711 lines
28 KiB
Perl
711 lines
28 KiB
Perl
.\" Copyright (c) 1986 The Regents of the University of California.
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)2.t 5.1 (Berkeley) 4/17/91
|
|
.\"
|
|
.\".ds RH "Basics
|
|
.bp
|
|
.nr H1 2
|
|
.nr H2 0
|
|
.bp
|
|
.LG
|
|
.B
|
|
.ce
|
|
2. BASICS
|
|
.sp 2
|
|
.R
|
|
.NL
|
|
.PP
|
|
The basic building block for communication is the \fIsocket\fP.
|
|
A socket is an endpoint of communication to which a name may
|
|
be \fIbound\fP. Each socket in use has a \fItype\fP
|
|
and one or more associated processes. Sockets exist within
|
|
\fIcommunication domains\fP.
|
|
A communication domain is an
|
|
abstraction introduced to bundle common properties of
|
|
processes communicating through sockets.
|
|
One such property is the scheme used to name sockets. For
|
|
example, in the UNIX communication domain sockets are
|
|
named with UNIX path names; e.g. a
|
|
socket may be named \*(lq/dev/foo\*(rq. Sockets normally
|
|
exchange data only with
|
|
sockets in the same domain (it may be possible to cross domain
|
|
boundaries, but only if some translation process is
|
|
performed). The
|
|
4.3BSD IPC facilities support three separate communication domains:
|
|
the UNIX domain, for on-system communication;
|
|
the Internet domain, which is used by
|
|
processes which communicate
|
|
using the the DARPA standard communication protocols;
|
|
and the NS domain, which is used by processes which
|
|
communicate using the Xerox standard communication
|
|
protocols*.
|
|
.FS
|
|
* See \fIInternet Transport Protocols\fP, Xerox System Integration
|
|
Standard (XSIS)028112 for more information. This document is
|
|
almost a necessity for one trying to write NS applications.
|
|
.FE
|
|
The underlying communication
|
|
facilities provided by these domains have a significant influence
|
|
on the internal system implementation as well as the interface to
|
|
socket facilities available to a user. An example of the
|
|
latter is that a socket \*(lqoperating\*(rq in the UNIX domain
|
|
sees a subset of the error conditions which are possible
|
|
when operating in the Internet (or NS) domain.
|
|
.NH 2
|
|
Socket types
|
|
.PP
|
|
Sockets are
|
|
typed according to the communication properties visible to a
|
|
user.
|
|
Processes are presumed to communicate only between sockets of
|
|
the same type, although there is
|
|
nothing that prevents communication between sockets of different
|
|
types should the underlying communication
|
|
protocols support this.
|
|
.PP
|
|
Four types of sockets currently are available to a user.
|
|
A \fIstream\fP socket provides for the bidirectional, reliable,
|
|
sequenced, and unduplicated flow of data without record boundaries.
|
|
Aside from the bidirectionality of data flow, a pair of connected
|
|
stream sockets provides an interface nearly identical to that of pipes\(dg.
|
|
.FS
|
|
\(dg In the UNIX domain, in fact, the semantics are identical and,
|
|
as one might expect, pipes have been implemented internally
|
|
as simply a pair of connected stream sockets.
|
|
.FE
|
|
.PP
|
|
A \fIdatagram\fP socket supports bidirectional flow of data which
|
|
is not promised to be sequenced, reliable, or unduplicated.
|
|
That is, a process
|
|
receiving messages on a datagram socket may find messages duplicated,
|
|
and, possibly,
|
|
in an order different from the order in which it was sent.
|
|
An important characteristic of a datagram
|
|
socket is that record boundaries in data are preserved. Datagram
|
|
sockets closely model the facilities found in many contemporary
|
|
packet switched networks such as the Ethernet.
|
|
.PP
|
|
A \fIraw\fP socket provides users access to
|
|
the underlying communication
|
|
protocols which support socket abstractions.
