An Advanced 4.3BSD Interprocess Communication
Tutorial
Samuel J. Leffler
Robert S. Fabry
William N. Joy
Phil Lapsley
Computer Systems Research Group
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, California 94720
Steve Miller
Chris Torek
Heterogeneous Systems Laboratory
Department of Computer Science
University of Maryland, College Park
College Park, Maryland 20742
ABSTRACT
This document provides an introduction to the
interprocess communication facilities included in
the 4.3BSD release of the UNIX* system.
It discusses the overall model for interpro-
cess communication and introduces the interprocess
communication primitives which have been added to
the system. The majority of the document consid-
ers the use of these primitives in developing
applications. The reader is expected to be famil-
iar with the C programming language as all exam-
ples are written in C.
_________________________
* UNIX is a Trademark of Bell Laboratories.
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1. INTRODUCTION
One of the most important additions to UNIX in 4.2BSD was
interprocess communication. These facilities were the
result of more than two years of discussion and research.
The facilities provided in 4.2BSD incorporated many of the
ideas from current research, while trying to maintain the
UNIX philosophy of simplicity and conciseness. The current
release of Berkeley UNIX, 4.3BSD, completes some of the IPC
facilities and provides an upward-compatible interface. It
is hoped that the interprocess communication facilities
included in 4.3BSD will establish a standard for UNIX. From
the response to the design, it appears many organizations
carrying out work with UNIX are adopting it.
UNIX has previously been very weak in the area of
interprocess communication. Prior to the 4BSD facilities,
the only standard mechanism which allowed two processes to
communicate were pipes (the mpx files which were part of
Version 7 were experimental). Unfortunately, pipes are very
restrictive in that the two communicating processes must be
related through a common ancestor. Further, the semantics
of pipes makes them almost impossible to maintain in a dis-
tributed environment.
Earlier attempts at extending the IPC facilities of
UNIX have met with mixed reaction. The majority of the
problems have been related to the fact that these facilities
have been tied to the UNIX file system, either through nam-
ing or implementation. Consequently, the IPC facilities
provided in 4.3BSD have been designed as a totally indepen-
dent subsystem. The 4.3BSD IPC allows processes to ren-
dezvous in many ways. Processes may rendezvous through a
UNIX file system-like name space (a space where all names
are path names) as well as through a network name space. In
fact, new name spaces may be added at a future time with
only minor changes visible to users. Further, the communi-
cation facilities have been extended to include more than
the simple byte stream provided by a pipe. These extensions
have resulted in a completely new part of the system which
users will need time to familiarize themselves with. It is
likely that as more use is made of these facilities they
will be refined; only time will tell.
This document provides a high-level description of the
IPC facilities in 4.3BSD and their use. It is designed to
complement the manual pages for the IPC primitives by exam-
ples of their use. The remainder of this document is orga-
nized in four sections. Section 2 introduces the IPC-
related system calls and the basic model of communication.
Section 3 describes some of the supporting library routines
users may find useful in constructing distributed applica-
tions. Section 4 is concerned with the client/server model
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used in developing applications and includes examples of the
two major types of servers. Section 5 delves into advanced
topics which sophisticated users are likely to encounter
when using the IPC facilities.
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2. BASICS
The basic building block for communication is the
socket. A socket is an endpoint of communication to which a
name may be bound. Each socket in use has a type and one or
more associated processes. Sockets exist within communica-
tion domains. A communication domain is an abstraction
introduced to bundle common properties of processes communi-
cating through sockets. One such property is the scheme
used to name sockets. For example, in the UNIX communica-
tion domain sockets are named with UNIX path names; e.g. a
socket may be named ``/dev/foo''. Sockets normally exchange
data only with sockets in the same domain (it may be possi-
ble 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 commu-
nication protocols*. The underlying communication facili-
ties provided by these domains have a significant influence
on the internal system implementation as well as the inter-
face to socket facilities available to a user. An example
of the latter is that a socket ``operating'' in the UNIX
domain sees a subset of the error conditions which are pos-
sible when operating in the Internet (or NS) domain.
2.1. Socket types
Sockets are typed according to the communication prop-
erties visible to a user. Processes are presumed to commu-
nicate only between sockets of the same type, although there
is nothing that prevents communication between sockets of
different types should the underlying communication proto-
cols support this.
Four types of sockets currently are available to a
user. A stream socket provides for the bidirectional, reli-
able, sequenced, and unduplicated flow of data without
record boundaries. Aside from the bidirectionality of data
flow, a pair of connected stream sockets provides an inter-
face nearly identical to that of pipes.
_________________________
* See Internet Transport Protocols, Xerox System Inte-
gration Standard (XSIS)028112 for more information.
This document is almost a necessity for one trying to
write NS applications.
In the UNIX domain, in fact, the semantics are identi-
cal and, as one might expect, pipes have been imple-
mented internally as simply a pair of connected stream
sockets.
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A datagram socket supports bidirectional flow of data
which is not promised to be sequenced, reliable, or undupli-
cated. 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.
A raw 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 pro-
vided 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.
A sequenced packet socket is similar to a stream
socket, with the exception that record boundaries are pre-
served. 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.
Another potential socket type which has interesting
properties is the reliably delivered message 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.
2.2. Socket creation
To create a socket the socket system call is used:
s = socket(domain, type, protocol);
This call requests that the system create a socket in the
specified domain and of the specified type. A particular
protocol may also be requested. If the protocol is left
unspecified (a value of 0), the system will select an appro-
priate protocol from those protocols which comprise the com-
munication domain and which may be used to support the
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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
. For the UNIX domain the constant is
AF_UNIX*; for the Internet 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:
s = socket(AF_INET, SOCK_STREAM, 0);
This call would result in a stream socket being created with
the TCP protocol providing the underlying communication sup-
port. To create a datagram socket for on-machine use the
call might be:
s = socket(AF_UNIX, SOCK_DGRAM, 0);
The default protocol (used when the protocol argument
to the socket 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.
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 (EPROTO-
TYPE).
2.3. Binding local names
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. Com-
municating processes are bound by an association. 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
tuples. UNIX domain sockets need not always be
bound to a name, but when bound there may never be duplicate
_________________________
* The manifest constants are named AF_whatever as they
indicate the ``address format'' to use in interpreting
names.
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Advanced 4.3BSD IPC Tutorial PS1:8-7
tuples. The
pathnames may not refer to files already existing on the
system in 4.3; the situation may change in future releases.
The bind system call allows a process to specify half
of an association, (or ), while the connect and accept primitives are used
to complete a socket's association.
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 connect and send calls will automatically bind
an appropriate address if they are used with an unbound
socket. The process of binding names to NS sockets is simi-
lar in most ways to that of binding names to Internet sock-
ets.
The bind system call is used as follows:
bind(s, name, namelen);
The bound name is a variable length byte string which is
interpreted by the supporting protocol(s). Its interpreta-
tion may vary from communication domain to communication
domain (this is one of the properties which comprise the
``domain''). 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 ``/tmp/foo''
to a UNIX domain socket, the following code would be used*:
#include
...
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));
Note that in determining the size of a UNIX domain address
null bytes are not counted, which is why strlen is used. In
the current implementation of UNIX domain IPC under 4.3BSD,
the file name referred to in addr.sun_path is created as a
socket in the system file space. The caller must, there-
fore, have write permission in the directory where
addr.sun_path is to reside, and this file should be deleted
_________________________
* Note that, although the tendency here is to call the
``addr'' structure ``sun'', doing so would cause prob-
lems if the code were ever ported to a Sun workstation.
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by the caller when it is no longer needed. Future versions
of 4BSD may not create this file.
In binding an Internet address things become more com-
plicated. The actual call is similar,
#include
#include
...
struct sockaddr_in sin;
...
bind(s, (struct sockaddr *) &sin, sizeof (sin));
but the selection of what to place in the address sin
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.
Binding an NS address to a socket is even more diffi-
cult, especially since the Internet library routines do not
work with NS hostnames. The actual call is again similar:
#include
#include
...
struct sockaddr_ns sns;
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
Again, discussion of what to place in a ``struct sock-
addr_ns'' will be deferred to section 3.
2.4. Connection establishment
Connection establishment is usually asymmetric, with
one process a ``client'' and the other a ``server''. The
server, when willing to offer its advertised services, binds
a socket to a well-known address associated with the service
and then passively ``listens'' 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 ``connection'' to the server's socket. On the
client side the connect call is used to initiate a connec-
tion. Using the UNIX domain, this might appear as,
struct sockaddr_un server;
...
connect(s, (struct sockaddr *)&server, strlen(server.sun_path) +
sizeof (server.sun_family));
while in the Internet domain,
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struct sockaddr_in server;
...
connect(s, (struct sockaddr *)&server, sizeof (server));
and in the NS domain,
struct sockaddr_ns server;
...
connect(s, (struct sockaddr *)&server, sizeof (server));
where server 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 neces-
sary; c.f. section 5.4. This is the usual way that local
addresses are bound to a socket.
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:
ETIMEDOUT
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 prob-
lems in the network resulted in transmissions being
lost.
ECONNREFUSED
The host refused service for some reason. This is usu-
ally due to a server process not being present at the
requested name.
ENETDOWN or EHOSTDOWN
These operational errors are returned based on status
information delivered to the client host by the under-
lying communication services.
ENETUNREACH or EHOSTUNREACH
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.
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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:
listen(s, 5);
The second parameter to the listen call specifies the maxi-
mum 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 lis-
ten 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.
