



IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                             PPSSDD::2200--11


                   AAnn IInnttrroodduuccttoorryy 44..44BBSSDD
            IInntteerrpprroocceessss CCoommmmuunniiccaattiioonn TTuuttoorriiaall


                      _S_t_u_a_r_t _S_e_c_h_r_e_s_t

              _C_o_m_p_u_t_e_r _S_c_i_e_n_c_e _R_e_s_e_a_r_c_h _G_r_o_u_p
                 _C_o_m_p_u_t_e_r _S_c_i_e_n_c_e _D_i_v_i_s_i_o_n
 _D_e_p_a_r_t_m_e_n_t _o_f _E_l_e_c_t_r_i_c_a_l _E_n_g_i_n_e_e_r_i_n_g _a_n_d _C_o_m_p_u_t_e_r _S_c_i_e_n_c_e
             _U_n_i_v_e_r_s_i_t_y _o_f _C_a_l_i_f_o_r_n_i_a_, _B_e_r_k_e_l_e_y


                          _A_B_S_T_R_A_C_T


       Berkeley  UNIX[+]  4.4BSD  offers  several  choices for
  interprocess  communication.   To  aid  the  programmer   in
  developing  programs which are comprised of cooperating pro-
  cesses, the different choices are discussed and a series  of
  example  programs are presented.  These programs demonstrate
  in a simple way the use of pipes, socketpairs,  sockets  and
  the use of datagram and stream communication.  The intent of
  this document is to present a few simple  example  programs,
  not to describe the networking system in full.



11..  GGooaallss

     Facilities  for  interprocess  communication  (IPC) and
networking were a major addition to  UNIX  in  the  Berkeley
UNIX  4.2BSD release.  These facilities required major addi-
tions and some changes to the system interface.   The  basic
idea  of  this interface is to make IPC similar to file I/O.
In UNIX a process has a set of I/O descriptors,  from  which
one reads and to which one writes.  Descriptors may refer to
normal files, to devices (including terminals), or to commu-
nication  channels.   The  use  of  a  descriptor  has three
phases: its creation, its use for reading and  writing,  and
its  destruction.   By  using  descriptors  to  write files,
rather than simply naming the target file in the write call,
one  gains  a  surprising amount of flexibility.  Often, the
program that creates a descriptor will be different from the
program that uses the descriptor.  For example the shell can
create a descriptor for the output of the `ls' command  that
will  cause the listing to appear in a file rather than on a
terminal.  Pipes are another form of  descriptor  that  have
been  used  in UNIX for some time.  Pipes allow one-way data
transmission from one process to another; the two  processes
____________________
   [+]UNIX is a trademark of AT&T Bell Laboratories.












PPSSDD::2200--22                             IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


and the pipe must be set up by a common ancestor.

     The  use  of  descriptors is not the only communication
interface provided by UNIX.  The signal  mechanism  sends  a
tiny amount of information from one process to another.  The
signaled process receives only  the  signal  type,  not  the
identity  of  the sender, and the number of possible signals
is small.  The signal semantics limit the flexibility of the
signaling  mechanism  as  a means of interprocess communica-
tion.

     The identification of IPC with I/O is quite  longstand-
ing in UNIX and has proved quite successful.  At first, how-
ever, IPC was limited to processes  communicating  within  a
single  machine.  With Berkeley UNIX 4.2BSD this expanded to
include IPC between machines.  This expansion  has  necessi-
tated  some  change in the way that descriptors are created.
Additionally, new possibilities for the meaning of read  and
write  have  been  admitted.   Originally  the  meanings, or
semantics, of these terms  were  fairly  simple.   When  you
wrote  something it was delivered.  When you read something,
you were blocked until the data arrived.   Other  possibili-
ties  exist,  however.  One can write without full assurance
of delivery if one can check later to catch occasional fail-
ures.  Messages can be kept as discrete units or merged into
a stream.  One can ask to read, but insist on not waiting if
nothing  is  immediately available.  These new possibilities
are allowed in the Berkeley UNIX IPC interface.

     Thus Berkeley UNIX 4.4BSD offers  several  choices  for
IPC.   This  paper  presents simple examples that illustrate
some of the choices.  The reader is presumed to be  familiar
with  the C programming language [Kernighan & Ritchie 1978],
but not necessarily with the system calls of the UNIX system
or with processes and interprocess communication.  The paper
reviews the notion of a process and the types of  communica-
tion  that  are supported by Berkeley UNIX 4.4BSD.  A series
of examples are presented that create processes that  commu-
nicate  with  one another.  The programs show different ways
of establishing channels  of  communication.   Finally,  the
calls  that actually transfer data are reviewed.  To clearly
present how communication can take place, the  example  pro-
grams  have been cleared of anything that might be construed
as useful work.  They can, therefore, serve  as  models  for
the  programmer  trying to construct programs which are com-
prised of cooperating processes.

22..  PPrroocceesssseess

     A _p_r_o_g_r_a_m is both a sequence of statements and a  rough
way  of  referring  to  the computation that occurs when the
compiled statements are run.  A _p_r_o_c_e_s_s can be thought of as
a  single  line  of  control  in  a  program.  Most programs










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                             PPSSDD::2200--33


execute some statements, go through a few loops,  branch  in
various  directions  and then end.  These are single process
programs.  Programs can also  have  a  point  where  control
splits into two independent lines, an action called _f_o_r_k_i_n_g_.
In UNIX these lines can never join again.   A  call  to  the
system  routine  _f_o_r_k_(_),  causes  a process to split in this
way.  The result of this call is that two  independent  pro-
cesses  will  be  running,  executing exactly the same code.
Memory values will be the same for all values set before the
fork, but, subsequently, each version will be able to change
only the value of its own copy of each variable.  Initially,
the  only  difference  between  the  two  will  be the value
returned by _f_o_r_k_(_)_.  The parent will receive  a  process  id
for  the  child,  the  child  will receive a zero.  Calls to
_f_o_r_k_(_)_, therefore, typically precede, or are included in, an
if-statement.

     A  process  views the rest of the system through a pri-
vate table of descriptors.  The  descriptors  can  represent
open  files  or  sockets  (sockets are communication objects
that will be discussed below).  Descriptors are referred  to
by  their  index  numbers  in  the  table.   The first three
descriptors are often known by special names,  _s_t_d_i_n_, _s_t_d_o_u_t
and _s_t_d_e_r_r.  These are the standard input, output and error.
When a process forks, its descriptor table is copied to  the
child.   Thus, if the parent's standard input is being taken
from a terminal (devices are also treated as files in UNIX),
the  child's  input  will  be  taken from the same terminal.
Whoever reads first will get the input.  If, before forking,
the  parent changes its standard input so that it is reading
from a new file, the child will take its input from the  new
file.   It  is  also  possible  to take input from a socket,
rather than from a file.

