NAME
socket
—
create an endpoint for
communication
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/socket.h>
int
socket
(int
domain, int type,
int protocol);
DESCRIPTION
socket
()
creates an endpoint for communication and returns a descriptor.
The domain parameter 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 local (previously UNIX) domain protocols PF_INET ARPA Internet protocols PF_INET6 IPv6 (Internet Protocol version 6) protocols PF_NS Xerox Network Systems protocols PF_APPLETALK AppleTalk protocols PF_BLUETOOTH Bluetooth protocols PF_CAN CAN bus protocols
The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:
SOCK_STREAM SOCK_DGRAM SOCK_RAW SOCK_SEQPACKET SOCK_RDM
The following flags can be or'ed to the type to condition the returned file descriptor: The following flags are valid:
SOCK_CLOEXEC
Set the close on exec property. |
SOCK_NONBLOCK
Sets non-blocking I/O. |
SOCK_NOSIGPIPE
Return EPIPE instead of raising
SIGPIPE . |
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 implemented only for PF_NS
.
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.
The protocol 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).
Sockets of type SOCK_STREAM
are full-duplex byte streams. 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) call. Once connected, data may be transferred using
read(2) and
write(2) calls or some variant of the
send(2) and recv(2) calls. 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). A SIGPIPE
signal is raised
if a process 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 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) 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
() 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.
- [
EPROTONOSUPPORT
] - The protocol family is not supported 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), getsockname(2), getsockopt(2), ioctl(2), listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2), shutdown(2), socketpair(2), write(2), getprotoent(3)
Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication Tutorial. (see /usr/share/doc/psd/20.ipctut)
Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve Miller, and Chris Torek, Advanced 4.4BSD IPC Tutorial. (see /usr/share/doc/psd/21.ipc)
HISTORY
The socket
() function call appeared in
4.2BSD.