With a socket marked as listening, a server may accept
a connection:
struct sockaddr_in from;
...
fromlen = sizeof (from);
newsock = accept(s, (struct sockaddr *)&from, &fromlen);
(For the UNIX domain, from would be declared as a struct
sockaddr_un, and for the NS domain, from would be declared
as a struct sockaddr_ns, but nothing different would need to
be done as far as fromlen 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 fromlen is ini-
tialized by the server to indicate how much space is associ-
ated with from, 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.
Accept normally blocks. That is, accept 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
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to the process. If the server process wants to accept con-
nections on more than one socket, or wants to avoid blocking
on the accept call, there are alternatives; they will be
considered in section 5.
2.5. Data transfer
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 spec-
ifying the peer. As one might expect, in this case, then
the normal read and write system calls are usable,
write(s, buf, sizeof (buf));
read(s, buf, sizeof (buf));
In addition to read and write, the new calls send and recv
may be used:
send(s, buf, sizeof (buf), flags);
recv(s, buf, sizeof (buf), flags);
While send and recv are virtually identical to read and
write, the extra flags argument is important. The flags,
defined in , may be specified as a non-zero
value if one or more of the following is required:
MSG_OOB send/receive out of band data
MSG_PEEK look at data without reading
MSG_DONTROUTE send data without routing packets
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 pack-
ets 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 recv call, any data pre-
sent is returned to the user, but treated as still
``unread''. That is, the next read or recv call applied to
the socket will return the data previously previewed.
2.6. Discarding sockets
Once a socket is no longer of interest, it may be dis-
carded by applying a close to the descriptor,
close(s);
If data is associated with a socket which promises reliable
delivery (e.g. a stream socket) when a close takes place,
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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 shutdown on
the socket prior to closing it. This call is of the form:
shutdown(s, how);
where how is 0 if the user is no longer interested in read-
ing data, 1 if no more data will be sent, or 2 if no data is
to be sent or received.
2.7. Connectionless sockets
To this point we have been concerned mostly with sock-
ets which follow a connection oriented model. However,
there is also support for connectionless interactions typi-
cal 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.
Datagram sockets are created as before. If a particu-
lar local address is needed, the bind 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 sendto primitive is used,
sendto(s, buf, buflen, flags, (struct sockaddr *)&to, tolen);
The s, buf, buflen, and flags parameters are used as before.
The to and tolen values are used to indicate the address of
the intended recipient of the message. When using an unre-
liable 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 errno will contain an error
number.
To receive messages on an unconnected datagram socket,
the recvfrom primitive is provided:
recvfrom(s, buf, buflen, flags, (struct sockaddr *)&from, &fromlen);
Once again, the fromlen parameter is handled in a value-
result fashion, initially containing the size of the from
buffer, and modified on return to indicate the actual size
of the address from which the datagram was received.
In addition to the two calls mentioned above, datagram
sockets may also use the connect call to associate a socket
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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).
Accept and listen are not used with datagram sockets.
While a datagram socket socket is connected, errors
from recent send calls may be returned asynchronously.
These errors may be reported on subsequent operations on the
socket, or a special socket option used with getsockopt,
SO_ERROR, may be used to interrogate the error status. A
select 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.
2.8. Input/Output multiplexing
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 select call:
#include
#include
...
fd_set readmask, writemask, exceptmask;
struct timeval timeout;
...
select(nfds, &readmask, &writemask, &exceptmask, &timeout);
Select 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 condi-
tions 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 select should
be a null pointer.
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.
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The macros FD_SET(fd, &mask) and FD_CLR(fd, &mask) have
been provided for adding and removing file descriptor fd in
the set mask. The set should be zeroed before use, and the
macro FD_ZERO(&mask) has been provided to clear the set
mask. The parameter nfds in the select call specifies the
range of file descriptors (i.e. one plus the value of the
largest descriptor) to be examined in a set.
A timeout value may be specified if the selection is
not to last more than a predetermined period of time. If
the fields in timeout are set to 0, the selection takes the
form of a poll, returning immediately. If the last parame-
ter is a null pointer, the selection will block indefi-
nitely*. Select normally returns the number of file
descriptors selected; if the select call returns due to the
timeout expiring, then the value 0 is returned. If the
select terminates because of an error or interruption, a -1
is returned with the error number in errno, and with the
file descriptor masks unchanged.
Assuming a successful return, the three sets will indi-
cate which file descriptors are ready to be read from, writ-
ten to, or have exceptional conditions pending. The status
of a file descriptor in a select mask may be tested with the
FD_ISSET(fd, &mask) macro, which returns a non-zero value if
fd is a member of the set mask, and 0 if it is not.
To determine if there are connections waiting on a
socket to be used with an accept call, select can be used,
followed by a FD_ISSET(fd, &mask) macro to check for read
readiness on the appropriate socket. If FD_ISSET returns a
non-zero value, indicating permission to read, then a con-
nection is pending on the socket.
As an example, to read data from two sockets, s1 and s2
as it is available from each and with a one-second timeout,
the following code might be used:
_________________________
* 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.
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#include
#include
...
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) {
An error occurred during the select, or
the select timed out.
}
if (FD_ISSET(s1, &read_template)) {
Socket #1 is ready to be read from.
}
if (FD_ISSET(s2, &read_template)) {
Socket #2 is ready to be read from.
}
}
In 4.2, the arguments to select were pointers to inte-
gers instead of pointers to fd_sets. 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; how-
ever, the methods illustrated above should be used in all
current programs.
Select provides a synchronous multiplexing scheme.
Asynchronous notification of output completion, input avail-
ability, and exceptional conditions is possible through use
of the SIGIO and SIGURG signals described in section 5.
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3. NETWORK LIBRARY ROUTINES
The discussion in section 2 indicated the possible need
to locate and construct network addresses when using the
interprocess communication facilities in a distributed envi-
ronment. To aid in this task a number of routines have been
added to the standard C run-time library. In this section
we will consider the new routines provided to manipulate
network addresses. While the 4.3BSD networking facilities
support both the DARPA standard Internet protocols and the
Xerox NS protocols, most of the routines presented in this
section do not apply to the NS domain. Unless otherwise
stated, it should be assumed that the routines presented in
this section do not apply to the NS domain.
Locating a service on a remote host requires many lev-
els of mapping before client and server may communicate. A
service is assigned a name which is intended for human con-
sumption; e.g. ``the login server on host monet''. This
name, and the name of the peer host, must then be translated
into network addresses which are not necessarily suitable
for human consumption. Finally, the address must then used
in locating a physical location and route to the service.
The specifics of these three mappings are likely to vary
between network architectures. For instance, it is desir-
able for a network to not require hosts to be named in such
a way that their physical location is known by the client
host. Instead, underlying services in the network may dis-
cover the actual location of the host at the time a client
host wishes to communicate. This ability to have hosts
named in a location independent manner may induce overhead
in connection establishment, as a discovery process must
take place, but allows a host to be physically mobile with-
out requiring it to notify its clientele of its current
location.
Standard routines are provided for: mapping host names
to network addresses, network names to network numbers, pro-
tocol names to protocol numbers, and service names to port
numbers and the appropriate protocol to use in communicating
with the server process. The file must be
included when using any of these routines.
3.1. Host names
An Internet host name to address mapping is represented
by the hostent structure:
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-17
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type (e.g., AF_INET) */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses, null terminated */
};
#define h_addr h_addr_list[0] /* first address, network byte order */
The routine gethostbyname(3N) takes an Internet host name
and returns a hostent structure, while the routine gethost-
byaddr(3N) maps Internet host addresses into a hostent
structure.
The official name of the host and its public aliases
are returned by these routines, along with the address type
(family) and a null terminated list of variable length
address. This list of addresses is required because it is
possible for a host to have many addresses, all having the
same name. The h_addr definition is provided for backward
compatibility, and is defined to be the first address in the
list of addresses in the hostent structure.
The database for these calls is provided either by the
file /etc/hosts (hosts(5)), or by use of a nameserver,
named(8). Because of the differences in these databases and
their access protocols, the information returned may differ.
When using the host table version of gethostbyname, only one
address will be returned, but all listed aliases will be
included. The nameserver version may return alternate
addresses, but will not provide any aliases other than one
given as argument.
Unlike Internet names, NS names are always mapped into
host addresses by the use of a standard NS Clearinghouse
service, a distributed name and authentication server. The
algorithms for mapping NS names to addresses via a Clearing-
house are rather complicated, and the routines are not part
of the standard libraries. The user-contributed Courier
(Xerox remote procedure call protocol) compiler contains
routines to accomplish this mapping; see the documentation
and examples provided therein for more information. It is
expected that almost all software that has to communicate
using NS will need to use the facilities of the Courier com-
piler.
An NS host address is represented by the following:
June 12, 1992
PS1:8-18 Advanced 4.3BSD IPC Tutorial
union ns_host {
u_char c_host[6];
u_short s_host[3];
};
union ns_net {
u_char c_net[4];
u_short s_net[2];
};
struct ns_addr {
union ns_net x_net;
union ns_host x_host;
u_short x_port;
};
The following code fragment inserts a known NS address into
a ns_addr:
#include
#include
#include
...
u_long netnum;
struct sockaddr_ns dst;
...
bzero((char *)&dst, sizeof(dst));
/*
* There is no convenient way to assign a long
* integer to a ``union ns_net'' at present; in
* the future, something will hopefully be provided,
* but this is the portable way to go for now.
* The network number below is the one for the NS net
* that the desired host (gyre) is on.