33..  PPiippeess

     Most users of UNIX know that they can pipe  the  output
of  a  program ``prog1'' to the input of another, ``prog2,''
by typing the command _`_`_p_r_o_g_1 _|  _p_r_o_g_2_._'_'   This  is  called
``piping''  the output of one program to another because the
mechanism used to transfer the  output  is  called  a  pipe.
When  the  user  types a command, the command is read by the
shell, which decides how to execute it.  If the  command  is
simple,  for  example, _`_`_p_r_o_g_1_,_'_' the shell forks a process,
which executes the program, prog1, and then dies.  The shell
waits  for  this  termination  and then prompts for the next
command.  If the command is a compound  command,  _`_`_p_r_o_g_1  _|
_p_r_o_g_2_,_'_'  the  shell  creates  two  processes connected by a
pipe. One process runs the program, prog1,  the  other  runs
prog2.  The pipe is an I/O mechanism with two ends, or sock-
ets.  Data that is written into one socket can be read  from
the other.











PPSSDD::2200--44                             IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


     Since  a program specifies its input and output only by
the descriptor table indices, which appear as  variables  or
constants,  the  input  source and output destination can be
changed without changing the text of the program.  It is  in
this  way  that  the  shell is able to set up pipes.  Before
executing prog1, the process can close whatever is at _s_t_d_o_u_t
and  replace  it  with  one  end  of a pipe.  Similarly, the
process that will execute prog2 can substitute the  opposite
end of the pipe for _s_t_d_i_n_.

     Let  us  now  examine a program that creates a pipe for
communication between its child and itself  (see  Figure  1;
all  figures  appear at the end of the document).  A pipe is
created by a parent  process,  which  then  forks.   When  a
process  forks, the parent's descriptor table is copied into
the child's.

     In Figure 1, the parent process makes  a  call  to  the
system  routine  _p_i_p_e_(_)_.   This  routine  creates a pipe and
places descriptors for the sockets for the two ends  of  the
pipe in the process's descriptor table.  _P_i_p_e_(_) is passed an
array into which it places the index numbers of the  sockets
it  created.   The  two ends are not equivalent.  The socket
whose index is returned in the low  word  of  the  array  is
opened for reading only, while the socket in the high end is
opened only for writing.  This corresponds to the fact  that
the  standard  input  is the first descriptor of a process's
descriptor table and the  standard  output  is  the  second.
After  creating  the pipe, the parent creates the child with
which it will share the pipe by calling  _f_o_r_k_(_)_.   Figure  2
illustrates  the  effect  of  a  fork.  The parent process's
descriptor table points to both ends of the pipe.  After the
fork,  both  parent's and child's descriptor tables point to
the pipe.  The child can then use the pipe to send a message
to the parent.

     Just  what  is  a  pipe?  It is a one-way communication
mechanism, with one end opened for reading and the other end
for  writing.   Therefore, parent and child need to agree on
which way to turn the pipe, from  parent  to  child  or  the
other  way  around.   Using  the same pipe for communication
both from parent to child and from child to parent would  be
possible  (since  both  processes  have  references  to both
ends), but very complicated.  If the parent and child are to
have  a  two-way conversation, the parent creates two pipes,
one for use in each direction.  (In  accordance  with  their
plans,  both parent and child in the example above close the
socket that they will not use.   It  is  not  required  that
unused  descriptors  be closed, but it is good practice.)  A
pipe is also a _s_t_r_e_a_m communication mechanism; that is,  all
messages sent through the pipe are placed in order and reli-
ably delivered.  When the reader asks for a  certain  number
of  bytes from this stream, he is given as many bytes as are










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                             PPSSDD::2200--55


available, up to the amount of the request. Note that  these
bytes  may  have  come from the same call to _w_r_i_t_e_(_) or from
several calls to _w_r_i_t_e_(_) which were concatenated.

44..  SSoocckkeettppaaiirrss

     Berkeley UNIX 4.4BSD provides a  slight  generalization
of pipes.  A pipe is a pair of connected sockets for one-way
stream communication.  One may obtain a  pair  of  connected
sockets for two-way stream communication by calling the rou-
tine _s_o_c_k_e_t_p_a_i_r_(_)_.  The program in Figure  3  calls  _s_o_c_k_e_t_-
_p_a_i_r_(_)  to  create  such a connection.  The program uses the
link for communication in both  directions.   Since  socket-
pairs are an extension of pipes, their use resembles that of
pipes.  Figure 4 illustrates the result of a fork  following
a call to _s_o_c_k_e_t_p_a_i_r_(_)_.

     _S_o_c_k_e_t_p_a_i_r_(_)  takes  as  arguments a specification of a
domain, a style of communication, and a protocol.  These are
the  parameters shown in the example.  Domains and protocols
will be discussed in the next section.  Briefly, a domain is
a  space  of  names that may be bound to sockets and implies
certain other conventions.  Currently, socketpairs have only
been  implemented  for  one  domain, called the UNIX domain.
The UNIX domain uses UNIX path names for naming sockets.  It
only  allows  communication  between  sockets  on  the  same
machine.

     Note  that  the   header   files   _<_s_y_s_/_s_o_c_k_e_t_._h_>   and
_<_s_y_s_/_t_y_p_e_s_._h_>_.  are required in this program.  The constants
AF_UNIX and SOCK_STREAM are defined in _<_s_y_s_/_s_o_c_k_e_t_._h_>_, which
in turn requires the file _<_s_y_s_/_t_y_p_e_s_._h_> for some of its def-
initions.

55..  DDoommaaiinnss aanndd PPrroottooccoollss

     Pipes and socketpairs are a simple solution for  commu-
nicating  between  a  parent and child or between child pro-
cesses.  What if we wanted to have processes  that  have  no
common  ancestor with whom to set up communication?  Neither
standard UNIX pipes nor socketpairs  are  the  answer  here,
since  both  mechanisms  require a common ancestor to set up
the communication.  We would like to have two processes sep-
arately  create  sockets and then have messages sent between
them.  This is often the case when providing or using a ser-
vice in the system.  This is also the case when the communi-
cating processes are on separate machines.  In Berkeley UNIX
4.4BSD  one  can  create individual sockets, give them names
and send messages between them.

     Sockets created by  different  programs  use  names  to
refer  to  one  another;  names generally must be translated
into addresses for use.  The space from which an address  is










PPSSDD::2200--66                             IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


drawn is referred to as a _d_o_m_a_i_n_.  There are several domains
for sockets.  Two that will be used in the examples here are
the  UNIX  domain  (or AF_UNIX, for Address Format UNIX) and
the Internet domain (or AF_INET).  UNIX  domain  IPC  is  an
experimental  facility  in  4.2BSD  and 4.3BSD.  In the UNIX
domain, a socket is given a path name within the file system
name  space.   A  file system node is created for the socket
and other processes may then refer to the socket  by  giving
the  proper  pathname.   UNIX domain names, therefore, allow
communication between any two processes  that  work  in  the
same file system.  The Internet domain is the UNIX implemen-
tation of the DARPA Internet standard protocols  IP/TCP/UDP.
Addresses  in  the Internet domain consist of a machine net-
work address and  an  identifying  number,  called  a  port.
Internet  domain names allow communication between machines.