|
|
These sockets are normally datagram oriented, though their
|
|
exact characteristics are dependent on the interface provided by
|
|
the protocol. Raw sockets are not intended for the general user; they
|
|
have been provided mainly for those interested in developing new
|
|
communication protocols, or for gaining access to some of the more
|
|
esoteric facilities of an existing protocol. The use of raw sockets
|
|
is considered in section 5.
|
|
.PP
|
|
A \fIsequenced packet\fP socket is similar to a stream socket,
|
|
with the exception that record boundaries are preserved. This
|
|
interface is provided only as part of the NS socket abstraction,
|
|
and is very important in most serious NS applications.
|
|
Sequenced-packet sockets allow the user to manipulate the
|
|
SPP or IDP headers on a packet or a group of packets either
|
|
by writing a prototype header along with whatever data is
|
|
to be sent, or by specifying a default header to be used with
|
|
all outgoing data, and allows the user to receive the headers
|
|
on incoming packets. The use of these options is considered in
|
|
section 5.
|
|
.PP
|
|
Another potential socket type which has interesting properties is
|
|
the \fIreliably delivered
|
|
message\fP socket.
|
|
The reliably delivered message socket has
|
|
similar properties to a datagram socket, but with
|
|
reliable delivery. There is currently no support for this
|
|
type of socket, but a reliably delivered message protocol
|
|
similar to Xerox's Packet Exchange Protocol (PEX) may be
|
|
simulated at the user level. More information on this topic
|
|
can be found in section 5.
|
|
.NH 2
|
|
Socket creation
|
|
.PP
|
|
To create a socket the \fIsocket\fP system call is used:
|
|
.DS
|
|
s = socket(domain, type, protocol);
|
|
.DE
|
|
This call requests that the system create a socket in the specified
|
|
\fIdomain\fP and of the specified \fItype\fP. A particular protocol may
|
|
also be requested. If the protocol is left unspecified (a value
|
|
of 0), the system will select an appropriate protocol from those
|
|
protocols which comprise the communication domain and which
|
|
may be used to support the requested socket type. The user is
|
|
returned a descriptor (a small integer number) which may be used
|
|
in later system calls which operate on sockets. The domain is specified as
|
|
one of the manifest constants defined in the file <\fIsys/socket.h\fP>.
|
|
For the UNIX domain the constant is AF_UNIX*; for the Internet
|
|
.FS
|
|
* The manifest constants are named AF_whatever as they indicate
|
|
the ``address format'' to use in interpreting names.
|
|
.FE
|
|
domain AF_INET; and for the NS domain, AF_NS.
|
|
The socket types are also defined in this file
|
|
and one of SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, or SOCK_SEQPACKET
|
|
must be specified.
|
|
To create a stream socket in the Internet domain the following
|
|
call might be used:
|
|
.DS
|
|
s = socket(AF_INET, SOCK_STREAM, 0);
|
|
.DE
|
|
This call would result in a stream socket being created with the TCP
|
|
protocol providing the underlying communication support. To
|
|
create a datagram socket for on-machine use the call might
|
|
be:
|
|
.DS
|
|
s = socket(AF_UNIX, SOCK_DGRAM, 0);
|
|
.DE
|
|
.PP
|
|
The default protocol (used when the \fIprotocol\fP argument to the
|
|
\fIsocket\fP call is 0) should be correct for most every
|
|
situation. However, it is possible to specify a protocol
|
|
other than the default; this will be covered in
|
|
section 5.
|
|
.PP
|
|
There are several reasons a socket call may fail. Aside from
|
|
the rare occurrence of lack of memory (ENOBUFS), a socket
|
|
request may fail due to a request for an unknown protocol
|
|
(EPROTONOSUPPORT), or a request for a type of socket for
|
|
which there is no supporting protocol (EPROTOTYPE).