*/
netnum = htonl(2266);
dst.sns_addr.x_net = *(union ns_net *) &netnum;
dst.sns_family = AF_NS;
/*
* host 2.7.1.0.2a.18 == "gyre:Computer Science:UofMaryland"
*/
dst.sns_addr.x_host.c_host[0] = 0x02;
dst.sns_addr.x_host.c_host[1] = 0x07;
dst.sns_addr.x_host.c_host[2] = 0x01;
dst.sns_addr.x_host.c_host[3] = 0x00;
dst.sns_addr.x_host.c_host[4] = 0x2a;
dst.sns_addr.x_host.c_host[5] = 0x18;
dst.sns_addr.x_port = htons(75);
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-19
3.2. Network names
As for host names, routines for mapping network names
to numbers, and back, are provided. These routines return a
netent structure:
/*
* Assumption here is that a network number
* fits in 32 bits -- probably a poor one.
*/
struct netent {
char *n_name; /* official name of net */
char **n_aliases; /* alias list */
int n_addrtype; /* net address type */
int n_net; /* network number, host byte order */
};
The routines getnetbyname(3N), getnetbynumber(3N), and get-
netent(3N) are the network counterparts to the host routines
described above. The routines extract their information
from /etc/networks.
NS network numbers are determined either by asking your
local Xerox Network Administrator (and hardcoding the infor-
mation into your code), or by querying the Clearinghouse for
addresses. The internetwork router is the only process that
needs to manipulate network numbers on a regular basis; if a
process wishes to communicate with a machine, it should ask
the Clearinghouse for that machine's address (which will
include the net number).
3.3. Protocol names
For protocols, which are defined in /etc/protocols, the
protoent structure defines the protocol-name mapping used
with the routines getprotobyname(3N), getprotobynumber(3N),
and getprotoent(3N):
struct protoent {
char *p_name; /* official protocol name */
char **p_aliases; /* alias list */
int p_proto; /* protocol number */
};
In the NS domain, protocols are indicated by the
"client type" field of a IDP header. No protocol database
exists; see section 5 for more information.
3.4. Service names
Information regarding services is a bit more compli-
cated. A service is expected to reside at a specific
``port'' and employ a particular communication protocol.
June 12, 1992
PS1:8-20 Advanced 4.3BSD IPC Tutorial
This view is consistent with the Internet domain, but incon-
sistent with other network architectures. Further, a ser-
vice may reside on multiple ports. If this occurs, the
higher level library routines will have to be bypassed or
extended. Services available are contained in the file
/etc/services. A service mapping is described by the ser-
vent structure,
struct servent {
char *s_name; /* official service name */
char **s_aliases; /* alias list */
int s_port; /* port number, network byte order */
char *s_proto; /* protocol to use */
};
The routine getservbyname(3N) maps service names to a ser-
vent structure by specifying a service name and, optionally,
a qualifying protocol. Thus the call
sp = getservbyname("telnet", (char *) 0);
returns the service specification for a telnet server using
any protocol, while the call
sp = getservbyname("telnet", "tcp");
returns only that telnet server which uses the TCP protocol.
The routines getservbyport(3N) and getservent(3N) are also
provided. The getservbyport routine has an interface simi-
lar to that provided by getservbyname; an optional protocol
name may be specified to qualify lookups.
In the NS domain, services are handled by a central
dispatcher provided as part of the Courier remote procedure
call facilities. Again, the reader is referred to the
Courier compiler documentation and to the Xerox standard*
for further details.
3.5. Miscellaneous
With the support routines described above, an Internet
application program should rarely have to deal directly with
addresses. This allows services to be developed as much as
possible in a network independent fashion. It is clear,
however, that purging all network dependencies is very dif-
ficult. So long as the user is required to supply network
addresses when naming services and sockets there will always
some network dependency in a program. For example, the nor-
mal code included in client programs, such as the remote
login program, is of the form shown in Figure 1. (This
_________________________
* Courier: The Remote Procedure Call Protocol, XSIS
038112.
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-21
example will be considered in more detail in section 4.)
If we wanted to make the remote login program indepen-
dent of the Internet protocols and addressing scheme we
would be forced to add a layer of routines which masked the
network dependent aspects from the mainstream login code.
For the current facilities available in the system this does
not appear to be worthwhile.
Aside from the address-related data base routines,
there are several other routines available in the run-time
library which are of interest to users. These are intended
mostly to simplify manipulation of names and addresses.
Table 1 summarizes the routines for manipulating variable
length byte strings and handling byte swapping of network
addresses and values.
Call Synopsis
bcmp(s1, s2, n) compare byte-strings; 0 if same, not 0 otherwise
bcopy(s1, s2, n) copy n bytes from s1 to s2
bzero(base, n) zero-fill n bytes starting at base
htonl(val) convert 32-bit quantity from host to network byte order
htons(val) convert 16-bit quantity from host to network byte order
ntohl(val) convert 32-bit quantity from network to host byte order
ntohs(val) convert 16-bit quantity from network to host byte order
Table 1. C run-time routines.
The byte swapping routines are provided because the
operating system expects addresses to be supplied in network
order. On some architectures, such as the VAX, host byte
ordering is different than network byte ordering. Conse-
quently, programs are sometimes required to byte swap quan-
tities. The library routines which return network addresses
provide them in network order so that they may simply be
copied into the structures provided to the system. This
implies users should encounter the byte swapping problem
only when interpreting network addresses. For example, if
an Internet port is to be printed out the following code
would be required:
printf("port number %d\n", ntohs(sp->s_port));
On machines where unneeded these routines are defined as
null macros.
June 12, 1992
PS1:8-22 Advanced 4.3BSD IPC Tutorial
#include
#include
#include
#include
#include
...
main(argc, argv)
int argc;
char *argv[];
{
struct sockaddr_in server;
struct servent *sp;
struct hostent *hp;
int s;
...
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogin: tcp/login: unknown service\n");
exit(1);
}
hp = gethostbyname(argv[1]);
if (hp == NULL) {
fprintf(stderr, "rlogin: %s: unknown host\n", argv[1]);
exit(2);
}
bzero((char *)&server, sizeof (server));
bcopy(hp->h_addr, (char *)&server.sin_addr, hp->h_length);
server.sin_family = hp->h_addrtype;
server.sin_port = sp->s_port;
s = socket(AF_INET, SOCK_STREAM, 0);
if (s < 0) {
perror("rlogin: socket");
exit(3);
}
...
/* Connect does the bind() for us */
if (connect(s, (char *)&server, sizeof (server)) < 0) {
perror("rlogin: connect");
exit(5);
}
...
}
Figure 1. Remote login client code.
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-23
4. CLIENT/SERVER MODEL
The most commonly used paradigm in constructing dis-
tributed applications is the client/server model. In this
scheme client applications request services from a server
process. This implies an asymmetry in establishing communi-
cation between the client and server which has been examined
in section 2. In this section we will look more closely at
the interactions between client and server, and consider
some of the problems in developing client and server appli-
cations.
The client and server require a well known set of con-
ventions before service may be rendered (and accepted).
This set of conventions comprises a protocol which must be
implemented at both ends of a connection. Depending on the
situation, the protocol may be symmetric or asymmetric. In
a symmetric protocol, either side may play the master or
slave roles. In an asymmetric protocol, one side is
immutably recognized as the master, with the other as the
slave. An example of a symmetric protocol is the TELNET
protocol used in the Internet for remote terminal emulation.
An example of an asymmetric protocol is the Internet file
transfer protocol, FTP. No matter whether the specific pro-
tocol used in obtaining a service is symmetric or asymmet-
ric, when accessing a service there is a ``client process''
and a ``server process''. We will first consider the prop-
erties of server processes, then client processes.
A server process normally listens at a well known
address for service requests. That is, the server process
remains dormant until a connection is requested by a
client's connection to the server's address. At such a time
the server process ``wakes up'' and services the client,
performing whatever appropriate actions the client requests
of it.
Alternative schemes which use a service server may be
used to eliminate a flock of server processes clogging the
system while remaining dormant most of the time. For Inter-
net servers in 4.3BSD, this scheme has been implemented via
inetd, the so called ``internet super-server.'' Inetd lis-
tens at a variety of ports, determined at start-up by read-
ing a configuration file. When a connection is requested to
a port on which inetd is listening, inetd executes the
appropriate server program to handle the client. With this
method, clients are unaware that an intermediary such as
inetd has played any part in the connection. Inetd will be
described in more detail in section 5.
A similar alternative scheme is used by most Xerox ser-
vices. In general, the Courier dispatch process (if used)
June 12, 1992
PS1:8-24 Advanced 4.3BSD IPC Tutorial
accepts connections from processes requesting services of
some sort or another. The client processes request a par-
ticular
triple. If the dispatcher knows of such a program, it is
started to handle the request; if not, an error is reported
to the client. In this way, only one port is required to
service a large variety of different requests. Again, the
Courier facilities are not available without the use and
installation of the Courier compiler. The information pre-
sented in this section applies only to NS clients and ser-
vices that do not use Courier.
4.1. Servers
In 4.3BSD most servers are accessed at well known
Internet addresses or UNIX domain names. For example, the
remote login server's main loop is of the form shown in Fig-
ure 2.
The first step taken by the server is look up its ser-
vice definition:
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogind: tcp/login: unknown service\n");
exit(1);
}
The result of the getservbyname call is used in later por-
tions of the code to define the Internet port at which it
listens for service requests (indicated by a connection).
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-25
main(argc, argv)
int argc;
char *argv[];
{
int f;
struct sockaddr_in from;
struct servent *sp;
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogind: tcp/login: unknown service\n");
exit(1);
}
...
#ifndef DEBUG
/* Disassociate server from controlling terminal */
...
#endif
sin.sin_port = sp->s_port; /* Restricted port -- see section 5 */
...
f = socket(AF_INET, SOCK_STREAM, 0);
...
if (bind(f, (struct sockaddr *) &sin, sizeof (sin)) < 0) {
...
