socket(2)socket(2)Namesocket - create an endpoint for communication
Syntax
#include <sys/types.h>
#include <sys/socket.h>
s = socket(af, type, protocol)
int s, af, type, protocol;
Description
The system call creates an endpoint for communication and returns a
descriptor.
The operation of sockets is controlled by socket-level options, defined
in the file and explained in the section, Socket-level Options. The
calls and are used to set and get options.
Arguments
The af parameter specifies an address format. Addresses specified in
later operations using the socket are interpreted according to these
formats. The formats are defined in the include file
AF_UNIX UNIX path names
AF_INET ARPA Internet addresses
AF_IMPLINK IMP “host at IMP” addresses
AF_DLI For access to broadcast devices (Ethernet)
The type argument specifies the semantics of communication. The
defined types are:
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
The SOCK_STREAM and SOCK_DGRAM types are available only if your system
includes the TCP/IP network. For example, if you can use the command
to log in to a remote ULTRIX node, your system supports these socket
types.
A SOCK_STREAM type provides sequenced, reliable, 2-way-connection-based
byte streams with an out-of-band data transmission mechanism. A
SOCK_DGRAM socket supports datagrams (connectionless, unreliable mes‐
sages of a fixed maximum length, typically small).
SOCK_RAW sockets provide access to internal network interfaces and are
available only to the super-user.
The SOCK_SEQPACKET type is the socket protocol to request when you want
to communicate with other Digital systems using DECnet.
Socket types are discussed further in following sections.
The protocol argument specifies the protocol to be used with the
socket. Normally, only a single protocol exists to support a particu‐
lar socket type using a given address format. 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.
For further information, see and
Socket Type SOCK_STREAM
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
can be sent or received on it. A connection to another socket is cre‐
ated with a call. Once connected, data can be transferred using and
calls or some variant of the and calls. When a session has been com‐
pleted, a may be performed. Out-of-band data can also be transmitted
as described in and received as described in
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 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 an extended period (for example, 5 minutes). A SIGPIPE signal is
raised if a process sends on a broken stream; this causes processes
that do not handle the signal to exit.
Socket Types SOCK_DGRAM and SOCK_RAW
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to corre‐
spondents named in calls. It is also possible to receive datagrams at
these sockets with
An call can be used to specify a process group to receive a SIGURG sig‐
nal when the out-of-band data arrives.
SOCK_DGRAM sockets are the only type of socket allowed by the Data Link
Interface.
Socket Type SOCK_SEQPACKET
SOCK_SEQPACKET sockets are similar to datagrams except that they are
guaranteed to be received in the sequence that they are sent. They are
also guaranteed to be error-free.
Socket-Level Options
The operation of sockets is controlled by socket-level options. These
options are defined in the file, The calls and are used to set and get
options.
Options other than SO_LINGER take an integer parameter that should be
nonzero, if the option is to be enabled, or zero (0), if it is to be
disabled. SO_LINGER uses a ``linger'' structure parameter defined in
This structure specifies the desired state of the option and the linger
interval (see the following).
SO_DEBUG Turn on recording of debugging information
SO_REUSEADDR Allow local address reuse
SO_KEEPALIVE Keep connections alive
SO_DONTROUTE Do not apply routing on outgoing messages
SO_LINGER Linger on close if data present
SO_BROADCAST Permit sending of broadcast messages
SO_ACCEPTCONN Socket has had listen()
SO_USELOOPBACK Bypass hardware when possible
SO_OOBINLINE Leave received OOB data in line
SO_DEBUG enables debugging in the underlying protocol modules.SO_REUSEADDR indicates the rules used in validating addresses supplied in a
call should allow reuse of local addresses.SO_KEEPALIVE enables the periodic transmission of messages on a connectedsocket. Should the connected party fail to respond to these messages, the
connection is considered broken and processes using the socket are notified
through a SIGIO signal. Note that in order to receive signals on a socket the
program must first call with a F_SETOWN request to set the process group of
the process to receive the signal, and with a F_SETFL request to set the
FASYNC flag, which enables the SIGIO signal to be sent.
SO_DONTROUTE indicates that outgoing messages should bypass the standard rout‐
ing facilities. Instead, messages are directed to the appropriate network
interface, according to the network portion of the destination address.
SO_LINGER controls the actions taken when unsent messages are queued on thesocket and a is performed. When using the to set the linger values, the
option value for the SO_LINGER command is the address of a linger structure:struct linger {
int l_onoff; /* option on/off */
int l_linger; /* linger time */
};
If thesocket promises reliable delivery of data and l_onoff is nonzero, the
system blocks the process on the attempt until it is able to transmit the dataor until it decides it is unable to deliver the information. A timeout
period, termed the linger interval, is specified in l_linger in seconds. If
l_onoff is set to zero (0) and a is issued, the system processes the close in
a manner that allows the process to continue as quickly as possible.SO_BROADCAST is used to enable or disable broadcasting on the socket.Return Values
A -1 is returned if an error occurs. Otherwise, the return value is a
descriptor to be used in other calls to refer to the socket.
Diagnostics
The call fails if:
[EAFNOSUPPORT]
The specified address family is not supported in this version
of the system.
[ESOCKTNOSUPPORT]
The specified socket type is not supported in this address
family.
[EPROTONOSUPPORT]
The specified protocol is not supported.
[EPROTOTYPE]
Request for a type of socket for which there is no supporting
protocol.
[EMFILE] The per-process descriptor table is full.
[ENOBUFS] No buffer space is available. The socket cannot be created.
See Alsoaccept(2), bind(2), close(2), connect(2), fcntl(2), getsockname(2),
getsockopt(2), ioctl(2), listen(2), read (2), recv(2), select(2),
send(2), setsockopt(2), shutdown(2), socketpair(2), protocols(5), ser‐
vices(5), write(2),
Guide to Network Programming,
Guide to the Data Link Interface
socket(2)