     Communication follows some particular ``style.''   Cur-
rently, communication is either through a _s_t_r_e_a_m or by _d_a_t_a_-
_g_r_a_m_.  Stream communication implies several things.   Commu-
nication  takes  place across a connection between two sock-
ets.  The communication is reliable, error-free, and, as  in
pipes, no message boundaries are kept. Reading from a stream
may result in reading the data  sent  from  one  or  several
calls  to  _w_r_i_t_e_(_)  or  only  part of the data from a single
call, if there is not enough room for the entire message, or
if  not  all  the  data from a large message has been trans-
ferred.   The  protocol  implementing  such  a  style   will
retransmit  messages  received  with  errors.  It  will also
return error messages if one tries to send a  message  after
the connection has been broken.  Datagram communication does
not use connections.  Each message  is  addressed  individu-
ally.   If  the  address  is  correct,  it will generally be
received, although this is not guaranteed.  Often  datagrams
are  used  for  requests  that  require  a response from the
recipient.  If no response arrives in a reasonable amount of
time,  the  request  is  repeated.  The individual datagrams
will be kept separate when they are read, that  is,  message
boundaries are preserved.

     The difference in performance between the two styles of
communication is generally less important than  the  differ-
ence in semantics.  The performance gain that one might find
in using datagrams must be  weighed  against  the  increased
complexity  of  the  program,  which must now concern itself
with lost or out of order messages.  If  lost  messages  may
simply  be ignored, the quantity of traffic may be a consid-
eration. The expense of setting up a connection is best jus-
tified by frequent use of the connection.  Since the perfor-
mance of a protocol changes as it  is  tuned  for  different
situations,  it is best to seek the most up-to-date informa-
tion when making choices for a program in which  performance
is crucial.











IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                             PPSSDD::2200--77


     A  protocol is a set of rules, data formats and conven-
tions that regulate the transfer of  data  between  partici-
pants in the communication.  In general, there is one proto-
col for each socket type  (stream,  datagram,  etc.)  within
each  domain.   The  code  that  implements a protocol keeps
track of the names that are bound to sockets, sets  up  con-
nections  and      transfers  data  between sockets, perhaps
sending the data across a network.   This  code  also  keeps
track  of the names that are bound to sockets.  It is possi-
ble for several  protocols,  differing  only  in  low  level
details, to implement the same style of communication within
a particular domain.  Although  it  is  possible  to  select
which  protocol  should  be  used, for nearly all uses it is
sufficient to request the default protocol.  This  has  been
done in all of the example programs.

     One  specifies  the  domain,  style  and  protocol of a
socket when it is created.  For example, in  Figure  5a  the
call  to  _s_o_c_k_e_t_(_)  causes the creation of a datagram socket
with the default protocol in the UNIX domain.

66..  DDaattaaggrraammss iinn tthhee UUNNIIXX DDoommaaiinn

     Let us now look at two  programs  that  create  sockets
separately.   The programs in Figures 5a and 5b use datagram
communication rather than a stream.  The structure  used  to
name  UNIX domain sockets is defined in the file _<_s_y_s_/_u_n_._h_>_.
The definition has also been included  in  the  example  for
clarity.

     Each  program creates a socket with a call to _s_o_c_k_e_t_(_)_.
These sockets are in the UNIX domain.  Once a name has  been
decided  upon  it is attached to a socket by the system call
_b_i_n_d_(_)_.  The program in Figure 5a uses the name  ``socket'',
which  it binds to its socket.  This name will appear in the
working directory of the program.  The routines in Figure 5b
use  its socket only for sending messages.  It does not cre-
ate a name for the socket because no other  process  has  to
refer to it.

     Names  in  the  UNIX  domain are path names.  Like file
path names they may be either absolute  (e.g.  ``/dev/imagi-
nary'')  or relative (e.g. ``socket'').  Because these names
are used to allow processes  to  rendezvous,  relative  path
names  can  pose  difficulties and should be used with care.
When a name is bound into the name space, a file (inode)  is
allocated  in  the file system.  If the inode is not deallo-
cated, the name will continue to exist even after the  bound
socket  is closed.  This can cause subsequent runs of a pro-
gram to find that a  name  is  unavailable,  and  can  cause
directories  to  fill  up with these objects.  The names are
removed by calling _u_n_l_i_n_k_(_)  or  using  the  _r_m(1)  command.
Names in the UNIX domain are only used for rendezvous.  They










PPSSDD::2200--88                             IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


are not used for  message  delivery  once  a  connection  is
established.   Therefore,  in  contrast  with  the  Internet
domain, unbound sockets need not be (and are not)  automati-
cally given addresses when they are connected.

     There is no established means of communicating names to
interested parties.  In the example, the program  in  Figure
5b  gets  the  name  of the socket to which it will send its
message through its command line arguments.  Once a line  of
communication  has  been  created, one can send the names of
additional, perhaps new, sockets over the link.   Facilities
will  have  to  be  built that will make the distribution of
names less of a problem than it now is.

77..  DDaattaaggrraammss iinn tthhee IInntteerrnneett DDoommaaiinn

     The examples in Figure 6a and 6b are very close to  the
previous  example  except that the socket is in the Internet
domain.  The  structure  of  Internet  domain  addresses  is
defined  in  the  file  _<_n_e_t_i_n_e_t_/_i_n_._h_>.   Internet addresses
specify a host address (a  32-bit  number)  and  a  delivery
slot,  or port, on that machine.  These ports are managed by
the system routines that implement  a  particular  protocol.
Unlike  UNIX  domain  names,  Internet  socket names are not
entered into the file system and,  therefore,  they  do  not
have  to be unlinked after the socket has been closed.  When
a message must be sent between machines it is  sent  to  the
protocol  routine  on  the destination machine, which inter-
prets the address to determine to which socket  the  message
should  be  delivered.   Several  different protocols may be
active on the same machine, but, in general, they  will  not
communicate with one another.  As a result, different proto-
cols are allowed  to  use  the  same  port  numbers.   Thus,
implicitly, an Internet address is a triple including a pro-
tocol as well as the port and machine address.  An  _a_s_s_o_c_i_a_-
_t_i_o_n  is a temporary or permanent specification of a pair of
communicating sockets.  An association is thus identified by
the  tuple  <_p_r_o_t_o_c_o_l_,  _l_o_c_a_l  _m_a_c_h_i_n_e  _a_d_d_r_e_s_s_, _l_o_c_a_l _p_o_r_t_,
_r_e_m_o_t_e _m_a_c_h_i_n_e _a_d_d_r_e_s_s_, _r_e_m_o_t_e _p_o_r_t>.  An association may be
transient when using datagram sockets; the association actu-
ally exists during a _s_e_n_d operation.