|
|
.NH 2
|
|
Binding local names
|
|
.PP
|
|
A socket is created without a name. Until a name is bound
|
|
to a socket, processes have no way to reference it and, consequently,
|
|
no messages may be received on it.
|
|
Communicating processes are bound
|
|
by an \fIassociation\fP. In the Internet and NS domains,
|
|
an association
|
|
is composed of local and foreign
|
|
addresses, and local and foreign ports,
|
|
while in the UNIX domain, an association is composed of
|
|
local and foreign path names (the phrase ``foreign pathname''
|
|
means a pathname created by a foreign process, not a pathname
|
|
on a foreign system).
|
|
In most domains, associations must be unique.
|
|
In the Internet domain there
|
|
may never be duplicate <protocol, local address, local port, foreign
|
|
address, foreign port> tuples. UNIX domain sockets need not always
|
|
be bound to a name, but when bound
|
|
there may never be duplicate <protocol, local pathname, foreign
|
|
pathname> tuples.
|
|
The pathnames may not refer to files
|
|
already existing on the system
|
|
in 4.3; the situation may change in future releases.
|
|
.PP
|
|
The \fIbind\fP system call allows a process to specify half of
|
|
an association, <local address, local port>
|
|
(or <local pathname>), while the \fIconnect\fP
|
|
and \fIaccept\fP primitives are used to complete a socket's association.
|
|
.PP
|
|
In the Internet domain,
|
|
binding names to sockets can be fairly complex.
|
|
Fortunately, it is usually not necessary to specifically bind an
|
|
address and port number to a socket, because the
|
|
\fIconnect\fP and \fIsend\fP calls will automatically
|
|
bind an appropriate address if they are used with an
|
|
unbound socket. The process of binding names to NS
|
|
sockets is similar in most ways to that of
|
|
binding names to Internet sockets.
|
|
.PP
|
|
The \fIbind\fP system call is used as follows:
|
|
.DS
|
|
bind(s, name, namelen);
|
|
.DE
|
|
The bound name is a variable length byte string which is interpreted
|
|
by the supporting protocol(s). Its interpretation may vary from
|
|
communication domain to communication domain (this is one of
|
|
the properties which comprise the \*(lqdomain\*(rq).
|
|
As mentioned, in the
|
|
Internet domain names contain an Internet address and port
|
|
number. NS domain names contain an NS address and
|
|
port number. In the UNIX domain, names contain a path name and
|
|
a family, which is always AF_UNIX. If one wanted to bind
|
|
the name \*(lq/tmp/foo\*(rq to a UNIX domain socket, the
|
|
following code would be used*:
|
|
.FS
|
|
* Note that, although the tendency here is to call the \*(lqaddr\*(rq
|
|
structure \*(lqsun\*(rq, doing so would cause problems if the code
|
|
were ever ported to a Sun workstation.
|
|
.FE
|
|
.DS
|
|
#include <sys/un.h>
|
|
...
|
|
struct sockaddr_un addr;
|
|
...
|
|
strcpy(addr.sun_path, "/tmp/foo");
|
|
addr.sun_family = AF_UNIX;
|
|
bind(s, (struct sockaddr *) &addr, strlen(addr.sun_path) +
|
|
sizeof (addr.sun_family));
|
|
.DE
|
|
Note that in determining the size of a UNIX domain address null
|
|
bytes are not counted, which is why \fIstrlen\fP is used. In
|
|
the current implementation of UNIX domain IPC under 4.3BSD,
|
|
the file name
|
|
referred to in \fIaddr.sun_path\fP is created as a socket
|
|
in the system file space.
|
|
The caller must, therefore, have
|
|
write permission in the directory where
|
|
\fIaddr.sun_path\fP is to reside, and this file should be deleted by the
|
|
caller when it is no longer needed. Future versions of 4BSD
|
|
may not create this file.
