socket.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)socket.2	6.1	4/27/89
.TH socket 2
.SH Name
socket \- create an endpoint for communication
.SH Syntax
.B #include <sys/types.h>
.br
.B #include <sys/socket.h>
.PP
.nf
.B s = socket(af, type, protocol)
.B int s, af, type, protocol;
.fi
.SH Description
.NXR "socket system call"
.NXR "socket" "creating"
.NXR "socket system call" "accept system call"
.NXR "socket system call" "bind system call"
.NXR "socket system call" "connect system call"
.NXR "socket system call" "getsockname system call"
.NXR "socket system call" "pipe system call"
.NXR "socket system call" "recv system call"
.NXR "socket system call" "socketpair system call"
The
.PN socket
system call
creates an endpoint for communication and returns a descriptor.
.PP
The operation of sockets is controlled by socket-level options, defined
in the file 
.PN <sys/socket.h>
and explained in the section, Socket-level Options.  The calls
.MS setsockopt 2
and
.MS getsockopt 2
are used to set and get options.
.SH Arguments
The
.I af
parameter specifies an address format. Addresses specified
in later operations using the socket are interpreted according
to these formats.  The
formats are defined in the include file 
.PN <sys/socket.h> : 
.NXR "socket system call" "address formats"
.PP
.EX
AF_UNIX	\fRUNIX path names\fP
AF_INET	\fRARPA Internet addresses\fP
AF_IMPLINK	\fRIMP \*(lqhost at IMP\*(rq addresses\fP
AF_DLI		\fRFor access to broadcast devices (Ethernet)\fP
.EE
.PP
The 
.I type
argument specifies the semantics of communication.  The 
defined types are:
.NXR "socket" "defined types"
.PP
.EX
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
.EE
.PP
The SOCK_STREAM and SOCK_DGRAM types are available only if
your system includes the TCP/IP network.  For example,
if you can use the
.PN rlogin
command to log in to a remote ULTRIX node, your system
supports these socket types.
.PP
A SOCK_STREAM type provides sequenced, reliable,
2-way-connection-based byte streams with an out-of-band data
transmission mechanism.
A SOCK_DGRAM socket supports
datagrams (connectionless, unreliable messages of
a fixed maximum length, typically small).
.NXR "datagram" "defined"
.PP
SOCK_RAW sockets provide access to internal network interfaces
and are available only to the super-user.  
.PP
The SOCK_SEQPACKET type is the socket protocol to request when
you want to communicate with other Digital systems using DECnet. 
.PP
Socket types are discussed further in following sections.
.PP
The
.I protocol
argument specifies the protocol to be used with the socket.
Normally, only a single protocol exists to support a particular
socket type using a given address format.  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.  For further information, see
.MS services 5
and
.MS protocols 5 .
.SH Socket Type SOCK_STREAM
.NXR "SOCK_STREAM socket type" "defined"
.PP
Sockets of type SOCK_STREAM
are full-duplex byte streams, similar
to pipes.  A stream socket must be in a
.I connected
state before any data can be sent or received
on it.  A connection to another socket is created with a
.PN connect 
call.  Once connected, data can be transferred using
.PN read 
and 
.PN write 
calls or some variant of the 
.PN send 
and 
.PN recv 
calls.  When a session has been completed, a
.PN close 
may be performed.
Out-of-band data can also be transmitted as described in
.MS send 2
and received as described in
.MS recv 2 .
.PP
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 \*(lqwarm\*(rq 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 (for example, 5 minutes).
A SIGPIPE signal is raised if a process sends
on a broken stream; this causes processes
that do not handle the signal to exit.
.SH Socket Types SOCK_DGRAM and SOCK_RAW
.NXR "SOCK_DGRAM socket type"
.NXR "SOCK_RAW socket type"
.PP
SOCK_DGRAM and SOCK_RAW
sockets allow sending of datagrams to correspondents
named in
.MS send 2
calls.  It is also possible to receive datagrams at
these sockets with
.MS recv 2 .
.PP
An 
.MS fcntl 2
call can be used to specify a process group to receive
a SIGURG signal when the out-of-band data arrives.
.PP
SOCK_DGRAM sockets are the only type of socket allowed by the Data
Link Interface.
.SH Socket Type SOCK_SEQPACKET
.NXR "SOCK_SEQPACKET socket type" "defined"
.PP
SOCK_SEQPACKET sockets are similar to datagrams except
that they are guaranteed to be received in the sequence
that they are sent.  They are also guaranteed to be
error-free.