}
...
listen(f, 5);
for (;;) {
int g, len = sizeof (from);
g = accept(f, (struct sockaddr *) &from, &len);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
if (fork() == 0) {
close(f);
doit(g, &from);
}
close(g);
}
}
Figure 2. Remote login server.
June 12, 1992
PS1:8-26 Advanced 4.3BSD IPC Tutorial
Step two is to disassociate the server from the con-
trolling terminal of its invoker:
for (i = 0; i < 3; ++i)
close(i);
open("/", O_RDONLY);
dup2(0, 1);
dup2(0, 2);
i = open("/dev/tty", O_RDWR);
if (i >= 0) {
ioctl(i, TIOCNOTTY, 0);
close(i);
}
This step is important as the server will likely not want to
receive signals delivered to the process group of the con-
trolling terminal. Note, however, that once a server has
disassociated itself it can no longer send reports of errors
to a terminal, and must log errors via syslog.
Once a server has established a pristine environment,
it creates a socket and begins accepting service requests.
The bind call is required to insure the server listens at
its expected location. It should be noted that the remote
login server listens at a restricted port number, and must
therefore be run with a user-id of root. This concept of a
``restricted port number'' is 4BSD specific, and is covered
in section 5.
The main body of the loop is fairly simple:
for (;;) {
int g, len = sizeof (from);
g = accept(f, (struct sockaddr *)&from, &len);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
if (fork() == 0) { /* Child */
close(f);
doit(g, &from);
}
close(g); /* Parent */
}
An accept call blocks the server until a client requests
service. This call could return a failure status if the
call is interrupted by a signal such as SIGCHLD (to be dis-
cussed in section 5). Therefore, the return value from
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-27
accept is checked to insure a connection has actually been
established, and an error report is logged via syslog if an
error has occurred.
With a connection in hand, the server then forks a
child process and invokes the main body of the remote login
protocol processing. Note how the socket used by the parent
for queuing connection requests is closed in the child,
while the socket created as a result of the accept is closed
in the parent. The address of the client is also handed the
doit routine because it requires it in authenticating
clients.
4.2. Clients
The client side of the remote login service was shown
earlier in Figure 1. One can see the separate, asymmetric
roles of the client and server clearly in the code. The
server is a passive entity, listening for client connec-
tions, while the client process is an active entity, initi-
ating a connection when invoked.
Let us consider more closely the steps taken by the
client remote login process. As in the server process, the
first step is to locate the service definition for a remote
login:
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogin: tcp/login: unknown service\n");
exit(1);
}
Next the destination host is looked up with a gethostbyname
call:
hp = gethostbyname(argv[1]);
if (hp == NULL) {
fprintf(stderr, "rlogin: %s: unknown host\n", argv[1]);
exit(2);
}
With this accomplished, all that is required is to establish
a connection to the server at the requested host and start
up the remote login protocol. The address buffer is
cleared, then filled in with the Internet address of the
foreign host and the port number at which the login process
resides on the foreign host:
bzero((char *)&server, sizeof (server));
bcopy(hp->h_addr, (char *) &server.sin_addr, hp->h_length);
server.sin_family = hp->h_addrtype;
server.sin_port = sp->s_port;
June 12, 1992
PS1:8-28 Advanced 4.3BSD IPC Tutorial
A socket is created, and a connection initiated. Note that
connect implicitly performs a bind call, since s is unbound.
s = socket(hp->h_addrtype, SOCK_STREAM, 0);
if (s < 0) {
perror("rlogin: socket");
exit(3);
}
...
if (connect(s, (struct sockaddr *) &server, sizeof (server)) < 0) {
perror("rlogin: connect");
exit(4);
}
The details of the remote login protocol will not be consid-
ered here.
4.3. Connectionless servers
While connection-based services are the norm, some ser-
vices are based on the use of datagram sockets. One, in
particular, is the ``rwho'' service which provides users
with status information for hosts connected to a local area
network. This service, while predicated on the ability to
broadcast information to all hosts connected to a particular
network, is of interest as an example usage of datagram
sockets.
A user on any machine running the rwho server may find
out the current status of a machine with the ruptime(1) pro-
gram. The output generated is illustrated in Figure 3.
arpa up 9:45, 5 users, load 1.15, 1.39, 1.31
cad up 2+12:04, 8 users, load 4.67, 5.13, 4.59
calder up 10:10, 0 users, load 0.27, 0.15, 0.14
dali up 2+06:28, 9 users, load 1.04, 1.20, 1.65
degas up 25+09:48, 0 users, load 1.49, 1.43, 1.41
ear up 5+00:05, 0 users, load 1.51, 1.54, 1.56
ernie down 0:24
esvax down 17:04
ingres down 0:26
kim up 3+09:16, 8 users, load 2.03, 2.46, 3.11
matisse up 3+06:18, 0 users, load 0.03, 0.03, 0.05
medea up 3+09:39, 2 users, load 0.35, 0.37, 0.50
merlin down 19+15:37
miro up 1+07:20, 7 users, load 4.59, 3.28, 2.12
monet up 1+00:43, 2 users, load 0.22, 0.09, 0.07
oz down 16:09
statvax up 2+15:57, 3 users, load 1.52, 1.81, 1.86
ucbvax up 9:34, 2 users, load 6.08, 5.16, 3.28
Figure 3. ruptime output.
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-29
Status information for each host is periodically broad-
cast by rwho server processes on each machine. The same
server process also receives the status information and uses
it to update a database. This database is then interpreted
to generate the status information for each host. Servers
operate autonomously, coupled only by the local network and
its broadcast capabilities.
Note that the use of broadcast for such a task is
fairly inefficient, as all hosts must process each message,
whether or not using an rwho server. Unless such a service
is sufficiently universal and is frequently used, the
expense of periodic broadcasts outweighs the simplicity.
The rwho server, in a simplified form, is pictured in
Figure 4. There are two separate tasks performed by the
server. The first task is to act as a receiver of status
information broadcast by other hosts on the network. This
job is carried out in the main loop of the program. Packets
received at the rwho port are interrogated to insure they've
been sent by another rwho server process, then are time
stamped with their arrival time and used to update a file
indicating the status of the host. When a host has not been
heard from for an extended period of time, the database
interpretation routines assume the host is down and indicate
such on the status reports. This algorithm is prone to
error as a server may be down while a host is actually up,
but serves our current needs.
The second task performed by the server is to supply
information regarding the status of its host. This involves
periodically acquiring system status information, packaging
it up in a message and broadcasting it on the local network
for other rwho servers to hear. The supply function is
triggered by a timer and runs off a signal. Locating the
system status information is somewhat involved, but uninter-
esting. Deciding where to transmit the resultant packet is
somewhat problematical, however.
Status information must be broadcast on the local net-
work. For networks which do not support the notion of
broadcast another scheme must be used to simulate or replace
broadcasting. One possibility is to enumerate the known
neighbors (based on the status messages received from other
rwho servers). This, unfortunately, requires some boot-
strapping information, for a server will have no idea what
machines are its neighbors until it receives status messages
from them. Therefore, if all machines on a net are freshly
booted, no machine will have any known neighbors and thus
never receive, or send, any status information. This is the
identical problem faced by the routing table management pro-
cess in propagating routing status information. The stan-
dard solution, unsatisfactory as it may be, is to inform one
or more servers of known neighbors and request that they
June 12, 1992
PS1:8-30 Advanced 4.3BSD IPC Tutorial
main()
{
...
sp = getservbyname("who", "udp");
net = getnetbyname("localnet");
sin.sin_addr = inet_makeaddr(INADDR_ANY, net);
sin.sin_port = sp->s_port;
...
s = socket(AF_INET, SOCK_DGRAM, 0);
...
on = 1;
if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on)) < 0) {
syslog(LOG_ERR, "setsockopt SO_BROADCAST: %m");
exit(1);
}
bind(s, (struct sockaddr *) &sin, sizeof (sin));
...
signal(SIGALRM, onalrm);
onalrm();
for (;;) {
struct whod wd;
int cc, whod, len = sizeof (from);
cc = recvfrom(s, (char *)&wd, sizeof (struct whod), 0,
(struct sockaddr *)&from, &len);
if (cc <= 0) {
if (cc < 0 && errno != EINTR)
syslog(LOG_ERR, "rwhod: recv: %m");
continue;
}
if (from.sin_port != sp->s_port) {
syslog(LOG_ERR, "rwhod: %d: bad from port",
ntohs(from.sin_port));
continue;
}
...
if (!verify(wd.wd_hostname)) {
syslog(LOG_ERR, "rwhod: malformed host name from %x",
ntohl(from.sin_addr.s_addr));
continue;
}
(void) sprintf(path, "%s/whod.%s", RWHODIR, wd.wd_hostname);
whod = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
...
(void) time(&wd.wd_recvtime);
(void) write(whod, (char *)&wd, cc);
(void) close(whod);
}
}
Figure 4. rwho server.
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-31
always communicate with these neighbors. If each server has
at least one neighbor supplied to it, status information may
then propagate through a neighbor to hosts which are not
(possibly) directly neighbors. If the server is able to
support networks which provide a broadcast capability, as
well as those which do not, then networks with an arbitrary
topology may share status information*.
It is important that software operating in a dis-
tributed environment not have any site-dependent information
compiled into it. This would require a separate copy of the
server at each host and make maintenance a severe headache.