|
|
.PP
|
|
In binding an Internet address things become more
|
|
complicated. The actual call is similar,
|
|
.DS
|
|
#include <sys/types.h>
|
|
#include <netinet/in.h>
|
|
...
|
|
struct sockaddr_in sin;
|
|
...
|
|
bind(s, (struct sockaddr *) &sin, sizeof (sin));
|
|
.DE
|
|
but the selection of what to place in the address \fIsin\fP
|
|
requires some discussion. We will come back to the problem
|
|
of formulating Internet addresses in section 3 when
|
|
the library routines used in name resolution are discussed.
|
|
.PP
|
|
Binding an NS address to a socket is even more
|
|
difficult,
|
|
especially since the Internet library routines do not
|
|
work with NS hostnames. The actual call is again similar:
|
|
.DS
|
|
#include <sys/types.h>
|
|
#include <netns/ns.h>
|
|
...
|
|
struct sockaddr_ns sns;
|
|
...
|
|
bind(s, (struct sockaddr *) &sns, sizeof (sns));
|
|
.DE
|
|
Again, discussion of what to place in a \*(lqstruct sockaddr_ns\*(rq
|
|
will be deferred to section 3.
|
|
.NH 2
|
|
Connection establishment
|
|
.PP
|
|
Connection establishment is usually asymmetric,
|
|
with one process a \*(lqclient\*(rq and the other a \*(lqserver\*(rq.
|
|
The server, when willing to offer its advertised services,
|
|
binds a socket to a well-known address associated with the service
|
|
and then passively \*(lqlistens\*(rq on its socket.
|
|
It is then possible for an unrelated process to rendezvous
|
|
with the server.
|
|
The client requests services from the server by initiating a
|
|
\*(lqconnection\*(rq to the server's socket.
|
|
On the client side the \fIconnect\fP call is
|
|
used to initiate a connection. Using the UNIX domain, this
|
|
might appear as,
|
|
.DS
|
|
struct sockaddr_un server;
|
|
...
|
|
connect(s, (struct sockaddr *)&server, strlen(server.sun_path) +
|
|
sizeof (server.sun_family));
|
|
.DE
|
|
while in the Internet domain,
|
|
.DS
|
|
struct sockaddr_in server;
|
|
...
|
|
connect(s, (struct sockaddr *)&server, sizeof (server));
|
|
.DE
|
|
and in the NS domain,
|
|
.DS
|
|
struct sockaddr_ns server;
|
|
...
|
|
connect(s, (struct sockaddr *)&server, sizeof (server));
|
|
.DE
|
|
where \fIserver\fP in the example above would contain either the UNIX
|
|
pathname, Internet address and port number, or NS address and
|
|
port number of the server to which the
|
|
client process wishes to speak.
|
|
If the client process's socket is unbound at the time of
|
|
the connect call,
|
|
the system will automatically select and bind a name to
|
|
the socket if necessary; c.f. section 5.4.
|
|
This is the usual way that local addresses are bound
|
|
to a socket.
|
|
.PP
|
|
An error is returned if the connection was unsuccessful
|
|
(any name automatically bound by the system, however, remains).
|
|
Otherwise, the socket is associated with the server and
|
|
data transfer may begin. Some of the more common errors returned
|
|
when a connection attempt fails are:
|
|
.IP ETIMEDOUT
|
|
.br
|
|
After failing to establish a connection for a period of time,
|
|
the system decided there was no point in retrying the
|
|
connection attempt any more. This usually occurs because
|
|
the destination host is down, or because problems in
|
|
the network resulted in transmissions being lost.
|
|
.IP ECONNREFUSED
|
|
.br
|
|
The host refused service for some reason.
|
|
This is usually
|
|
due to a server process
|
|
not being present at the requested name.
|
|
.IP "ENETDOWN or EHOSTDOWN"
|
|
.br
|
|
These operational errors are
|
|
returned based on status information delivered to
|
|
the client host by the underlying communication services.
