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Copyright 2022 Garrett D'Amore
socket - create an endpoint for communication

cc [ flag ... ] file ... -lsocket  -lnsl  [ library ... ]
#include <sys/types.h>
#include <sys/socket.h>

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

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

The domain argument specifies the protocol family within which communication takes place. The protocol family is generally the same as the address family for the addresses supplied in later operations on the socket. These families are defined in <sys/socket.h>.

The currently supported protocol families are: PF_UNIX

PF_INET PF_INET6

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

SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
SOCK_RDM

The type may be augmented by a bitwise-inclusive-OR of flags from the following list, defined in <sys/socket.h>. SOCK_CLOEXEC

SOCK_NDELAY SOCK_NONBLOCK

There must be an entry in the netconfig(5) file for at least each protocol family and type required. If a non-zero protocol has been specified but no exact match for the protocol family, type, and protocol is found, then the first entry containing the specified family and type with a protocol value of zero will be used.

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 not implemented for any protocol family. SOCK_RAW sockets provide access to internal network interfaces. The types SOCK_RAW, which is available only to a user with the net_rawaccess privilege, and SOCK_RDM, for which no implementation currently exists, are not described here.

The protocol parameter is a protocol-family-specific value which specifies a particular protocol to be used with the socket. Normally this value is zero, as commonly only a single protocol exists to support a particular socket type within a given protocol family. However, multiple protocols may exist, in which case a particular protocol may be specified in this manner.

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(3SOCKET) call. Once connected, data may be transferred using read(2) and write(2) calls or some variant of the send(3SOCKET) and recv(3SOCKET) calls. When a session has been completed, a close(2) may be performed. Out-of-band data may also be transmitted as described on the send(3SOCKET) manual page and received as described on the recv(3SOCKET) manual page.

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 a extended period (for instance 5 minutes). A SIGPIPE signal is raised if a thread sends on a broken stream; this causes naive processes, 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(2) 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 datagrams to be sent to correspondents named in sendto(3SOCKET) calls. Datagrams are generally received with recvfrom(3SOCKET), 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 can also enable non-blocking I/O.

The operation of sockets is controlled by socket level options. These options are defined in the file <sys/socket.h>. setsockopt(3SOCKET) and getsockopt(3SOCKET) are used to set and get options, respectively.

Upon successful completion, a descriptor referencing the socket is returned. Otherwise, -1 is returned and errno is set to indicate the error. The socket() function will fail if: EACCES EAGAIN EAFNOSUPPORT EMFILE ENOMEM ENOSR EPFNOSUPPORT EPROTONOSUPPORT EPROTOTYPE EINVAL See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE

MT-Level Safe

close (2), exec (2), fcntl (2), ioctl (2), open (2), read (2), write (2), in.h (3HEAD), socket.h (3HEAD), accept (3SOCKET), bind (3SOCKET), connect (3SOCKET), getsockname (3SOCKET), getsockopt (3SOCKET), listen (3SOCKET), recv (3SOCKET), send (3SOCKET), setsockopt (3SOCKET), shutdown (3SOCKET), socketpair (3SOCKET), attributes (7) Historically, AF_* was commonly used in places where PF_* was meant. New code should be careful to use PF_* as necessary.