4.3BSD attempts to isolate host-specific information from
applications by providing system calls which return the nec-
essary information*. A mechanism exists, in the form of an
ioctl call, for finding the collection of networks to which
a host is directly connected. Further, a local network
broadcasting mechanism has been implemented at the socket
level. Combining these two features allows a process to
broadcast on any directly connected local network which sup-
ports the notion of broadcasting in a site independent man-
ner. This allows 4.3BSD to solve the problem of deciding
how to propagate status information in the case of rwho, or
more generally in broadcasting: Such status information is
broadcast to connected networks at the socket level, where
the connected networks have been obtained via the appropri-
ate ioctl calls. The specifics of such broadcastings are
complex, however, and will be covered in section 5.
_________________________
* One must, however, be concerned about ``loops''.
That is, if a host is connected to multiple networks,
it will receive status information from itself. This
can lead to an endless, wasteful, exchange of informa-
tion.
* An example of such a system call is the gethost-
name(2) call which returns the host's ``official''
name.
June 12, 1992
PS1:8-32 Advanced 4.3BSD IPC Tutorial
5. ADVANCED TOPICS
A number of facilities have yet to be discussed. For
most users of the IPC the mechanisms already described will
suffice in constructing distributed applications. However,
others will find the need to utilize some of the features
which we consider in this section.
5.1. Out of band data
The stream socket abstraction includes the notion of
``out of band'' data. Out of band data is a logically inde-
pendent transmission channel associated with each pair of
connected stream sockets. Out of band data is delivered to
the user independently of normal data. The abstraction
defines that the out of band data facilities must support
the reliable delivery of at least one out of band message at
a time. This message may contain at least one byte of data,
and at least one message may be pending delivery to the user
at any one time. For communications protocols which support
only in-band signaling (i.e. the urgent data is delivered in
sequence with the normal data), the system normally extracts
the data from the normal data stream and stores it sepa-
rately. This allows users to choose between receiving the
urgent data in order and receiving it out of sequence with-
out having to buffer all the intervening data. It is possi-
ble to ``peek'' (via MSG_PEEK) at out of band data. If the
socket has a process group, a SIGURG signal is generated
when the protocol is notified of its existence. A process
can set the process group or process id to be informed by
the SIGURG signal via the appropriate fcntl call, as
described below for SIGIO. If multiple sockets may have out
of band data awaiting delivery, a select call for excep-
tional conditions may be used to determine those sockets
with such data pending. Neither the signal nor the select
indicate the actual arrival of the out-of-band data, but
only notification that it is pending.
In addition to the information passed, a logical mark
is placed in the data stream to indicate the point at which
the out of band data was sent. The remote login and remote
shell applications use this facility to propagate signals
between client and server processes. When a signal flushs
any pending output from the remote process(es), all data up
to the mark in the data stream is discarded.
To send an out of band message the MSG_OOB flag is sup-
plied to a send or sendto calls, while to receive out of
band data MSG_OOB should be indicated when performing a
recvfrom or recv call. To find out if the read pointer is
currently pointing at the mark in the data stream, the SIO-
CATMARK ioctl is provided:
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-33
ioctl(s, SIOCATMARK, &yes);
If yes is a 1 on return, the next read will return data
after the mark. Otherwise (assuming out of band data has
arrived), the next read will provide data sent by the client
prior to transmission of the out of band signal. The rou-
tine used in the remote login process to flush output on
receipt of an interrupt or quit signal is shown in Figure 5.
It reads the normal data up to the mark (to discard it),
then reads the out-of-band byte.
#include
#include
...
oob()
{
int out = FWRITE, mark;
char waste[BUFSIZ];
/* flush local terminal output */
ioctl(1, TIOCFLUSH, (char *)&out);
for (;;) {
if (ioctl(rem, SIOCATMARK, &mark) < 0) {
perror("ioctl");
break;
}
if (mark)
break;
(void) read(rem, waste, sizeof (waste));
}
if (recv(rem, &mark, 1, MSG_OOB) < 0) {
perror("recv");
...
}
...
}
Figure 5. Flushing terminal I/O on receipt of out of band data.
A process may also read or peek at the out-of-band data
without first reading up to the mark. This is more diffi-
cult when the underlying protocol delivers the urgent data
in-band with the normal data, and only sends notification of
its presence ahead of time (e.g., the TCP protocol used to
implement streams in the Internet domain). With such proto-
cols, the out-of-band byte may not yet have arrived when a
recv is done with the MSG_OOB flag. In that case, the call
will return an error of EWOULDBLOCK. Worse, there may be
enough in-band data in the input buffer that normal flow
control prevents the peer from sending the urgent data until
the buffer is cleared. The process must then read enough of
the queued data that the urgent data may be delivered.
June 12, 1992
PS1:8-34 Advanced 4.3BSD IPC Tutorial
Certain programs that use multiple bytes of urgent data
and must handle multiple urgent signals (e.g., telnet(1C))
need to retain the position of urgent data within the
stream. This treatment is available as a socket-level
option, SO_OOBINLINE; see setsockopt(2) for usage. With
this option, the position of urgent data (the ``mark'') is
retained, but the urgent data immediately follows the mark
within the normal data stream returned without the MSG_OOB
flag. Reception of multiple urgent indications causes the
mark to move, but no out-of-band data are lost.
5.2. Non-Blocking Sockets
It is occasionally convenient to make use of sockets
which do not block; that is, I/O requests which cannot com-
plete immediately and would therefore cause the process to
be suspended awaiting completion are not executed, and an
error code is returned. Once a socket has been created via
the socket call, it may be marked as non-blocking by fcntl
as follows:
#include
...
int s;
...
s = socket(AF_INET, SOCK_STREAM, 0);
...
if (fcntl(s, F_SETFL, FNDELAY) < 0)
perror("fcntl F_SETFL, FNDELAY");
exit(1);
}
...
When performing non-blocking I/O on sockets, one must
be careful to check for the error EWOULDBLOCK (stored in the
global variable errno), which occurs when an operation would
normally block, but the socket it was performed on is marked
as non-blocking. In particular, accept, connect, send,
recv, read, and write can all return EWOULDBLOCK, and pro-
cesses should be prepared to deal with such return codes.
If an operation such as a send cannot be done in its
entirety, but partial writes are sensible (for example, when
using a stream socket), the data that can be sent immedi-
ately will be processed, and the return value will indicate
the amount actually sent.
5.3. Interrupt driven socket I/O
The SIGIO signal allows a process to be notified via a
signal when a socket (or more generally, a file descriptor)
has data waiting to be read. Use of the SIGIO facility
requires three steps: First, the process must set up a
SIGIO signal handler by use of the signal or sigvec calls.
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-35
Second, it must set the process id or process group id which
is to receive notification of pending input to its own pro-
cess id, or the process group id of its process group (note
that the default process group of a socket is group zero).
This is accomplished by use of an fcntl call. Third, it
must enable asynchronous notification of pending I/O
requests with another fcntl call. Sample code to allow a
given process to receive information on pending I/O requests
as they occur for a socket s is given in Figure 6. With the
addition of a handler for SIGURG, this code can also be used
to prepare for receipt of SIGURG signals.
#include
...
int io_handler();
...
signal(SIGIO, io_handler);
/* Set the process receiving SIGIO/SIGURG signals to us */
if (fcntl(s, F_SETOWN, getpid()) < 0) {
perror("fcntl F_SETOWN");
exit(1);
}
/* Allow receipt of asynchronous I/O signals */
if (fcntl(s, F_SETFL, FASYNC) < 0) {
perror("fcntl F_SETFL, FASYNC");
exit(1);
}
Figure 6. Use of asynchronous notification of I/O requests.
5.4. Signals and process groups
Due to the existence of the SIGURG and SIGIO signals
each socket has an associated process number, just as is
done for terminals. This value is initialized to zero, but
may be redefined at a later time with the F_SETOWN fcntl,
such as was done in the code above for SIGIO. To set the
socket's process id for signals, positive arguments should
be given to the fcntl call. To set the socket's process
group for signals, negative arguments should be passed to
fcntl. Note that the process number indicates either the
associated process id or the associated process group; it is
impossible to specify both at the same time. A similar
fcntl, F_GETOWN, is available for determining the current
process number of a socket.
Another signal which is useful when constructing server
processes is SIGCHLD. This signal is delivered to a process
when any child processes have changed state. Normally
June 12, 1992
PS1:8-36 Advanced 4.3BSD IPC Tutorial
servers use the signal to ``reap'' child processes that have
exited without explicitly awaiting their termination or
periodic polling for exit status. For example, the remote
login server loop shown in Figure 2 may be augmented as
shown in Figure 7.
int reaper();
...
signal(SIGCHLD, reaper);
listen(f, 5);
for (;;) {
int g, len = sizeof (from);
g = accept(f, (struct sockaddr *)&from, &len,);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
...
}
...
#include
reaper()
{
union wait status;
while (wait3(&status, WNOHANG, 0) > 0)
;
}
Figure 7. Use of the SIGCHLD signal.
If the parent server process fails to reap its chil-
dren, a large number of ``zombie'' processes may be created.
5.5. Pseudo terminals
Many programs will not function properly without a ter-
minal for standard input and output. Since sockets do not
provide the semantics of terminals, it is often necessary to
have a process communicating over the network do so through
a pseudo-terminal. A pseudo- terminal is actually a pair of
devices, master and slave, which allow a process to serve as
an active agent in communication between processes and
users. Data written on the slave side of a pseudo-terminal
is supplied as input to a process reading from the master
side, while data written on the master side are processed as
terminal input for the slave. In this way, the process
manipulating the master side of the pseudo-terminal has con-
trol over the information read and written on the slave side
as if it were manipulating the keyboard and reading the
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-37
screen on a real terminal. The purpose of this abstraction
is to preserve terminal semantics over a network connec-
tion-- that is, the slave side appears as a normal terminal
to any process reading from or writing to it.
