SOCKET(2) System Calls Manual SOCKET(2)
socket - create an endpoint for communication
int socket(domain, type, protocol)
int domain, type, protocol;
socket() creates an endpoint for communication and returns a descrip-
The domain parameter specifies a communications domain within which
communication will take place; this selects the protocol family which
should be used. The protocol family generally is the same as the
address family for the addresses supplied in later operations on the
socket. These families are defined in the include file <<sys/socket.h>>.
The currently understood formats are
PF_UNIX (UNIX system internal protocols),
PF_INET (ARPA Internet protocols), and
PF_IMPLINK (IMP "host at IMP" link layer).
The socket has the indicated type, which specifies the semantics of
communication. Currently defined types are:
A SOCK_STREAM type provides sequenced, reliable, two-way connection
based byte streams. An out-of-band data transmission mechanism may be
supported. A SOCK_DGRAM socket supports datagrams (connectionless,
unreliable messages of a fixed (typically small) maximum length). A
SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way con-
nection-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
not implemented for any protocol family. SOCK_RAW sockets provide
access to internal network interfaces. The types SOCK_RAW, which is
available only to the super-user, and SOCK_RDM, for which no implemen-
tation currently exists, 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 specified in this manner. The protocol number to use is particular
to the "communication domain" in which communication is to take place;
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
pipes. 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 cre-
ated with a connect(2) call. Once connected, data may be transferred
using read(2V) and write(2V) calls or some variant of the send(2) and
recv(2) calls. When a session has been completed a close(2V), 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 insure
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 pro-
tocols optionally keep sockets "warm" by forcing transmissions roughly
every minute in the absence of other activity. An error is then indi-
cated if no response can be elicited on an otherwise idle connection
for a extended period (for instance 5 minutes). A SIGPIPE signal is
raised if a process sends on a broken stream; this causes naive pro-
cesses, which do not handle the signal, to exit.
SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM
sockets. The only difference is that read(2V) calls will return only
the amount of data requested, and any remaining in the arriving packet
will be discarded.
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to corre-
spondents named in send(2) calls. Datagrams are generally received
with recv(2), which returns the next datagram with its return address.
An fcntl(2V) 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 with SIGIO
The operation of sockets is controlled by socket level options. These
options are defined in the file socket.h. getsockopt(2) and setsock-
opt() are used to get and set options, respectively.
socket() returns a non-negative descriptor on success. On failure, it
returns -1 and sets errno to indicate the error.
EACCES Permission to create a socket of the specified type
and/or protocol is denied.
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 type or the specified protocol is not
supported within this domain.
EPROTOTYPE The protocol is the wrong type for the socket.
accept(2), bind(2), close(2V), connect(2), fcntl(2V), getsockname(2),
getsockopt(2), ioctl(2), listen(2), read(2V), recv(2), select(2),
send(2), shutdown(2), socketpair(2), write(2V), protocols(5)
21 January 1990 SOCKET(2)