     The protocol for a socket is chosen when the socket  is
created.   The local machine address for a socket can be any
valid network address of the machine, if it  has  more  than
one,  or it can be the wildcard value INADDR_ANY.  The wild-
card value is used in  the  program  in  Figure  6a.   If  a
machine  has  several  network  addresses, it is likely that
messages sent to any of the addresses should be  deliverable
to  a  socket.   This will be the case if the wildcard value
has been chosen.  Note that even if the  wildcard  value  is
chosen,  a program sending messages to the named socket must
specify a valid network address.   One  can  be  willing  to










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                             PPSSDD::2200--99


receive  from  ``anywhere,''  but  one cannot send a message
``anywhere.''  The program in Figure 6b is given the  desti-
nation host name as a command line argument.  To determine a
network address to which it can send the message,  it  looks
up  the  host  address  by the call to _g_e_t_h_o_s_t_b_y_n_a_m_e_(_).  The
returned structure  includes  the  host's  network  address,
which  is  copied into the structure specifying the destina-
tion of the message.

     The port number can be thought of as the  number  of  a
mailbox,  into  which  the  protocol  places one's messages.
Certain daemons, offering certain advertised services,  have
reserved  or ``well-known'' port numbers.  These fall in the
range from 1 to 1023.  Higher numbers are available to  gen-
eral  users.  Only servers need to ask for a particular num-
ber.  The system will assign an unused port number  when  an
address  is  bound  to  a  socket.   This may happen when an
explicit _b_i_n_d call is made with a port number of 0, or  when
a  _c_o_n_n_e_c_t  or _s_e_n_d is performed on an unbound socket.  Note
that port numbers are not automatically reported back to the
user.  After calling _b_i_n_d_(_)_, asking for port 0, one may call
_g_e_t_s_o_c_k_n_a_m_e_(_) to discover what port was  actually  assigned.
The  routine  _g_e_t_s_o_c_k_n_a_m_e_(_)  will  not work for names in the
UNIX domain.

     The format of the socket address is specified  in  part
by  standards within the Internet domain.  The specification
includes the order of the bytes  in  the  address.   Because
machines differ in the internal representation they ordinar-
ily use to represent integers, printing out the port  number
as  returned by _g_e_t_s_o_c_k_n_a_m_e_(_) may result in a misinterpreta-
tion.  To print out the number, it is necessary to  use  the
routine  _n_t_o_h_s_(_) (for _n_e_t_w_o_r_k _t_o _h_o_s_t_: _s_h_o_r_t) to convert the
number from the network representation to the host's  repre-
sentation.   On some machines, such as 68000-based machines,
this is a null operation.  On others, such  as  VAXes,  this
results  in  a swapping of bytes.  Another routine exists to
convert a short integer from the host format to the  network
format,  called  _h_t_o_n_s_(_);  similar  routines  exist for long
integers.  For further information, refer to the  entry  for
_b_y_t_e_o_r_d_e_r in section 3 of the manual.

88..  CCoonnnneeccttiioonnss

     To  send data between stream sockets (having communica-
tion style SOCK_STREAM),  the  sockets  must  be  connected.
Figures  7a and 7b show two programs that create such a con-
nection.  The program in 7a is relatively simple.  To initi-
ate  a  connection,  this  program  simply  creates a stream
socket, then calls _c_o_n_n_e_c_t_(_), specifying the address of  the
socket  to  which  it wishes its socket connected.  Provided
that the target socket exists and is prepared  to  handle  a
connection, connection will be complete, and the program can










PPSSDD::2200--1100                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


begin to send messages.  Messages will be delivered in order
without  message  boundaries, as with pipes.  The connection
is destroyed when either socket is closed  (or  soon  there-
after).  If a process persists in sending messages after the
connection is closed,  a  SIGPIPE  signal  is  sent  to  the
process  by the operating system.  Unless explicit action is
taken to handle the signal (see the manual page  for  _s_i_g_n_a_l
or  _s_i_g_v_e_c),  the  process will terminate and the shell will
print the message ``broken pipe.''

     Forming a connection is asymmetrical; one process, such
as  the  program  in Figure 7a, requests a connection with a
particular socket,  the  other  process  accepts  connection
requests.  Before a connection can be accepted a socket must
be created and an address bound to it.   This  situation  is
illustrated in the top half of Figure 8.  Process 2 has cre-
ated a socket and bound a port number to it.  Process 1  has
created an unnamed socket.  The address bound to process 2's
socket is then made known to process 1 and, perhaps to  sev-
eral  other  potential  communicants  as well.  If there are
several possible communicants, this one socket might receive
several requests for connections.  As a result, a new socket
is created for each connection.  This new socket is the end-
point for communication within this process for this connec-
tion.  A connection may be destroyed by closing  the  corre-
sponding socket.

     The program in Figure 7b is a rather trivial example of
a server.  It creates a socket to which  it  binds  a  name,
which  it  then advertises.  (In this case it prints out the
socket number.)  The program then calls  _l_i_s_t_e_n_(_)  for  this
socket.   Since  several clients may attempt to connect more
or less simultaneously, a queue of  pending  connections  is
maintained  in the system address space.  _L_i_s_t_e_n_(_) marks the
socket as willing to accept connections and initializes  the
queue.   When a connection is requested, it is listed in the
queue.  If the  queue  is  full,  an  error  status  may  be
returned to the requester.  The maximum length of this queue
is specified by the second argument of _l_i_s_t_e_n_(_); the maximum
length  is  limited by the system.  Once the listen call has
been completed, the program enters  an  infinite  loop.   On
each pass through the loop, a new connection is accepted and
removed from the queue, and, hence, a  new  socket  for  the
connection  is  created.   The bottom half of Figure 8 shows
the result of Process 1 connecting with the named socket  of
Process  2,  and  Process 2 accepting the connection.  After
the connection is created, the service, in this case  print-
ing out the messages, is performed and the connection socket
closed.  The _a_c_c_e_p_t_(_) call will take  a  pending  connection
request from the queue if one is available, or block waiting
for a  request.   Messages  are  read  from  the  connection
socket.  Reads from an active connection will normally block
until data is  available.   The  number  of  bytes  read  is










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--1111


returned.   When  a  connection  is destroyed, the read call
returns immediately.  The number of bytes returned  will  be
zero.

     The  program  in Figure 7c is a slight variation on the
server in Figure 7b.  It avoids blocking when there  are  no
pending connection requests by calling _s_e_l_e_c_t_(_) to check for
pending requests before calling _a_c_c_e_p_t_(_)_.  This strategy  is
useful  when  connections  may  be received on more than one
socket, or when data may arrive on other  connected  sockets
before another connection request.

     The  programs in Figures 9a and 9b show a program using
stream communication in the UNIX  domain.   Streams  in  the
UNIX  domain can be used for this sort of program in exactly
the same way as Internet domain streams, except for the form
of  the  names  and  the restriction of the connections to a
single file system.  There are some differences, however, in
the  functionality of streams in the two domains, notably in
the handling of _o_u_t_-_o_f_-_b_a_n_d data (discussed briefly  below).
These differences are beyond the scope of this paper.