For example, the remote login server uses pseudo-
terminals for remote login sessions. A user logging in to a
machine across the network is provided a shell with a slave
pseudo-terminal as standard input, output, and error. The
server process then handles the communication between the
programs invoked by the remote shell and the user's local
client process. When a user sends a character that gener-
ates an interrupt on the remote machine that flushes termi-
nal output, the pseudo-terminal generates a control message
for the server process. The server then sends an out of
band message to the client process to signal a flush of data
at the real terminal and on the intervening data buffered in
the network.
Under 4.3BSD, the name of the slave side of a pseudo-
terminal is of the form /dev/ttyxy, where x is a single let-
ter starting at `p' and continuing to `t'. y is a hexadeci-
mal digit (i.e., a single character in the range 0 through 9
or `a' through `f'). The master side of a pseudo-terminal
is /dev/ptyxy, where x and y correspond to the slave side of
the pseudo-terminal.
In general, the method of obtaining a pair of master
and slave pseudo-terminals is to find a pseudo-terminal
which is not currently in use. The master half of a pseudo-
terminal is a single-open device; thus, each master may be
opened in turn until an open succeeds. The slave side of
the pseudo-terminal is then opened, and is set to the proper
terminal modes if necessary. The process then forks; the
child closes the master side of the pseudo-terminal, and
execs the appropriate program. Meanwhile, the parent closes
the slave side of the pseudo-terminal and begins reading and
writing from the master side. Sample code making use of
pseudo-terminals is given in Figure 8; this code assumes
that a connection on a socket s exists, connected to a peer
who wants a service of some kind, and that the process has
disassociated itself from any previous controlling terminal.
5.6. Selecting specific protocols
If the third argument to the socket call is 0, socket
will select a default protocol to use with the returned
socket of the type requested. The default protocol is usu-
ally correct, and alternate choices are not usually avail-
able. However, when using ``raw'' sockets to communicate
directly with lower-level protocols or hardware interfaces,
the protocol argument may be important for setting up demul-
tiplexing. For example, raw sockets in the Internet family
may be used to implement a new protocol above IP, and the
June 12, 1992
PS1:8-38 Advanced 4.3BSD IPC Tutorial
gotpty = 0;
for (c = 'p'; !gotpty && c <= 's'; c++) {
line = "/dev/ptyXX";
line[sizeof("/dev/pty")-1] = c;
line[sizeof("/dev/ptyp")-1] = '0';
if (stat(line, &statbuf) < 0)
break;
for (i = 0; i < 16; i++) {
line[sizeof("/dev/ptyp")-1] = "0123456789abcdef"[i];
master = open(line, O_RDWR);
if (master > 0) {
gotpty = 1;
break;
}
}
}
if (!gotpty) {
syslog(LOG_ERR, "All network ports in use");
exit(1);
}
line[sizeof("/dev/")-1] = 't';
slave = open(line, O_RDWR); /* slave is now slave side */
if (slave < 0) {
syslog(LOG_ERR, "Cannot open slave pty %s", line);
exit(1);
}
ioctl(slave, TIOCGETP, &b); /* Set slave tty modes */
b.sg_flags = CRMOD|XTABS|ANYP;
ioctl(slave, TIOCSETP, &b);
i = fork();
if (i < 0) {
syslog(LOG_ERR, "fork: %m");
exit(1);
} else if (i) { /* Parent */
close(slave);
...
} else { /* Child */
(void) close(s);
(void) close(master);
dup2(slave, 0);
dup2(slave, 1);
dup2(slave, 2);
if (slave > 2)
(void) close(slave);
...
}
Figure 8. Creation and use of a pseudo terminal
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-39
socket will receive packets only for the protocol specified.
To obtain a particular protocol one determines the protocol
number as defined within the communication domain. For the
Internet domain one may use one of the library routines dis-
cussed in section 3, such as getprotobyname:
#include
#include
#include
#include
...
pp = getprotobyname("newtcp");
s = socket(AF_INET, SOCK_STREAM, pp->p_proto);
This would result in a socket s using a stream based connec-
tion, but with protocol type of ``newtcp'' instead of the
default ``tcp.''
In the NS domain, the available socket protocols are
defined in . To create a raw socket for Xerox
Error Protocol messages, one might use:
#include
#include
#include
...
s = socket(AF_NS, SOCK_RAW, NSPROTO_ERROR);
5.7. Address binding
As was mentioned in section 2, binding addresses to
sockets in the Internet and NS domains can be fairly com-
plex. As a brief reminder, these associations are composed
of local and foreign addresses, and local and foreign ports.
Port numbers are allocated out of separate spaces, one for
each system and one for each domain on that system. Through
the bind system call, a process may specify half of an asso-
ciation, the part, while the
connect and accept primitives are used to complete a
socket's association by specifying the part. Since the association is created in two
steps the association uniqueness requirement indicated pre-
viously could be violated unless care is taken. Further, it
is unrealistic to expect user programs to always know proper
values to use for the local address and local port since a
host may reside on multiple networks and the set of allo-
cated port numbers is not directly accessible to a user.
To simplify local address binding in the Internet
domain the notion of a ``wildcard'' address has been pro-
vided. When an address is specified as INADDR_ANY (a mani-
fest constant defined in ), the system inter-
prets the address as ``any valid address''. For example, to
June 12, 1992
PS1:8-40 Advanced 4.3BSD IPC Tutorial
bind a specific port number to a socket, but leave the local
address unspecified, the following code might be used:
#include
#include
...
struct sockaddr_in sin;
...
s = socket(AF_INET, SOCK_STREAM, 0);
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof (sin));
Sockets with wildcarded local addresses may receive messages
directed to the specified port number, and sent to any of
the possible addresses assigned to a host. For example, if
a host has addresses 128.32.0.4 and 10.0.0.78, and a socket
is bound as above, the process will be able to accept con-
nection requests which are addressed to 128.32.0.4 or
10.0.0.78. If a server process wished to only allow hosts
on a given network connect to it, it would bind the address
of the host on the appropriate network.
In a similar fashion, a local port may be left unspeci-
fied (specified as zero), in which case the system will
select an appropriate port number for it. This shortcut
will work both in the Internet and NS domains. For example,
to bind a specific local address to a socket, but to leave
the local port number unspecified:
hp = gethostbyname(hostname);
if (hp == NULL) {
...
}
bcopy(hp->h_addr, (char *) sin.sin_addr, hp->h_length);
sin.sin_port = htons(0);
bind(s, (struct sockaddr *) &sin, sizeof (sin));
The system selects the local port number based on two crite-
ria. The first is that on 4BSD systems, Internet ports
below IPPORT_RESERVED (1024) (for the Xerox domain, 0
through 3000) are reserved for privileged users (i.e., the
super user); Internet ports above IPPORT_USERRESERVED
(50000) are reserved for non-privileged servers. The second
is that the port number is not currently bound to some other
socket. In order to find a free Internet port number in the
privileged range the rresvport library routine may be used
as follows to return a stream socket in with a privileged
port number:
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-41
int lport = IPPORT_RESERVED - 1;
int s;
s = rresvport(&lport);
if (s < 0) {
if (errno == EAGAIN)
fprintf(stderr, "socket: all ports in use\n");
else
perror("rresvport: socket");
...
}
The restriction on allocating ports was done to allow pro-
cesses executing in a ``secure'' environment to perform
authentication based on the originating address and port
number. For example, the rlogin(1) command allows users to
log in across a network without being asked for a password,
if two conditions hold: First, the name of the system the
user is logging in from is in the file /etc/hosts.equiv on
the system he is logging in to (or the system name and the
user name are in the user's .rhosts file in the user's home
directory), and second, that the user's rlogin process is
coming from a privileged port on the machine from which he
is logging. The port number and network address of the
machine from which the user is logging in can be determined
either by the from result of the accept call, or from the
getpeername call.
In certain cases the algorithm used by the system in
selecting port numbers is unsuitable for an application.
This is because associations are created in a two step pro-
cess. For example, the Internet file transfer protocol,
FTP, specifies that data connections must always originate
from the same local port. However, duplicate associations
are avoided by connecting to different foreign ports. In
this situation the system would disallow binding the same
local address and port number to a socket if a previous data
connection's socket still existed. To override the default
port selection algorithm, an option call must be performed
prior to address binding:
...
int on = 1;
...
setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
bind(s, (struct sockaddr *) &sin, sizeof (sin));
With the above call, local addresses may be bound which are
already in use. This does not violate the uniqueness
requirement as the system still checks at connect time to be
sure any other sockets with the same local address and port
do not have the same foreign address and port. If the asso-
ciation already exists, the error EADDRINUSE is returned.
June 12, 1992
PS1:8-42 Advanced 4.3BSD IPC Tutorial
5.8. Broadcasting and determining network configuration
By using a datagram socket, it is possible to send
broadcast packets on many networks supported by the system.
The network itself must support broadcast; the system pro-
vides no simulation of broadcast in software. Broadcast
messages can place a high load on a network since they force
every host on the network to service them. Consequently,
the ability to send broadcast packets has been limited to
sockets which are explicitly marked as allowing broadcast-
ing. Broadcast is typically used for one of two reasons: it
is desired to find a resource on a local network without
prior knowledge of its address, or important functions such
as routing require that information be sent to all accessi-
ble neighbors.
