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Create an endpoint for communication.

#include <sys/types.h>
#include <sys/socket.h>

socket(int domain, int type, int protocol);


The socket() system call creates an endpoint for communication and returns a descriptor.

The domain argument specifies a communications domain within which communication 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 include:

Internet version 4 protocols,
Internal Routing protocol,
Link layer interface,
Internal key-management function,
Internet version 6 protocols,
Netgraph sockets

The socket has the indicated type, which specifies the semantics of communication. Currently defined types include:

Stream socket,
Datagram socket,
Provides access to the Raw-IP layer

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 connection-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 unimplemented. SOCK_RAW sockets provide access to internal network protocols and interfaces. A RAW socket is created like this:

int fd = socket(AF_INET, SOCK_RAW, XXX);

where XXX is one of following IP protocols.


Raw socket use is beyond the scope of this document.  For further details see the extensive treatment in "UNIX Network Programming, The Sockets Networking API, Volume 1, Stevens, Fenner, Rudoff".

The protocol argument 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 created with a connect system call. Once connected, data may be transferred using read and write calls or some variant of the send and recv functions. (Some protocol families, such as the Internet family, support the notion of an "implied" "connect", which permits data to be sent piggybacked onto a connect operation by using the sendto system call.) When a session has been completed a close may be performed. Out-of-band data may also be transmitted as described in send and received as described in recv.

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 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).

SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sockets. The only difference is that read 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 correspondents named in send calls. Datagrams are generally received with recvfrom, which returns the next datagram with its return address.

The operation of sockets is controlled by socket level options. These options are defined in the file <sys/socket.h>. The setsockopt and getsockopt system calls are used to set and get options, respectively.

Return Values

A value of -1 is returned if an error occurs, otherwise the return value is a descriptor referencing the socket.

The socket() system call fails if:

ENODEV The TCP/IP stack is not loaded or initialized.
EPROTONOSUPPORT The protocol type or the specified protocol is not supported within this domain.
EMFILE The per-process descriptor table is full.
ENFILE The system file table is full.
EACCES Permission to create a socket of the specified type and/or protocol is denied.
ENOBUFS Insufficient buffer space is available. The socket cannot be created until sufficient resources are freed.


Versions Link to
INtime 4.0 netlib.lib
See Also