.SH Socket-Level Options
.NXR "socket system call" "options"
.PP
The operation of sockets is controlled by socket-level
options.
These options are defined in the file,
.PN <sys/socket.h> .
The calls 
.PN setsockopt 
and
.PN getsockopt 
are used to set and get options.
.PP
Options other than SO_LINGER take an integer parameter that
should be nonzero, if the option is to be enabled, or zero (0), if
it is to be disabled.
SO_LINGER uses a ``linger'' structure parameter
defined in 
.PN <sys/socket.h> .  
This structure specifies the desired state of the option and the linger
interval (see the following).
.PP
.EX
SO_DEBUG             \fRTurn on recording of debugging information\fP
SO_REUSEADDR         \fRAllow local address reuse\fP
SO_KEEPALIVE         \fRKeep connections alive\fP
SO_DONTROUTE         \fRDo not apply routing on outgoing messages\fP
SO_LINGER            \fRLinger on close if data present\fP
SO_BROADCAST         \fRPermit sending of broadcast messages\fP
SO_ACCEPTCONN        \fRSocket has had listen()\fP
SO_USELOOPBACK       \fRBypass hardware when possible\fP
SO_OOBINLINE         \fRLeave received OOB data in line\fP
.EE
.RE
.PP
SO_DEBUG enables debugging in the underlying protocol modules.
.PP
SO_REUSEADDR indicates the rules used in validating addresses supplied
in a
.PN bind 
call should allow reuse of local addresses.  
.PP
SO_KEEPALIVE enables the
periodic transmission of messages on a connected socket.  Should the
connected party fail to respond to these messages, the connection is
considered broken and processes using the socket are notified through 
a SIGIO signal.  
Note that in order to receive signals on a socket the
program must first call
.MS fcntl 2
with a F_SETOWN request to set the process group
of the process
to receive the signal, and with a F_SETFL request to set the
FASYNC flag, which enables the SIGIO signal to be sent.
.PP
SO_DONTROUTE indicates that outgoing messages should
bypass the standard routing facilities.  Instead, messages are directed
to the appropriate network interface, according to the network portion
of the destination address.  
.PP
SO_LINGER controls the actions taken when unsent messages
are queued on the socket and a 
.PN close 
is performed.
When using the 
.PN setsockopt 
to set the linger values,
the option value for the SO_LINGER command is the address of a linger
structure:
.EX
struct  linger {
        int     l_onoff;       /* option on/off */
        int     l_linger;      /* linger time */
};
.EE
If the socket promises reliable delivery of data and l_onoff is nonzero,
the system blocks the process on the 
.PN close
attempt until it is able to transmit the data or until it decides it
is unable to deliver the information.  A timeout period, termed the
linger interval, is specified in l_linger in seconds.
If l_onoff is set to zero (0) and a 
.PN close
is issued, the system processes the close in a manner that allows
the process to continue as quickly as possible.
.PP
SO_BROADCAST is used to enable or disable broadcasting on the socket.
.SH Return Values
.NXR "socket system call" "return value"
A \-1 is returned if an error occurs.   Otherwise, the return
value is a descriptor to be used in other calls to refer to
the socket.
.SH Diagnostics
.NXR "socket system call" "diagnostics"
The
.PN socket
call fails if:
.TP 10
[EAFNOSUPPORT]
The specified address family is not supported in this version
of the system.
.br
.ne 2
.TP 10
[ESOCKTNOSUPPORT]
The specified socket type is not supported in this address family.
.br
.ne 3
.TP 10
[EPROTONOSUPPORT]
The specified protocol is not supported.
.TP 10
[EPROTOTYPE]
Request for a type of socket for which there is no supporting
protocol.
.TP 10
[EMFILE]
The per-process descriptor table is full.
.TP 10
[ENOBUFS]
No buffer space is available.  The socket cannot be created.
.SH See Also
accept(2), bind(2), close(2), connect(2), fcntl(2), getsockname(2),
getsockopt(2), ioctl(2), listen(2), read (2), recv(2), select(2),
send(2), setsockopt(2), shutdown(2), socketpair(2), protocols(5), 
services(5), write(2),
.br
\fIGuide to Network Programming\fR,
.br
\fIGuide to the Data Link Interface\fR