SOCKET(2) BSD System Calls Manual SOCKET(2)
NAME
socket -- create an endpoint for communication
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
AF_UNIX (UNIX internal protocols),
AF_INET (ARPA Internet protocols),
AF_ISO (ISO protocols),
AF_NS (Xerox Network Systems protocols), and
AF_IMPLINK (IMP host at IMP link layer).
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
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, superuser,
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 proto-col protocol
col 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, 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) 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 insure that data is not lost or dupli-cated. duplicated.
cated. If a piece of data for which the peer protocol has buffer space cannot be successfully trans-mitted transmitted
mitted 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 (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>. Setsockopt(2) and getsockopt(2) 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 specified address family is not supported.
[EISCONN] The per-process descriptor table is full.
[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 suffi-cient sufficient
cient resources are freed.
[ENOMEM] Insufficient memory was available to fulfill the request.
[EPROTONOSUPPORT] The protocol type or the specified protocol is not supported within this domain.
[EPROTOTYPE] The socket type is not supported by the protocol.
If a new protocol family is defined, the socreate process is free to return any desired error code.
The socket() system call will pass this error code along (even if it is undefined).
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
The include file <sys/types.h> is necessary.
SEE ALSO
accept(2), bind(2), connect(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), inet(4), inet6(4), unix(4),
compat(5)
An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's
Supplementary Documents Volume 1.
BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume
1.
HISTORY
The socket() function call appeared in 4.2BSD.
4.2 Berkeley Distribution June 4, 1993 4.2 Berkeley Distribution
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