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 close-on-exec flag,
FD_CLOEXEC set. This is similar in purpose to the
O_CLOEXEC flag to open(2). The file descriptor will be
closed on any successful calls to the exec(2) family of
functions.


SOCK_CLOFORK
Creates the socket with the close-on-fork flag,
FD_CLOFORK, set. This is similar in purpose to the
O_CLOFORK flag to open(2). The file descriptor will be
closed in any children created with the fork(2) family of
functions.


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.

June 21, 2024 SOCKET(3SOCKET)