
band data may also be transmitted as described in send(2) and received as
described in recv(2).
The communications protocols used to implement a SOCK_STREAM ensure that
data is not lost or duplicated. If a piece of data for which the peer
protocol has buffer space cannot be successfully transmitted within a
reasonable length of time, then the connection is considered broken and
calls will indicate an error with -1 returns and with ETIMEDOUT as the
specific code in the global variable errno. The protocols optionally
keep sockets ``warm'' by forcing transmissions roughly every minute in
the absence of other activity. An error is then indicated if no response
can be elicited on an otherwise idle connection for an extended period
(e.g., 5 minutes). A SIGPIPE signal is raised if a process sends on a
broken stream; this causes naive processes, which do not handle the sig-
nal, to exit.
SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sock-
ets. The only difference is that read(2) calls will return only the
amount of data requested, and any remaining in the arriving packet will
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspon-
dents named in send(2) calls. Datagrams are generally received with
recvfrom(2), which returns the next datagram with its return address.
An fcntl(2) call can be used to specify a process group to receive a
SIGURG signal when the out-of-band data arrives. It may also enable non-
blocking I/O and asynchronous notification of I/O events via SIGIO.
The operation of sockets is controlled by socket level options. These
options are defined in the file <sys/socket.h>. The setsockopt(2) and
getsockopt(2) system calls are used to set and get options, respectively.
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
The socket() call fails if:
[EACCES] Permission to create a socket of the specified type
and/or protocol is denied.
[EAFNOSUPPORT] The address family (domain) is not supported or the
specified domain is not supported by this protocol
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOBUFS] Insufficient buffer space is available. The socket
cannot be created until sufficient resources are
[EPROTONOSUPPORT] The protocol family is not supported or the specified
protocol is not supported within this domain.
[EPROTOTYPE] The socket type is not supported by the protocol.
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2),
listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2),
shutdown(2), socketpair(2), write(2), getprotoent(3)
Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication
Tutorial. (see /usr/share/doc/reference/ref3/sockets)
Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve
Miller, and Chris Torek, Advanced 4.4BSD IPC Tutorial. (see
The socket() function call appeared in 4.2BSD.
NetBSD 10.99 June 28, 2022 NetBSD 10.99