SOCKET(2) BSD System Calls Manual SOCKET(2)
socket -- create an endpoint for communication
Standard C Library (libc, -lc)
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 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. Cur-
rently 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 is protocol specific, and
presently unimplemented. 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 trans-
ferred using read(2) and write(2) calls or some variant of the send(2) and recv(2) func-
tions. (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 per-
formed. 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 suc-
cessfully 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 forc-
ing 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 dif-
ference is that read(2) calls will return only the amount of data requested, and any remain-
ing 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.
A -1 is returned if an error occurs, otherwise the return value is a descriptor referencing
The socket() system call fails if:
[EACCES] Permission to create a socket of the specified type and/or protocol is
[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
[EPROTOTYPE] The socket type is not supported by the protocol.
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),
getprotoent(3), netgraph(4), protocols(5)
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"BSD Interprocess Communication Tutorial", PS1, 8.
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.
The socket() system call appeared in 4.2BSD.
BSD March 19, 2013 BSD