SOCKET(2) FreeBSD System Calls Manual SOCKET(2)
NAME
socket - create an endpoint for communication
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
int
socket(int domain, int type, int protocol);
DESCRIPTION
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 are:
PF_LOCAL Host-internal protocols (alias for PF_UNIX),
PF_UNIX Host-internal protocols,
PF_INET Internet version 4 protocols,
PF_INET6 Internet version 6 protocols,
PF_ROUTE Internal routing protocol,
PF_LINK Link layer interface,
PF_KEY Internal key-management function,
PF_NATM Asynchronous transfer mode protocols,
PF_NETGRAPH Netgraph sockets,
PF_IEEE80211 IEEE 802.11 wireless link-layer protocols (WiFi),
PF_BLUETOOTH Bluetooth protocols,
PF_INET_SDP OFED socket direct protocol (IPv4),
PF_INET6_SDP OFED socket direct protocol (IPv6)
Each protocol family is connected to an address family, which has the
same name except that the prefix is "AF_" in place of "PF_". Other
protocol families may be also defined, beginning with "PF_", with
corresponding address families.
The socket has the indicated type, which specifies the semantics of
communication. Currently defined types are:
SOCK_STREAM Stream socket,
SOCK_DGRAM Datagram socket,
SOCK_RAW Raw-protocol interface,
SOCK_RDM Reliably-delivered packet,
SOCK_SEQPACKET Sequenced packet stream
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 may have protocol-specific properties.
SOCK_RAW sockets provide access to internal network protocols and
interfaces. The types SOCK_RAW, which is available only to the super-
user, and SOCK_RDM, which is planned, but not yet implemented, are not
described here.
Additionally, the following flags are allowed in the type argument:
SOCK_CLOEXEC Set close-on-exec on the new descriptor,
SOCK_NONBLOCK Set non-blocking mode on the new socket
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; see
protocols(5).
The protocol argument may be set to zero (0) to request the default
implementation of a socket type for the protocol, if any.
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(2) system call. Once connected, data may be transferred
using read(2) and write(2) calls or some variant of the send(2) and
recv(2) 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(2) system call.)
When a session has been completed a close(2) may be performed. Out-of-
band data may also be transmitted as described in send(2) and received as
described in recv(2).
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 (e.g.
5 minutes). By default, a SIGPIPE signal is raised if a process sends on
a broken stream, but this behavior may be inhibited via setsockopt(2).
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 sending of datagrams to
correspondents named in send(2) calls. Datagrams are generally received
with recvfrom(2), which returns the next datagram with its return
address.
An fcntl(2) system call can be used to specify a process group to receive
a SIGURG signal when the out-of-band data arrives. It may also enable
non-blocking I/O and asynchronous notification of I/O events via SIGIO.
The operation of sockets is controlled by socket level options. These
options are defined in the file <sys/socket.h>. The setsockopt(2) and
getsockopt(2) system calls are used to set and get options, respectively.
RETURN VALUES
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
ERRORS
The socket() system call fails if:
[EACCES] Permission to create a socket of the specified type
and/or protocol is denied.
[EAFNOSUPPORT] The address family (domain) is not supported or the
specified domain is not supported by this protocol
family.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOBUFS] Insufficient buffer space is available. The socket
cannot be created until sufficient resources are
freed.
[EPERM] User has insufficient privileges to carry out the
requested operation.
[EPROTONOSUPPORT] The protocol type or the specified protocol is not
supported within this domain.
[EPROTOTYPE] The socket type is not supported by the protocol.
SEE ALSO
accept(2), bind(2), connect(2), getpeername(2), getsockname(2),
getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
shutdown(2), socketpair(2), write(2), CMSG_DATA(3), getprotoent(3),
netgraph(4), protocols(5)
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"BSD Interprocess Communication Tutorial", PS1, 8.
STANDARDS
The socket() function conforms to IEEE Std 1003.1-2008 ("POSIX.1"). The
POSIX standard specifies only the AF_INET, AF_INET6, and AF_UNIX
constants for address families, and requires the use of AF_* constants
for the domain argument of socket(). The SOCK_CLOEXEC flag is expected
to conform to the next revision of the POSIX standard. The SOCK_RDM
type, the PF_* constants, and other address families are FreeBSD
extensions.
HISTORY
The socket() system call appeared in 4.2BSD.
FreeBSD 13.1-RELEASE-p6 August 19, 2018 FreeBSD 13.1-RELEASE-p6
man2web Home...