To send a broadcast message, a datagram socket should
be created:
s = socket(AF_INET, SOCK_DGRAM, 0);
or
s = socket(AF_NS, SOCK_DGRAM, 0);
The socket is marked as allowing broadcasting,
int on = 1;
setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof (on));
and at least a port number should be bound to the socket:
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof (sin));
or, for the NS domain,
sns.sns_family = AF_NS;
netnum = htonl(net);
sns.sns_addr.x_net = *(union ns_net *) &netnum; /* insert net number */
sns.sns_addr.x_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sns, sizeof (sns));
The destination address of the message to be broadcast
depends on the network(s) on which the message is to be
broadcast. The Internet domain supports a shorthand nota-
tion for broadcast on the local network, the address
INADDR_BROADCAST (defined in . To determine
the list of addresses for all reachable neighbors requires
knowledge of the networks to which the host is connected.
Since this information should be obtained in a host-
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-43
independent fashion and may be impossible to derive, 4.3BSD
provides a method of retrieving this information from the
system data structures. The SIOCGIFCONF ioctl call returns
the interface configuration of a host in the form of a sin-
gle ifconf structure; this structure contains a ``data
area'' which is made up of an array of of ifreq structures,
one for each network interface to which the host is con-
nected. These structures are defined in as fol-
lows:
struct ifconf {
int ifc_len; /* size of associated buffer */
union {
caddr_t ifcu_buf;
struct ifreq *ifcu_req;
} ifc_ifcu;
};
#define ifc_buf ifc_ifcu.ifcu_buf/* buffer address */
#define ifc_req ifc_ifcu.ifcu_req/* array of structures returned */
#define IFNAMSIZ 16
struct ifreq {
char ifr_name[IFNAMSIZ]; /* if name, e.g. "en0" */
union {
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
struct sockaddr ifru_broadaddr;
short ifru_flags;
caddr_t ifru_data;
} ifr_ifru;
};
#define ifr_addr ifr_ifru.ifru_addr/* address */
#define ifr_dstaddr ifr_ifru.ifru_dstaddr/* other end of p-to-p link */
#define ifr_broadaddr ifr_ifru.ifru_broadaddr/* broadcast address */
#define ifr_flags ifr_ifru.ifru_flags/* flags */
#define ifr_data ifr_ifru.ifru_data/* for use by interface */
The actual call which obtains the interface configuration is
struct ifconf ifc;
char buf[BUFSIZ];
ifc.ifc_len = sizeof (buf);
ifc.ifc_buf = buf;
if (ioctl(s, SIOCGIFCONF, (char *) &ifc) < 0) {
...
}
After this call buf will contain one ifreq structure for
each network to which the host is connected, and ifc.ifc_len
will have been modified to reflect the number of bytes used
June 12, 1992
PS1:8-44 Advanced 4.3BSD IPC Tutorial
by the ifreq structures.
For each structure there exists a set of ``interface
flags'' which tell whether the network corresponding to that
interface is up or down, point to point or broadcast, etc.
The SIOCGIFFLAGS ioctl retrieves these flags for an inter-
face specified by an ifreq structure as follows:
struct ifreq *ifr;
ifr = ifc.ifc_req;
for (n = ifc.ifc_len / sizeof (struct ifreq); --n >= 0; ifr++) {
/*
* We must be careful that we don't use an interface
* devoted to an address family other than those intended;
* if we were interested in NS interfaces, the
* AF_INET would be AF_NS.
*/
if (ifr->ifr_addr.sa_family != AF_INET)
continue;
if (ioctl(s, SIOCGIFFLAGS, (char *) ifr) < 0) {
...
}
/*
* Skip boring cases.
*/
if ((ifr->ifr_flags & IFF_UP) == 0 ||
(ifr->ifr_flags & IFF_LOOPBACK) ||
(ifr->ifr_flags & (IFF_BROADCAST | IFF_POINTTOPOINT)) == 0)
continue;
Once the flags have been obtained, the broadcast
address must be obtained. In the case of broadcast networks
this is done via the SIOCGIFBRDADDR ioctl, while for point-
to-point networks the address of the destination host is
obtained with SIOCGIFDSTADDR.
struct sockaddr dst;
if (ifr->ifr_flags & IFF_POINTTOPOINT) {
if (ioctl(s, SIOCGIFDSTADDR, (char *) ifr) < 0) {
...
}
bcopy((char *) ifr->ifr_dstaddr, (char *) &dst, sizeof (ifr->ifr_dstaddr));
} else if (ifr->ifr_flags & IFF_BROADCAST) {
if (ioctl(s, SIOCGIFBRDADDR, (char *) ifr) < 0) {
...
}
bcopy((char *) ifr->ifr_broadaddr, (char *) &dst, sizeof (ifr->ifr_broadaddr));
}
June 12, 1992
Advanced 4.3BSD IPC Tutorial PS1:8-45
After the appropriate ioctl's have obtained the broad-
cast or destination address (now in dst), the sendto call
may be used:
sendto(s, buf, buflen, 0, (struct sockaddr *)&dst, sizeof (dst));
}
In the above loop one sendto occurs for every interface to
which the host is connected that supports the notion of
broadcast or point-to-point addressing. If a process only
wished to send broadcast messages on a given network, code
similar to that outlined above would be used, but the loop
would need to find the correct destination address.
Received broadcast messages contain the senders address
and port, as datagram sockets are bound before a message is
allowed to go out.
5.9. Socket Options
It is possible to set and get a number of options on
sockets via the setsockopt and getsockopt system calls.
These options include such things as marking a socket for
broadcasting, not to route, to linger on close, etc. The
general forms of the calls are:
setsockopt(s, level, optname, optval, optlen);
and
getsockopt(s, level, optname, optval, optlen);
The parameters to the calls are as follows: s is the
socket on which the option is to be applied. Level speci-
fies the protocol layer on which the option is to be
applied; in most cases this is the ``socket level'', indi-
cated by the symbolic constant SOL_SOCKET, defined in
. The actual option is specified in optname,
and is a symbolic constant also defined in .
Optval and Optlen point to the value of the option (in most
cases, whether the option is to be turned on or off), and
the length of the value of the option, respectively. For
getsockopt, optlen is a value-result parameter, initially
set to the size of the storage area pointed to by optval,
and modified upon return to indicate the actual amount of
storage used.
An example should help clarify things. It is sometimes
useful to determine the type (e.g., stream, datagram, etc.)
of an existing socket; programs under inetd (described
below) may need to perform this task. This can be accom-
plished as follows via the SO_TYPE socket option and the
getsockopt call:
June 12, 1992
PS1:8-46 Advanced 4.3BSD IPC Tutorial
#include
#include
int type, size;
size = sizeof (int);
if (getsockopt(s, SOL_SOCKET, SO_TYPE, (char *) &type, &size) < 0) {
...
}
After the getsockopt call, type will be set to the value of
the socket type, as defined in . If, for
example, the socket were a datagram socket, type would have
the value corresponding to SOCK_DGRAM.
5.10. NS Packet Sequences
The semantics of NS connections demand that the user
both be able to look inside the network header associated
with any incoming packet and be able to specify what should
go in certain fields of an outgoing packet. Using different
calls to setsockopt, it is possible to indicate whether pro-
totype headers will be associated by the user with each out-
going packet (SO_HEADERS_ON_OUTPUT), to indicate whether the
headers received by the system should be delivered to the
user (SO_HEADERS_ON_INPUT), or to indicate default informa-
tion that should be associated with all outgoing packets on
a given socket (SO_DEFAULT_HEADERS).
The contents of a SPP header (minus the IDP header)
are:
struct sphdr {
u_char sp_cc; /* connection control */
#define SP_SP 0x80 /* system packet */
#define SP_SA 0x40 /* send acknowledgement */
#define SP_OB 0x20 /* attention (out of band data) */
#define SP_EM 0x10 /* end of message */
u_char sp_dt; /* datastream type */
u_short sp_sid; /* source connection identifier */
u_short sp_did; /* destination connection identifier */
u_short sp_seq; /* sequence number */
u_short sp_ack; /* acknowledge number */
u_short sp_alo; /* allocation number */
};
Here, the items of interest are the datastream type and the
connection control fields. The semantics of the datastream
type are defined by the application(s) in question; the
value of this field is, by default, zero, but it can be used
to indicate things such as Xerox's Bulk Data Transfer Proto-
col (in which case it is set to one). The connection
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Advanced 4.3BSD IPC Tutorial PS1:8-47
control field is a mask of the flags defined just below it.
The user may set or clear the end-of-message bit to indicate
that a given message is the last of a given substream type,
or may set/clear the attention bit as an alternate way to
indicate that a packet should be sent out-of-band. As an
example, to associate prototype headers with outgoing SPP
packets, consider:
#include
#include
#include
#include
...
struct sockaddr_ns sns, to;
int s, on = 1;
struct databuf {
struct sphdr proto_spp; /* prototype header */
char buf[534]; /* max. possible data by Xerox std. */
} buf;
...
s = socket(AF_NS, SOCK_SEQPACKET, 0);
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_OUTPUT, &on, sizeof(on));
...
buf.proto_spp.sp_dt = 1; /* bulk data */
buf.proto_spp.sp_cc = SP_EM; /* end-of-message */
strcpy(buf.buf, "hello world\n");
sendto(s, (char *) &buf, sizeof(struct sphdr) + strlen("hello world\n"),
(struct sockaddr *) &to, sizeof(to));
...
Note that one must be careful when writing headers; if the
prototype header is not written with the data with which it
is to be associated, the kernel will treat the first few
bytes of the data as the header, with unpredictable results.