|
|
.IP "ENETUNREACH or EHOSTUNREACH"
|
|
.br
|
|
These operational errors can occur either because the network
|
|
or host is unknown (no route to the network or host is present),
|
|
or because of status information returned by intermediate
|
|
gateways or switching nodes. Many times the status returned
|
|
is not sufficient to distinguish a network being down from a
|
|
host being down, in which case the system
|
|
indicates the entire network is unreachable.
|
|
.PP
|
|
For the server to receive a client's connection it must perform
|
|
two steps after binding its socket.
|
|
The first is to indicate a willingness to listen for
|
|
incoming connection requests:
|
|
.DS
|
|
listen(s, 5);
|
|
.DE
|
|
The second parameter to the \fIlisten\fP call specifies the maximum
|
|
number of outstanding connections which may be queued awaiting
|
|
acceptance by the server process; this number
|
|
may be limited by the system. Should a connection be
|
|
requested while the queue is full, the connection will not be
|
|
refused, but rather the individual messages which comprise the
|
|
request will be ignored. This gives a harried server time to
|
|
make room in its pending connection queue while the client
|
|
retries the connection request. Had the connection been returned
|
|
with the ECONNREFUSED error, the client would be unable to tell
|
|
if the server was up or not. As it is now it is still possible
|
|
to get the ETIMEDOUT error back, though this is unlikely. The
|
|
backlog figure supplied with the listen call is currently limited
|
|
by the system to a maximum of 5 pending connections on any
|
|
one queue. This avoids the problem of processes hogging system
|
|
resources by setting an infinite backlog, then ignoring
|
|
all connection requests.
|
|
.PP
|
|
With a socket marked as listening, a server may \fIaccept\fP
|
|
a connection:
|
|
.DS
|
|
struct sockaddr_in from;
|
|
...
|
|
fromlen = sizeof (from);
|
|
newsock = accept(s, (struct sockaddr *)&from, &fromlen);
|
|
.DE
|
|
(For the UNIX domain, \fIfrom\fP would be declared as a
|
|
\fIstruct sockaddr_un\fP, and for the NS domain, \fIfrom\fP
|
|
would be declared as a \fIstruct sockaddr_ns\fP,
|
|
but nothing different would need
|
|
to be done as far as \fIfromlen\fP is concerned. In the examples
|
|
which follow, only Internet routines will be discussed.) A new
|
|
descriptor is returned on receipt of a connection (along with
|
|
a new socket). If the server wishes to find out who its client is,
|
|
it may supply a buffer for the client socket's name. The value-result
|
|
parameter \fIfromlen\fP is initialized by the server to indicate how
|
|
much space is associated with \fIfrom\fP, then modified on return
|
|
to reflect the true size of the name. If the client's name is not
|
|
of interest, the second parameter may be a null pointer.
|
|
.PP
|
|
\fIAccept\fP normally blocks. That is, \fIaccept\fP
|
|
will not return until a connection is available or the system call
|
|
is interrupted by a signal to the process. Further, there is no
|
|
way for a process to indicate it will accept connections from only
|
|
a specific individual, or individuals. It is up to the user process
|
|
to consider who the connection is from and close down the connection
|
|
if it does not wish to speak to the process. If the server process
|
|
wants to accept connections on more than one socket, or wants to avoid blocking
|
|
on the accept call, there are alternatives; they will be considered
|
|
in section 5.
|
|
.NH 2
|
|
Data transfer
|
|
.PP
|
|
With a connection established, data may begin to flow. To send
|
|
and receive data there are a number of possible calls.