99..  RReeaaddss,, WWrriitteess,, RReeccvvss,, eettcc..

     UNIX  4.4BSD  has  several system calls for reading and
writing information.  The simplest  calls  are  _r_e_a_d_(_)   and
_w_r_i_t_e_(_)_.  _W_r_i_t_e_(_) takes as arguments the index of a descrip-
tor, a pointer to a buffer containing the data and the  size
of the data.  The descriptor may indicate either a file or a
connected socket.  ``Connected'' can mean either a connected
stream  socket  (as  described  in  Section 8) or a datagram
socket for which a _c_o_n_n_e_c_t_(_) call  has  provided  a  default
destination  (see  the  _c_o_n_n_e_c_t_(_) manual page).  _R_e_a_d_(_) also
takes a descriptor that indicates either a file or a socket.
_W_r_i_t_e_(_)  requires a connected socket since no destination is
specified in the parameters of the system call.  _R_e_a_d_(_)  can
be  used  for  either  a connected or an unconnected socket.
These calls are, therefore, quite flexible and may  be  used
to  write applications that require no assumptions about the
source of their input or the destination  of  their  output.
There  are  variations on _r_e_a_d_(_)  and _w_r_i_t_e_(_) that allow the
source and destination of the input and output to  use  sev-
eral  separate  buffers,  while retaining the flexibility to
handle both  files  and  sockets.   These  are  _r_e_a_d_v_(_)  and
_w_r_i_t_e_v_(_)_, for read and write _v_e_c_t_o_r_.

     It  is  sometimes  necessary to send high priority data
over a connection that may have unread low priority data  at
the other end.  For example, a user interface process may be
interpreting commands and sending them on to another process
through  a  stream  connection.  The user interface may have
filled the stream with as yet unprocessed requests when  the
user  types  a  command  to cancel all outstanding requests.










PPSSDD::2200--1122                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


Rather than have the high priority data wait to be processed
after  the  low  priority data, it is possible to send it as
_o_u_t_-_o_f_-_b_a_n_d (OOB) data.  The  notification  of  pending  OOB
data  results  in the generation of a SIGURG signal, if this
signal has been enabled (see the manual page for  _s_i_g_n_a_l  or
_s_i_g_v_e_c).  See [Leffler 1986] for a more complete description
of the OOB mechanism.  There are a pair of calls similar  to
_r_e_a_d  and  _w_r_i_t_e  that  allow options, including sending and
receiving OOB information; these  are   _s_e_n_d_(_)  and  _r_e_c_v_(_)_.
These  calls  are  used  only  with  sockets;  specifying  a
descriptor for a file will result in the return of an  error
status.  These calls also allow _p_e_e_k_i_n_g at data in a stream.
That is, they allow a process to read data without  removing
the  data  from  the stream.  One use of this facility is to
read ahead in a stream to determine the  size  of  the  next
item  to be read.  When not using these options, these calls
have the same functions as _r_e_a_d_(_) and _w_r_i_t_e_(_)_.

     To send datagrams, one must be allowed to  specify  the
destination.   The call _s_e_n_d_t_o_(_) takes a destination address
as an argument and is therefore used for sending  datagrams.
The  call  _r_e_c_v_f_r_o_m_(_) is often used to read datagrams, since
this call returns the address of the sender, if it is avail-
able,  along  with  the data.  If the identity of the sender
does not matter, one may use _r_e_a_d_(_) or _r_e_c_v_(_)_.

     Finally, there are a pair of calls that allow the send-
ing  and  receiving  of messages from multiple buffers, when
the address of the recipient must be specified.   These  are
_s_e_n_d_m_s_g_(_)  and  _r_e_c_v_m_s_g_(_)_.   These  calls are actually quite
general and have other uses, including, in the UNIX  domain,
the  transmission  of  a file descriptor from one process to
another.

     The various options for reading and writing  are  shown
in  Figure  10, together with their parameters.  The parame-
ters for each system call reflect the differences  in  func-
tion  of the different calls.  In the examples given in this
paper, the calls _r_e_a_d_(_) and _w_r_i_t_e_(_) have been used  whenever
possible.

1100..  CChhooiicceess

     This  paper has presented examples of some of the forms
of communication supported by Berkeley UNIX  4.4BSD.   These
have been presented in an order chosen for ease of presenta-
tion.  It is useful to review these options emphasizing  the
factors that make each attractive.

     Pipes  have  the advantage of portability, in that they
are supported in all UNIX systems.  They also are relatively
simple  to  use.  Socketpairs share this simplicity and have
the   additional   advantage   of   allowing   bidirectional










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--1133


communication.  The major shortcoming of these mechanisms is
that they require communicating processes to be  descendants
of  a common process.  They do not allow intermachine commu-
nication.

     The two communication domains, UNIX and Internet, allow
processes  with  no  common ancestor to communicate.  Of the
two, only the Internet domain allows  communication  between
machines.  This makes the Internet domain a necessary choice
for processes running on separate machines.

     The choice between datagrams and  stream  communication
is  best made by carefully considering the semantic and per-
formance requirements of the application.   Streams  can  be
both  advantageous and disadvantageous.  One disadvantage is
that a process is only allowed  a  limited  number  of  open
streams,  as  there are usually only 64 entries available in
the open descriptor table.  This can  cause  problems  if  a
single  server  must  talk  with  a large number of clients.
Another is that for delivering a short  message  the  stream
setup  and teardown time can be unnecessarily long.  Weighed
against this are the reliability  built  into  the  streams.
This  will often be the deciding factor in favor of streams.

1111..  WWhhaatt ttoo ddoo NNeexxtt

     Many of the examples presented here can serve as models
for  multiprocess  programs  and  for  programs  distributed
across several machines.  In developing a  new  multiprocess
program, it is often easiest to first write the code to cre-
ate the processes and communication paths.  After this  code
is  debugged,  the  code  specific to the application can be
added.

     An introduction to  the  UNIX  system  and  programming
using  UNIX system calls can be found in [Kernighan and Pike
1984].  Further documentation of the  Berkeley  UNIX  4.4BSD
IPC  mechanisms can be found in [Leffler et al. 1986].  More
detailed information about particular calls and protocols is
provided  in  sections  2,  3 and 4 of the UNIX Programmer's
Manual [CSRG 1986].   In  particular  the  following  manual
pages are relevant:


    creating and naming sockets   socket(2), bind(2)
    establishing connections      listen(2), accept(2), connect(2)
    transferring data             read(2), write(2), send(2), recv(2)
    addresses                     inet(4F)
    protocols                     tcp(4P), udp(4P).














PPSSDD::2200--1144                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC



    AAcckknnoowwlleeddggeemmeennttss

     I  would  like to thank Sam Leffler and Mike Karels
for their help in understanding the IPC  mechanisms  and
all the people whose comments have helped in writing and
improving this report.

