= announce, dial, listen: make network connections     include "sys.m";    sys := load Sys Sys->PATH;     Connection: adt    {        dfd:  ref FD;  # data file        cfd:  ref FD;  # control file        dir:  string;  # pathname of line directory    };        announce: fn(addr: string):           (int, Connection);    dial:     fn(addr, local: string):    (int, Connection);    listen:   fn(c: Connection):          (int, Connection); These  routines  in  the  Sys  module  are still used by someexisting code.  They are  deprecated,  and  new  applicationsshould use the Dial module described in dial(2). These routines establish network connections.  Their descrip‐tion uses the following definitions: addr   is a network address in one of the following forms:        network!netaddr!service       network!netaddr       netaddr network       Any directory listed in /net (eg, tcp), or the special       token, net.  The name net acts as a free variable that       stands for any network in common  between  the  source       and  netaddr.   A  network name can be preceded by the       full path name of a directory of networks,  using  the       form /dir/network (eg, /net.alt/tcp). netaddr       A  host  name,  a domain name, a network address, or a       meta-name of the form $attribute, which is replaced by       value  from  the corresponding attribute-value pair in       the connection server data base (see db(6)). The functions dial and announce translate a given addr to  anactual network address using the connection server cs(8).  Ifa logical name addr corresponds to several network addresses,for instance if a destination machine has several interfaces,cs will return them all.  In particular, if addr is  net,  cswill  return  addresses  on  all  networks that are common tosource and destination.  The translation  procedure  accessescs  using  its interface file cs, which is sought as follows:first, in an explicit directory /dir if one was given in net‐work;  second, in the standard directory /net; and finally inthe directory /net.alt (dial only).  If the connection servercannot be found, the addr is used as-is. If  a connection attempt is successful, the dir member of theresulting Connection will be the path name of a  line  direc‐tory  that  has files for accessing the connection.  One linedirectory exists for each possible connection.  The data filein  the  line  directory  is opened to make a connection, andread and written to communicate with  the  destination.   Thectl  file  in the line directory can be used to send commandsto the line.  See ip(3) for messages that can be  written  tothe  ctl  file.   The last close of the data or ctl file willclose the connection.  The remote file in the line  directorycontains  the  address  called;  the  file local contains thelocal address assigned. The dial routine makes a call to destination addr on a multi‐plexed  network.   If  the  connection server returns severaladdresses, dial tries each in turn,  until  a  connection  ismade  or  no addresses remain to be tried.  It returns a Con‐nection containing a file descriptor dfd open for reading andwriting  the  data  file  in  the  line directory, and a filedescriptor cfd open for reading and writing the ctl file.  Iflocal  is non-empty, and the network allows the local addressto be set, as is the case with UDP and TCP port numbers,  thelocal address will be set to local. Announce  and  listen  are the complements of dial.  Announceestablishes a network name to which  incoming  calls  can  bemade.   In  addr, netaddr gives the name or address of one ofthe local host's interfaces on which to listen for  calls  tothe  given  service;  it  can be * to listen for calls on anyinterface on network.  Announce returns a  Connection  struc‐ture in which only the cfd descriptor is open, on the controlfile representing the announcement.  Listen takes as its onlyargument  the  Connection  structure  of a successful call toannounce.  When a call is received, listen  returns  an  openConnection  structure  as  if from dial, except that only thecfd descriptor is open, dfd is nil, and the caller must  openthe data file for itself. == examplesMake  a  call  and  return an open file descriptor to use forcommunications:        callkremvax(): (int, Connection)       {            return sys->dial("tcp!kremvax!80", nil);       } Call the local certificate signer:        dialsigner(service: string): (int, Connection)       {            return sys->dial("net!$SIGNER!inflogin", nil);       } == source/emu/port/inferno.c/emu/port/dial.c/os/port/inferno.c/os/port/dial.c == diagnosticsThe integer valued functions return 0 on success  and  -1  onerror; the system error string is set.  The integer componentof the tuple returned by the other functions follows the sameconvention. == bugsNote that listen does not open the data file.