SOCKET(3SOCKET) Sockets Library Functions SOCKET(3SOCKET)

NAME

socket - create an endpoint for communication

SYNOPSIS

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

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

DESCRIPTION

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
UNIX system internal protocols


PF_INET
Internet Protocol Version 4 (IPv4)


PF_INET6
Internet Protocol Version 6 (IPv6)


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
Creates the socket with the FD_CLOEXEC flag set, causing the
underlying file descriptor to be closed prior to any future
calls to exec(2). This is similar in purpose to the O_CLOEXEC
flag to open(2).


SOCK_NDELAY
Creates the socket with the O_NDELAY flag set, causing the
socket to provide nonblocking semantics as described for
O_NDELAY in open(2). SOCK_NONBLOCK should normally be used
in preference to SOCK_NDELAY, and takes precedence if both
are set. See open(2) for further details.


SOCK_NONBLOCK
Creates the socket with the O_NONBLOCK flag set, causing the
socket to provide nonblocking semantics as described for
O_NONBLOCK in open(2).


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.

RETURN VALUES

Upon successful completion, a descriptor referencing the socket is
returned. Otherwise, -1 is returned and errno is set to indicate the
error.

ERRORS

The socket() function will fail if:

EACCES
Permission to create a socket of the specified type or
protocol is denied.


EAGAIN
There were insufficient resources available to
complete the operation.


EAFNOSUPPORT
The specified address family is not supported by the
protocol family.


EMFILE
The per-process descriptor table is full.


ENOMEM
Insufficient user memory is available.


ENOSR
There were insufficient STREAMS resources available to
complete the operation.


EPFNOSUPPORT
The specified protocol family is not supported.


EPROTONOSUPPORT
The protocol type is not supported by the address
family.


EPROTOTYPE
The socket type is not supported by the protocol.


EINVAL
One or more of the specified flags is not supported.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|MT-Level | Safe |
+---------------+-----------------+

SEE ALSO

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)

NOTES

Historically, AF_* was commonly used in places where PF_* was meant. New
code should be careful to use PF_* as necessary.

July 6, 2022 SOCKET(3SOCKET)