To turn off the above association, and to indicate that
packet headers received by the system should be passed up to
the user, one might use:
#include
#include
#include
#include
...
struct sockaddr sns;
int s, on = 1, off = 0;
...
s = socket(AF_NS, SOCK_SEQPACKET, 0);
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_OUTPUT, &off, sizeof(off));
setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_INPUT, &on, sizeof(on));
...
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Output is handled somewhat differently in the IDP
world. The header of an IDP-level packet looks like:
struct idp {
u_short idp_sum; /* Checksum */
u_short idp_len; /* Length, in bytes, including header */
u_char idp_tc; /* Transport Control (i.e., hop count) */
u_char idp_pt; /* Packet Type (i.e., level 2 protocol) */
struct ns_addr idp_dna; /* Destination Network Address */
struct ns_addr idp_sna; /* Source Network Address */
};
The primary field of interest in an IDP header is the packet
type field. The standard values for this field are (as
defined in ):
#define NSPROTO_RI 1 /* Routing Information */
#define NSPROTO_ECHO 2 /* Echo Protocol */
#define NSPROTO_ERROR 3 /* Error Protocol */
#define NSPROTO_PE 4 /* Packet Exchange */
#define NSPROTO_SPP 5 /* Sequenced Packet */
For SPP connections, the contents of this field are automat-
ically set to NSPROTO_SPP; for IDP packets, this value
defaults to zero, which means ``unknown''.
Setting the value of that field with SO_DEFAULT_HEADERS
is easy:
#include
#include
#include
#include
...
struct sockaddr sns;
struct idp proto_idp; /* prototype header */
int s, on = 1;
...
s = socket(AF_NS, SOCK_DGRAM, 0);
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
proto_idp.idp_pt = NSPROTO_PE; /* packet exchange */
setsockopt(s, NSPROTO_IDP, SO_DEFAULT_HEADERS, (char *) &proto_idp,
sizeof(proto_idp));
...
Using SO_HEADERS_ON_OUTPUT is somewhat more difficult.
When SO_HEADERS_ON_OUTPUT is turned on for an IDP socket,
the socket becomes (for all intents and purposes) a raw
socket. In this case, all the fields of the prototype
header (except the length and checksum fields, which are
computed by the kernel) must be filled in correctly in order
for the socket to send and receive data in a sensible
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Advanced 4.3BSD IPC Tutorial PS1:8-49
manner. To be more specific, the source address must be set
to that of the host sending the data; the destination
address must be set to that of the host for whom the data is
intended; the packet type must be set to whatever value is
desired; and the hopcount must be set to some reasonable
value (almost always zero). It should also be noted that
simply sending data using write will not work unless a con-
nect or sendto call is used, in spite of the fact that it is
the destination address in the prototype header that is
used, not the one given in either of those calls. For
almost all IDP applications , using SO_DEFAULT_HEADERS is
easier and more desirable than writing headers.
5.11. Three-way Handshake
The semantics of SPP connections indicates that a
three-way handshake, involving changes in the datastream
type, should -- but is not absolutely required to -- take
place before a SPP connection is closed. Almost all SPP
connections are ``well-behaved'' in this manner; when commu-
nicating with any process, it is best to assume that the
three-way handshake is required unless it is known for cer-
tain that it is not required. In a three-way close, the
closing process indicates that it wishes to close the con-
nection by sending a zero-length packet with end-of-message
set and with datastream type 254. The other side of the
connection indicates that it is OK to close by sending a
zero-length packet with end-of-message set and datastream
type 255. Finally, the closing process replies with a zero-
length packet with substream type 255; at this point, the
connection is considered closed. The following code frag-
ments are simplified examples of how one might handle this
three-way handshake at the user level; in the future, sup-
port for this type of close will probably be provided as
part of the C library or as part of the kernel. The first
code fragment below illustrates how a process might handle
three-way handshake if it sees that the process it is commu-
nicating with wants to close the connection:
June 12, 1992
PS1:8-50 Advanced 4.3BSD IPC Tutorial
#include
#include
#include
#include
...
#ifndef SPPSST_END
#define SPPSST_END 254
#define SPPSST_ENDREPLY 255
#endif
struct sphdr proto_sp;
int s;
...
read(s, buf, BUFSIZE);
if (((struct sphdr *)buf)->sp_dt == SPPSST_END) {
/*
* SPPSST_END indicates that the other side wants to
* close.
*/
proto_sp.sp_dt = SPPSST_ENDREPLY;
proto_sp.sp_cc = SP_EM;
setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
sizeof(proto_sp));
write(s, buf, 0);
/*
* Write a zero-length packet with datastream type = SPPSST_ENDREPLY
* to indicate that the close is OK with us. The packet that we
* don't see (because we don't look for it) is another packet
* from the other side of the connection, with SPPSST_ENDREPLY
* on it it, too. Once that packet is sent, the connection is
* considered closed; note that we really ought to retransmit
* the close for some time if we do not get a reply.
*/
close(s);
}
...
To indicate to another process that we would like to close
the connection, the following code would suffice:
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Advanced 4.3BSD IPC Tutorial PS1:8-51
#include
#include
#include
#include
...
#ifndef SPPSST_END
#define SPPSST_END 254
#define SPPSST_ENDREPLY 255
#endif
struct sphdr proto_sp;
int s;
...
proto_sp.sp_dt = SPPSST_END;
proto_sp.sp_cc = SP_EM;
setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
sizeof(proto_sp));
write(s, buf, 0); /* send the end request */
proto_sp.sp_dt = SPPSST_ENDREPLY;
setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
sizeof(proto_sp));
/*
* We assume (perhaps unwisely)
* that the other side will send the
* ENDREPLY, so we'll just send our final ENDREPLY
* as if we'd seen theirs already.
*/
write(s, buf, 0);
close(s);
...
5.12. Packet Exchange
The Xerox standard protocols include a protocol that is
both reliable and datagram-oriented. This protocol is known
as Packet Exchange (PEX or PE) and, like SPP, is layered on
top of IDP. PEX is important for a number of things:
Courier remote procedure calls may be expedited through the
use of PEX, and many Xerox servers are located by doing a
PEX ``BroadcastForServers'' operation. Although there is no
implementation of PEX in the kernel, it may be simulated at
the user level with some clever coding and the use of one
peculiar getsockopt. A PEX packet looks like:
/*
* The packet-exchange header shown here is not defined
* as part of any of the system include files.
*/
struct pex {
struct idp p_idp; /* idp header */
u_short ph_id[2]; /* unique transaction ID for pex */
u_short ph_client; /* client type field for pex */
};
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PS1:8-52 Advanced 4.3BSD IPC Tutorial
The ph_id field is used to hold a ``unique id'' that is used
in duplicate suppression; the ph_client field indicates the
PEX client type (similar to the packet type field in the IDP
header). PEX reliability stems from the fact that it is an
idempotent (``I send a packet to you, you send a packet to
me'') protocol. Processes on each side of the connection
may use the unique id to determine if they have seen a given
packet before (the unique id field differs on each packet
sent) so that duplicates may be detected, and to indicate
which message a given packet is in response to. If a packet
with a given unique id is sent and no response is received
in a given amount of time, the packet is retransmitted until
it is decided that no response will ever be received. To
simulate PEX, one must be able to generate unique ids --
something that is hard to do at the user level with any real
guarantee that the id is really unique. Therefore, a means
(via getsockopt) has been provided for getting unique ids
from the kernel. The following code fragment indicates how
to get a unique id:
long uniqueid;
int s, idsize = sizeof(uniqueid);
...
s = socket(AF_NS, SOCK_DGRAM, 0);
...
/* get id from the kernel -- only on IDP sockets */
getsockopt(s, NSPROTO_PE, SO_SEQNO, (char *)&uniqueid, &idsize);
...
The retransmission and duplicate suppression code required
to simulate PEX fully is left as an exercise for the reader.
5.13. Inetd
One of the daemons provided with 4.3BSD is inetd, the
so called ``internet super-server.'' Inetd is invoked at
boot time, and determines from the file /etc/inetd.conf the
servers for which it is to listen. Once this information
has been read and a pristine environment created, inetd pro-
ceeds to create one socket for each service it is to listen
for, binding the appropriate port number to each socket.
Inetd then performs a select on all these sockets for
read availability, waiting for somebody wishing a connection
to the service corresponding to that socket. Inetd then
performs an accept on the socket in question, forks, dups
the new socket to file descriptors 0 and 1 (stdin and std-
out), closes other open file descriptors, and execs the
appropriate server.
Servers making use of inetd are considerably simpli-
fied, as inetd takes care of the majority of the IPC work
required in establishing a connection. The server invoked
by inetd expects the socket connected to its client on file
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Advanced 4.3BSD IPC Tutorial PS1:8-53
descriptors 0 and 1, and may immediately perform any opera-
tions such as read, write, send, or recv. Indeed, servers
may use buffered I/O as provided by the ``stdio'' conven-
tions, as long as as they remember to use fflush when appro-
priate.
One call which may be of interest to individuals writ-
ing servers under inetd is the getpeername call, which
returns the address of the peer (process) connected on the
other end of the socket. For example, to log the Internet
address in ``dot notation'' (e.g., ``128.32.0.4'') of a
client connected to a server under inetd, the following code
might be used:
struct sockaddr_in name;
int namelen = sizeof (name);
...
if (getpeername(0, (struct sockaddr *)&name, &namelen) < 0) {
syslog(LOG_ERR, "getpeername: %m");
exit(1);
} else
syslog(LOG_INFO, "Connection from %s", inet_ntoa(name.sin_addr));
...
While the getpeername call is especially useful when writing
programs to run with inetd, it can be used under other cir-
cumstances. Be warned, however, that getpeername will fail
on UNIX domain sockets.
June 12, 1992
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