     This work was sponsored by the Defense Advanced Re-
search Projects Agency (DoD), ARPA Order No. 4031, moni-
tored by the Naval  Electronics  Systems  Command  under
contract  No.  N00039-C-0235.  The views and conclusions
contained in this document are those of the  author  and
should not be interpreted as representing official poli-
cies, either expressed or implied, of  the  Defense  Re-
search Projects Agency or of the US Government.



    RReeffeerreenncceess

    B.W. Kernighan & R. Pike, 1984,
    _T_h_e _U_N_I_X _P_r_o_g_r_a_m_m_i_n_g _E_n_v_i_r_o_n_m_e_n_t_.
    Englewood Cliffs, N.J.: Prentice-Hall.

    B.W. Kernighan & D.M. Ritchie, 1978,
    _T_h_e _C _P_r_o_g_r_a_m_m_i_n_g _L_a_n_g_u_a_g_e_,
    Englewood Cliffs, N.J.: Prentice-Hall.

    S.J. Leffler, R.S. Fabry, W.N. Joy, P. Lapsley, S. Miller & C. Torek, 1986,
    _A_n _A_d_v_a_n_c_e_d _4_._4_B_S_D _I_n_t_e_r_p_r_o_c_e_s_s _C_o_m_m_u_n_i_c_a_t_i_o_n _T_u_t_o_r_i_a_l_.
    Computer Systems Research Group,
    Department of Electrical Engineering and Computer Science,
    University of California, Berkeley.

    Computer Systems Research Group, 1986,
    _U_N_I_X _P_r_o_g_r_a_m_m_e_r_'_s _M_a_n_u_a_l_, _4_._4 _B_e_r_k_e_l_e_y _S_o_f_t_w_a_r_e _D_i_s_t_r_i_b_u_t_i_o_n_.
    Computer Systems Research Group,
    Department of Electrical Engineering and Computer Science,
    University of California, Berkeley.























IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--1155



#include <stdio.h>

#define DATA "Bright star, would I were steadfast as thou art . . ."

/*
 * This program creates a pipe, then forks.  The child communicates to the
 * parent over the pipe. Notice that a pipe is a one-way communications
 * device.  I can write to the output socket (sockets[1], the second socket
 * of the array returned by pipe()) and read from the input socket
 * (sockets[0]), but not vice versa.
 */

main()
{
     int sockets[2], child;

     /* Create a pipe */
     if (pipe(sockets) < 0) {
          perror("opening stream socket pair");
          exit(10);
     }

     if ((child = fork()) == -1)
          perror("fork");
     else if (child) {
          char buf[1024];

          /* This is still the parent.  It reads the child's message. */
          close(sockets[1]);
          if (read(sockets[0], buf, 1024) < 0)
               perror("reading message");
          printf("-->%s\n", buf);
          close(sockets[0]);
     } else {
          /* This is the child.  It writes a message to its parent. */
          close(sockets[0]);
          if (write(sockets[1], DATA, sizeof(DATA)) < 0)
               perror("writing message");
          close(sockets[1]);
     }
}
                  Figure 1  Use of a pipe




















PPSSDD::2200--1166                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


                   Parent



                     |
                 +---+-----+
                 +---+-----+--------------+
                 +---------+---           |
                 +---------+              |
                 +---------+              |
                 +---------+              |
                                          |
                                          |
                         +                |
                         --------------- -|
                         -----------P-I-P-E-

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


              Parent                      Child


                |                            |
                |                            |
            +---+-----+--------------++-+----+----+
            +---------+---           || +---------+
            +---------+              || +---------+
            +---------+              || +---------+
            +---------+              || +---------+
            +---------+              || +---------+
                                     ||
                    |-               ||
                    --------------- -||
                    ------------P-I-P-E --+

     Figure 2  Sharing a pipe between parent and child


























IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--1177


#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>

#define DATA1 "In Xanadu, did Kublai Khan . . ."
#define DATA2 "A stately pleasure dome decree . . ."

/*
 * This program creates a pair of connected sockets then forks and
 * communicates over them.  This is very similar to communication with pipes,
 * however, socketpairs are two-way communications objects. Therefore I can
 * send messages in both directions.
 */

main()
{
     int sockets[2], child;
     char buf[1024];

     if (socketpair(AF_UNIX, SOCK_STREAM, 0, sockets) < 0) {
          perror("opening stream socket pair");
          exit(1);
     }

     if ((child = fork()) == -1)
          perror("fork");
     else if (child) {   /* This is the parent. */
          close(sockets[0]);
          if (read(sockets[1], buf, 1024, 0) < 0)
               perror("reading stream message");
          printf("-->%s\n", buf);
          if (write(sockets[1], DATA2, sizeof(DATA2)) < 0)
               perror("writing stream message");
          close(sockets[1]);
     } else {       /* This is the child. */
          close(sockets[1]);
          if (write(sockets[0], DATA1, sizeof(DATA1)) < 0)
               perror("writing stream message");
          if (read(sockets[0], buf, 1024, 0) < 0)
               perror("reading stream message");
          printf("-->%s\n", buf);
          close(sockets[0]);
     }
}
               Figure 3  Use of a socketpair


















PPSSDD::2200--1188                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


                   Parent



                     |
                 +---+-----+
                 +---+-----+--------------+
                 +---------+---           |
                 +---------+              |
                 +---------+              |
                 +---------+              |
                                          |
                                          |
                         +                |
                        +++++++++++++    -|
                                   -

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


              Parent                      Child


                |                            |
                |                            |
            +---+-----+--------------++-+----+----+
            +---------+---           || +---------+
            +---------+              || +---------+
            +---------+              || +---------+
            +---------+              || +---------+
            +---------+              || +---------+
                                     ||
                    |-               ||
                    +++++++++++++   -||
                    +++++++++++++   --+

  Figure 4  Sharing a socketpair between parent and child


























IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--1199


#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>

/*
 * In the included file <sys/un.h> a sockaddr_un is defined as follows
 * struct sockaddr_un {
 *   short     sun_family;
 *   char sun_path[108];
 * };
 */

#include <stdio.h>

#define NAME "socket"

/*
 * This program creates a UNIX domain datagram socket, binds a name to it,
 * then reads from the socket.
 */
main()
{
     int sock, length;
     struct sockaddr_un name;
     char buf[1024];

     /* Create socket from which to read. */
     sock = socket(AF_UNIX, SOCK_DGRAM, 0);
     if (sock < 0) {
          perror("opening datagram socket");
          exit(1);
     }
     /* Create name. */
     name.sun_family = AF_UNIX;
     strcpy(name.sun_path, NAME);
     if (bind(sock, &name, sizeof(struct sockaddr_un))) {
          perror("binding name to datagram socket");
          exit(1);
     }
     printf("socket -->%s\n", NAME);
     /* Read from the socket */
     if (read(sock, buf, 1024) < 0)
          perror("receiving datagram packet");
     printf("-->%s\n", buf);
     close(sock);
     unlink(NAME);
}
          Figure 5a  Reading UNIX domain datagrams















PPSSDD::2200--2200                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>

#define DATA "The sea is calm tonight, the tide is full . . ."