|
|
With the peer entity at each end of a connection
|
|
anchored, a user can send or receive a message without specifying
|
|
the peer. As one might expect, in this case, then
|
|
the normal \fIread\fP and \fIwrite\fP system calls are usable,
|
|
.DS
|
|
write(s, buf, sizeof (buf));
|
|
read(s, buf, sizeof (buf));
|
|
.DE
|
|
In addition to \fIread\fP and \fIwrite\fP,
|
|
the new calls \fIsend\fP and \fIrecv\fP
|
|
may be used:
|
|
.DS
|
|
send(s, buf, sizeof (buf), flags);
|
|
recv(s, buf, sizeof (buf), flags);
|
|
.DE
|
|
While \fIsend\fP and \fIrecv\fP are virtually identical to
|
|
\fIread\fP and \fIwrite\fP,
|
|
the extra \fIflags\fP argument is important. The flags,
|
|
defined in \fI<sys/socket.h>\fP, may be
|
|
specified as a non-zero value if one or more
|
|
of the following is required:
|
|
.DS
|
|
.TS
|
|
l l.
|
|
MSG_OOB send/receive out of band data
|
|
MSG_PEEK look at data without reading
|
|
MSG_DONTROUTE send data without routing packets
|
|
.TE
|
|
.DE
|
|
Out of band data is a notion specific to stream sockets, and one
|
|
which we will not immediately consider. The option to have data
|
|
sent without routing applied to the outgoing packets is currently
|
|
used only by the routing table management process, and is
|
|
unlikely to be of interest to the casual user. The ability
|
|
to preview data is, however, of interest. When MSG_PEEK
|
|
is specified with a \fIrecv\fP call, any data present is returned
|
|
to the user, but treated as still \*(lqunread\*(rq. That
|
|
is, the next \fIread\fP or \fIrecv\fP call applied to the socket will
|
|
return the data previously previewed.
|
|
.NH 2
|
|
Discarding sockets
|
|
.PP
|
|
Once a socket is no longer of interest, it may be discarded
|
|
by applying a \fIclose\fP to the descriptor,
|
|
.DS
|
|
close(s);
|
|
.DE
|
|
If data is associated with a socket which promises reliable delivery
|
|
(e.g. a stream socket) when a close takes place, the system will
|
|
continue to attempt to transfer the data.
|
|
However, after a fairly long period of
|
|
time, if the data is still undelivered, it will be discarded.
|
|
Should a user have no use for any pending data, it may
|
|
perform a \fIshutdown\fP on the socket prior to closing it.
|
|
This call is of the form:
|
|
.DS
|
|
shutdown(s, how);
|
|
.DE
|
|
where \fIhow\fP is 0 if the user is no longer interested in reading
|
|
data, 1 if no more data will be sent, or 2 if no data is to
|
|
be sent or received.
|
|
.NH 2
|
|
Connectionless sockets
|
|
.PP
|
|
To this point we have been concerned mostly with sockets which
|
|
follow a connection oriented model. However, there is also
|
|
support for connectionless interactions typical of the datagram
|
|
facilities found in contemporary packet switched networks.
|
|
A datagram socket provides a symmetric interface to data
|
|
exchange. While processes are still likely to be client
|
|
and server, there is no requirement for connection establishment.
|
|
Instead, each message includes the destination address.
|
|
.PP
|
|
Datagram sockets are created as before.
|
|
If a particular local address is needed,
|
|
the \fIbind\fP operation must precede the first data transmission.
|
|
Otherwise, the system will set the local address and/or port
|
|
when data is first sent.
|
|
To send data, the \fIsendto\fP primitive is used,
|
|
.DS
|
|
sendto(s, buf, buflen, flags, (struct sockaddr *)&to, tolen);
|
|
.DE
|
|
The \fIs\fP, \fIbuf\fP, \fIbuflen\fP, and \fIflags\fP
|
|
parameters are used as before.
|
|
The \fIto\fP and \fItolen\fP
|
|
values are used to indicate the address of the intended recipient of the
|
|
message. When
|
|
using an unreliable datagram interface, it is
|
|
unlikely that any errors will be reported to the sender. When
|
|
information is present locally to recognize a message that can
|
|
not be delivered (for instance when a network is unreachable),
|
|
the call will return \-1 and the global value \fIerrno\fP will
|
|
contain an error number.
