Name

socket − 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 socket 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 <sys/socket.h> and explained in the section, Socket-level Options.  The calls setsockopt() and getsockopt() 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 <sys/socket.h>:

AF_UNIXUNIX path names
AF_INETARPA Internet addresses
AF_IMPLINKIMP “host at IMP” addresses
AF_DLIFor 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 rlogin 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 messages 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 particular 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 services() and protocols(.).

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 created with a connect call.  Once connected, data can be transferred using read and write calls or some variant of the send and recv calls.  When a session has been completed, a close may be performed. Out-of-band data can also be transmitted as described in send() and received as described in recv(.).

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 (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 correspondents named in send() calls.  It is also possible to receive datagrams at these sockets with recv(.).

An fcntl() call can be used to specify a process group to receive a SIGURG signal 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, <sys/socket.h>. The calls setsockopt and getsockopt 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 <sys/socket.h>. 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_REUSEADDRAllow local address reuse
SO_KEEPALIVEKeep connections alive
SO_DONTROUTEDo not apply routing on outgoing messages
SO_LINGERLinger on close if data present
SO_BROADCASTPermit sending of broadcast messages
SO_ACCEPTCONNSocket has had listen()
SO_USELOOPBACKBypass hardware when possible
SO_OOBINLINELeave 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 bind call should allow reuse of local addresses.

SO_KEEPALIVE enables the periodic transmission of messages on a connected socket.  Should the connected party fail to respond to these messages, the connection is considered broken and processes using the socket are notified through a SIGPIPE signal. 

SO_DONTROUTE indicates that outgoing messages should bypass the standard routing 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 the socket and a close is performed. When using the setsockopt 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 the socket promises reliable delivery of data and l_onoff is nonzero, the system blocks the process on the close attempt until it is able to transmit the data or 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 close 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 Value

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 socket 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 Also

accept(2), bind(2), close(2), connect(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), services(5), write(2),
“A 4.2 BSD Interprocess Communication Primer,” ULTRIX Supplementary Documents, Vol. III:System Manager,
Guide to the Data Link Interface