/*
 * Here I send a datagram to a receiver whose name I get from the command
 * line arguments.  The form of the command line is udgramsend pathname
 */

main(argc, argv)
     int argc;
     char *argv[];
{
     int sock;
     struct sockaddr_un name;

     /* Create socket on which to send. */
     sock = socket(AF_UNIX, SOCK_DGRAM, 0);
     if (sock < 0) {
          perror("opening datagram socket");
          exit(1);
     }
     /* Construct name of socket to send to. */
     name.sun_family = AF_UNIX;
     strcpy(name.sun_path, argv[1]);
     /* Send message. */
     if (sendto(sock, DATA, sizeof(DATA), 0,
         &name, sizeof(struct sockaddr_un)) < 0) {
          perror("sending datagram message");
     }
     close(sock);
}
         Figure 5b  Sending a UNIX domain datagrams



























IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--2211


#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>

/*
 * In the included file <netinet/in.h> a sockaddr_in is defined as follows:
 * struct sockaddr_in {
 *   short     sin_family;
 *   u_short   sin_port;
 *   struct in_addr sin_addr;
 *   char sin_zero[8];
 * };
 *
 * This program creates a datagram socket, binds a name to it, then reads
 * from the socket.
 */
main()
{
     int sock, length;
     struct sockaddr_in name;
     char buf[1024];

     /* Create socket from which to read. */
     sock = socket(AF_INET, SOCK_DGRAM, 0);
     if (sock < 0) {
          perror("opening datagram socket");
          exit(1);
     }
     /* Create name with wildcards. */
     name.sin_family = AF_INET;
     name.sin_addr.s_addr = INADDR_ANY;
     name.sin_port = 0;
     if (bind(sock, &name, sizeof(name))) {
          perror("binding datagram socket");
          exit(1);
     }
     /* Find assigned port value and print it out. */
     length = sizeof(name);
     if (getsockname(sock, &name, &length)) {
          perror("getting socket name");
          exit(1);
     }
     printf("Socket has port #%d\n", ntohs(name.sin_port));
     /* Read from the socket */
     if (read(sock, buf, 1024) < 0)
          perror("receiving datagram packet");
     printf("-->%s\n", buf);
     close(sock);
}
        Figure 6a  Reading Internet domain datagrams












PPSSDD::2200--2222                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>

#define DATA "The sea is calm tonight, the tide is full . . ."

/*
 * Here I send a datagram to a receiver whose name I get from the command
 * line arguments.  The form of the command line is dgramsend hostname
 * portnumber
 */

main(argc, argv)
     int argc;
     char *argv[];
{
     int sock;
     struct sockaddr_in name;
     struct hostent *hp, *gethostbyname();

     /* Create socket on which to send. */
     sock = socket(AF_INET, SOCK_DGRAM, 0);
     if (sock < 0) {
          perror("opening datagram socket");
          exit(1);
     }
     /*
      * Construct name, with no wildcards, of the socket to send to.
      * Getnostbyname() returns a structure including the network address
      * of the specified host.  The port number is taken from the command
      * line.
      */
     hp = gethostbyname(argv[1]);
     if (hp == 0) {
          fprintf(stderr, "%s: unknown host0, argv[1]);
          exit(2);
     }
     bcopy(hp->h_addr, &name.sin_addr, hp->h_length);
     name.sin_family = AF_INET;
     name.sin_port = htons(atoi(argv[2]));
     /* Send message. */
     if (sendto(sock, DATA, sizeof(DATA), 0, &name, sizeof(name)) < 0)
          perror("sending datagram message");
     close(sock);
}
       Figure 6b  Sending an Internet domain datagram















IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--2233


#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>

#define DATA "Half a league, half a league . . ."

/*
 * This program creates a socket and initiates a connection with the socket
 * given in the command line.  One message is sent over the connection and
 * then the socket is closed, ending the connection. The form of the command
 * line is streamwrite hostname portnumber
 */

main(argc, argv)
     int argc;
     char *argv[];
{
     int sock;
     struct sockaddr_in server;
     struct hostent *hp, *gethostbyname();
     char buf[1024];

     /* Create socket */
     sock = socket(AF_INET, SOCK_STREAM, 0);
     if (sock < 0) {
          perror("opening stream socket");
          exit(1);
     }
     /* Connect socket using name specified by command line. */
     server.sin_family = AF_INET;
     hp = gethostbyname(argv[1]);
     if (hp == 0) {
          fprintf(stderr, "%s: unknown host0, argv[1]);
          exit(2);
     }
     bcopy(hp->h_addr, &server.sin_addr, hp->h_length);
     server.sin_port = htons(atoi(argv[2]));

     if (connect(sock, &server, sizeof(server)) < 0) {
          perror("connecting stream socket");
          exit(1);
     }
     if (write(sock, DATA, sizeof(DATA)) < 0)
          perror("writing on stream socket");
     close(sock);
}
 Figure 7a  Initiating an Internet domain stream connection














PPSSDD::2200--2244                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#define TRUE 1

/*
 * This program creates a socket and then begins an infinite loop. Each time
 * through the loop it accepts a connection and prints out messages from it.
 * When the connection breaks, or a termination message comes through, the
 * program accepts a new connection.
 */

main()
{
     int sock, length;
     struct sockaddr_in server;
     int msgsock;
     char buf[1024];
     int rval;
     int i;

     /* Create socket */
     sock = socket(AF_INET, SOCK_STREAM, 0);
     if (sock < 0) {
          perror("opening stream socket");
          exit(1);
     }
     /* Name socket using wildcards */
     server.sin_family = AF_INET;
     server.sin_addr.s_addr = INADDR_ANY;
     server.sin_port = 0;
     if (bind(sock, &server, sizeof(server))) {
          perror("binding stream socket");
          exit(1);
     }
     /* Find out assigned port number and print it out */
     length = sizeof(server);
     if (getsockname(sock, &server, &length)) {
          perror("getting socket name");
          exit(1);
     }
     printf("Socket has port #%d\n", ntohs(server.sin_port));

     /* Start accepting connections */
     listen(sock, 5);
     do {
          msgsock = accept(sock, 0, 0);
          if (msgsock == -1)
               perror("accept");
          else do {
               bzero(buf, sizeof(buf));










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--2255


               if ((rval = read(msgsock, buf, 1024)) < 0)
                    perror("reading stream message");
               i = 0;
               if (rval == 0)
                    printf("Ending connection\n");
               else
                    printf("-->%s\n", buf);
          } while (rval != 0);
          close(msgsock);
     } while (TRUE);
     /*
      * Since this program has an infinite loop, the socket "sock" is
      * never explicitly closed.  However, all sockets will be closed
      * automatically when a process is killed or terminates normally.
      */
}
 Figure 7b  Accepting an Internet domain stream connection


#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#define TRUE 1

/*
 * This program uses select() to check that someone is trying to connect
 * before calling accept().
 */

main()
{
     int sock, length;
     struct sockaddr_in server;
     int msgsock;
     char buf[1024];
     int rval;
     fd_set ready;
     struct timeval to;