|
|
.PP
|
|
To receive messages on an unconnected datagram socket, the
|
|
\fIrecvfrom\fP primitive is provided:
|
|
.DS
|
|
recvfrom(s, buf, buflen, flags, (struct sockaddr *)&from, &fromlen);
|
|
.DE
|
|
Once again, the \fIfromlen\fP parameter is handled in
|
|
a value-result fashion, initially containing the size of
|
|
the \fIfrom\fP buffer, and modified on return to indicate
|
|
the actual size of the address from which the datagram was received.
|
|
.PP
|
|
In addition to the two calls mentioned above, datagram
|
|
sockets may also use the \fIconnect\fP call to associate
|
|
a socket with a specific destination address. In this case, any
|
|
data sent on the socket will automatically be addressed
|
|
to the connected peer, and only data received from that
|
|
peer will be delivered to the user. Only one connected
|
|
address is permitted for each socket at one time;
|
|
a second connect will change the destination address,
|
|
and a connect to a null address (family AF_UNSPEC)
|
|
will disconnect.
|
|
Connect requests on datagram sockets return immediately,
|
|
as this simply results in the system recording
|
|
the peer's address (as compared to a stream socket, where a
|
|
connect request initiates establishment of an end to end
|
|
connection). \fIAccept\fP and \fIlisten\fP are not
|
|
used with datagram sockets.
|
|
.PP
|
|
While a datagram socket socket is connected,
|
|
errors from recent \fIsend\fP calls may be returned
|
|
asynchronously.
|
|
These errors may be reported on subsequent operations
|
|
on the socket,
|
|
or a special socket option used with \fIgetsockopt\fP, SO_ERROR,
|
|
may be used to interrogate the error status.
|
|
A \fIselect\fP for reading or writing will return true
|
|
when an error indication has been received.
|
|
The next operation will return the error, and the error status is cleared.
|
|
Other of the less
|
|
important details of datagram sockets are described
|
|
in section 5.
|
|
.NH 2
|
|
Input/Output multiplexing
|
|
.PP
|
|
One last facility often used in developing applications
|
|
is the ability to multiplex i/o requests among multiple
|
|
sockets and/or files. This is done using the \fIselect\fP
|
|
call:
|
|
.DS
|
|
#include <sys/time.h>
|
|
#include <sys/types.h>
|
|
...
|
|
|
|
fd_set readmask, writemask, exceptmask;
|
|
struct timeval timeout;
|
|
...
|
|
select(nfds, &readmask, &writemask, &exceptmask, &timeout);
|
|
.DE
|
|
\fISelect\fP takes as arguments pointers to three sets, one for
|
|
the set of file descriptors for which the caller wishes to
|
|
be able to read data on, one for those descriptors to which
|
|
data is to be written, and one for which exceptional conditions
|
|
are pending; out-of-band data is the only
|
|
exceptional condition currently implemented by the socket
|
|
If the user is not interested
|
|
in certain conditions (i.e., read, write, or exceptions),
|
|
the corresponding argument to the \fIselect\fP should
|
|
be a null pointer.
|
|
.PP
|
|
Each set is actually a structure containing an array of
|
|
long integer bit masks; the size of the array is set
|
|
by the definition FD_SETSIZE.
|
|
The array is be
|
|
long enough to hold one bit for each of FD_SETSIZE file descriptors.
|
|
.PP
|
|
The macros FD_SET(\fIfd, &mask\fP) and
|
|
FD_CLR(\fIfd, &mask\fP)
|
|
have been provided for adding and removing file descriptor
|
|
\fIfd\fP in the set \fImask\fP. The
|
|
set should be zeroed before use, and
|
|
the macro FD_ZERO(\fI&mask\fP) has been provided
|
|
to clear the set \fImask\fP.