     /* Create socket */
     sock = socket(AF_INET, SOCK_STREAM, 0);
     if (sock < 0) {
          perror("opening stream socket");
          exit(1);
     }
     /* Name socket using wildcards */
     server.sin_family = AF_INET;
     server.sin_addr.s_addr = INADDR_ANY;
     server.sin_port = 0;
     if (bind(sock, &server, sizeof(server))) {










PPSSDD::2200--2266                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


          perror("binding stream socket");
          exit(1);
     }
     /* Find out assigned port number and print it out */
     length = sizeof(server);
     if (getsockname(sock, &server, &length)) {
          perror("getting socket name");
          exit(1);
     }
     printf("Socket has port #%d\n", ntohs(server.sin_port));

     /* Start accepting connections */
     listen(sock, 5);
     do {
          FD_ZERO(&ready);
          FD_SET(sock, &ready);
          to.tv_sec = 5;
          if (select(sock + 1, &ready, 0, 0, &to) < 0) {
               perror("select");
               continue;
          }
          if (FD_ISSET(sock, &ready)) {
               msgsock = accept(sock, (struct sockaddr *)0, (int *)0);
               if (msgsock == -1)
                    perror("accept");
               else do {
                    bzero(buf, sizeof(buf));
                    if ((rval = read(msgsock, buf, 1024)) < 0)
                         perror("reading stream message");
                    else if (rval == 0)
                         printf("Ending connection\n");
                    else
                         printf("-->%s\n", buf);
               } while (rval > 0);
               close(msgsock);
          } else
               printf("Do something else\n");
     } while (TRUE);
}
 Figure 7c  Using select() to check for pending connections























IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--2277


         Process 1                   Process 2



                |                           |
            +---+----+-                 +---+----+-
            +---+----+-                 +---+----+-
            +--------+----+        -+---+--------+-
            +--------+-   |         |   +--------+-
            +--------+-   |         |   +--------+-
           -+--------+-   |         |  -+--------+-
                          |         |
                          |        ||
                          |           NAME


++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


          Process 1                   Process 2



                |                            |
            +---+-----+                 +----+----+
            +---+-----+---      ------+-+----+----+
            +---------+               | +---------+
            +---------+               | +---------+
            +---------+               | +---------+
            +---------+               | +---------+
                                      |  |
                                      | -
                    +                 |     NAME
                    +++++++++++++   --+
                                      |


         Figure 8  Establishing a stream connection

























PPSSDD::2200--2288                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>

#define DATA "Half a league, half a league . . ."

/*
 * This program connects to the socket named in the command line and sends a
 * one line message to that socket. The form of the command line is
 * ustreamwrite pathname
 */
main(argc, argv)
     int argc;
     char *argv[];
{
     int sock;
     struct sockaddr_un server;
     char buf[1024];

     /* Create socket */
     sock = socket(AF_UNIX, SOCK_STREAM, 0);
     if (sock < 0) {
          perror("opening stream socket");
          exit(1);
     }
     /* Connect socket using name specified by command line. */
     server.sun_family = AF_UNIX;
     strcpy(server.sun_path, argv[1]);

     if (connect(sock, &server, sizeof(struct sockaddr_un)) < 0) {
          close(sock);
          perror("connecting stream socket");
          exit(1);
     }
     if (write(sock, DATA, sizeof(DATA)) < 0)
          perror("writing on stream socket");
}
   Figure 9a  Initiating a UNIX domain stream connection


#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>

#define NAME "socket"

/*
 * This program creates a socket in the UNIX domain and binds a name to it.
 * After printing the socket's name it begins a loop. Each time through the
 * loop it accepts a connection and prints out messages from it.  When the
 * connection breaks, or a termination message comes through, the program










IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC                            PPSSDD::2200--2299


 * accepts a new connection.
 */
main()
{
     int sock, msgsock, rval;
     struct sockaddr_un server;
     char buf[1024];

     /* Create socket */
     sock = socket(AF_UNIX, SOCK_STREAM, 0);
     if (sock < 0) {
          perror("opening stream socket");
          exit(1);
     }
     /* Name socket using file system name */
     server.sun_family = AF_UNIX;
     strcpy(server.sun_path, NAME);
     if (bind(sock, &server, sizeof(struct sockaddr_un))) {
          perror("binding stream socket");
          exit(1);
     }
     printf("Socket has name %s\n", server.sun_path);
     /* Start accepting connections */
     listen(sock, 5);
     for (;;) {
          msgsock = accept(sock, 0, 0);
          if (msgsock == -1)
               perror("accept");
          else do {
               bzero(buf, sizeof(buf));
               if ((rval = read(msgsock, buf, 1024)) < 0)
                    perror("reading stream message");
               else if (rval == 0)
                    printf("Ending connection\n");
               else
                    printf("-->%s\n", buf);
          } while (rval > 0);
          close(msgsock);
     }
     /*
      * The following statements are not executed, because they follow an
      * infinite loop.  However, most ordinary programs will not run
      * forever.  In the UNIX domain it is necessary to tell the file
      * system that one is through using NAME.  In most programs one uses
      * the call unlink() as below. Since the user will have to kill this
      * program, it will be necessary to remove the name by a command from
      * the shell.
      */
     close(sock);
     unlink(NAME);
}
    Figure 9b  Accepting a UNIX domain stream connection











PPSSDD::2200--3300                            IInnttrroodduuccttoorryy 44..44BBSSDD IIPPCC


     /*
      * The variable descriptor may be the descriptor of either a file
      * or of a socket.
      */
     cc = read(descriptor, buf, nbytes)
     int cc, descriptor; char *buf; int nbytes;

     /*
      * An iovec can include several source buffers.
      */
     cc = readv(descriptor, iov, iovcnt)
     int cc, descriptor; struct iovec *iov; int iovcnt;

     cc = write(descriptor, buf, nbytes)
     int cc, descriptor; char *buf; int nbytes;

     cc = writev(descriptor, iovec, ioveclen)
     int cc, descriptor; struct iovec *iovec; int ioveclen;

     /*
      * The variable ``sock'' must be the descriptor of a socket.
      * Flags may include MSG_OOB and MSG_PEEK.
      */
     cc = send(sock, msg, len, flags)
     int cc, sock; char *msg; int len, flags;

     cc = sendto(sock, msg, len, flags, to, tolen)
     int cc, sock; char *msg; int len, flags;
     struct sockaddr *to; int tolen;

     cc = sendmsg(sock, msg, flags)
     int cc, sock; struct msghdr msg[]; int flags;

     cc = recv(sock, buf, len, flags)
     int cc, sock; char *buf; int len, flags;

     cc = recvfrom(sock, buf, len, flags, from, fromlen)
     int cc, sock; char *buf; int len, flags;
     struct sockaddr *from; int *fromlen;

     cc = recvmsg(sock, msg, flags)
     int cc, socket; struct msghdr msg[]; int flags;

      Figure 10  Varieties of read and write commands