|
|
The parameter \fInfds\fP in the \fIselect\fP call specifies the range
|
|
of file descriptors (i.e. one plus the value of the largest
|
|
descriptor) to be examined in a set.
|
|
.PP
|
|
A timeout value may be specified if the selection
|
|
is not to last more than a predetermined period of time. If
|
|
the fields in \fItimeout\fP are set to 0, the selection takes
|
|
the form of a
|
|
\fIpoll\fP, returning immediately. If the last parameter is
|
|
a null pointer, the selection will block indefinitely*.
|
|
.FS
|
|
* To be more specific, a return takes place only when a
|
|
descriptor is selectable, or when a signal is received by
|
|
the caller, interrupting the system call.
|
|
.FE
|
|
\fISelect\fP normally returns the number of file descriptors selected;
|
|
if the \fIselect\fP call returns due to the timeout expiring, then
|
|
the value 0 is returned.
|
|
If the \fIselect\fP terminates because of an error or interruption,
|
|
a \-1 is returned with the error number in \fIerrno\fP,
|
|
and with the file descriptor masks unchanged.
|
|
.PP
|
|
Assuming a successful return, the three sets will
|
|
indicate which
|
|
file descriptors are ready to be read from, written to, or
|
|
have exceptional conditions pending.
|
|
The status of a file descriptor in a select mask may be
|
|
tested with the \fIFD_ISSET(fd, &mask)\fP macro, which
|
|
returns a non-zero value if \fIfd\fP is a member of the set
|
|
\fImask\fP, and 0 if it is not.
|
|
.PP
|
|
To determine if there are connections waiting
|
|
on a socket to be used with an \fIaccept\fP call,
|
|
\fIselect\fP can be used, followed by
|
|
a \fIFD_ISSET(fd, &mask)\fP macro to check for read
|
|
readiness on the appropriate socket. If \fIFD_ISSET\fP
|
|
returns a non-zero value, indicating permission to read, then a
|
|
connection is pending on the socket.
|
|
.PP
|
|
As an example, to read data from two sockets, \fIs1\fP and
|
|
\fIs2\fP as it is available from each and with a one-second
|
|
timeout, the following code
|
|
might be used:
|
|
.DS
|
|
#include <sys/time.h>
|
|
#include <sys/types.h>
|
|
...
|
|
fd_set read_template;
|
|
struct timeval wait;
|
|
...
|
|
for (;;) {
|
|
wait.tv_sec = 1; /* one second */
|
|
wait.tv_usec = 0;
|
|
|
|
FD_ZERO(&read_template);
|
|
|
|
FD_SET(s1, &read_template);
|
|
FD_SET(s2, &read_template);
|
|
|
|
nb = select(FD_SETSIZE, &read_template, (fd_set *) 0, (fd_set *) 0, &wait);
|
|
if (nb <= 0) {
|
|
\fIAn error occurred during the \fPselect\fI, or
|
|
the \fPselect\fI timed out.\fP
|
|
}
|
|
|
|
if (FD_ISSET(s1, &read_template)) {
|
|
\fISocket #1 is ready to be read from.\fP
|
|
}
|
|
|
|
if (FD_ISSET(s2, &read_template)) {
|
|
\fISocket #2 is ready to be read from.\fP
|
|
}
|
|
}
|
|
.DE
|
|
.PP
|
|
In 4.2, the arguments to \fIselect\fP were pointers to integers
|
|
instead of pointers to \fIfd_set\fPs. This type of call
|
|
will still work as long as the number of file descriptors
|
|
being examined is less than the number of bits in an
|
|
integer; however, the methods illustrated above should
|
|
be used in all current programs.
|
|
.PP
|
|
\fISelect\fP provides a synchronous multiplexing scheme.
|
|
Asynchronous notification of output completion, input availability,
|
|
and exceptional conditions is possible through use of the
|
|
SIGIO and SIGURG signals described in section 5.
|